Part i, pp. 57–67, Pl. 1.)

—— Odoriferous glands of invertebrata. (Proc. Acad. Philadelphia, 1849, iv, 234–236, 1 Pl.; Ann. and Mag. N. H., Ser. 2, 1850, v, pp. 154–156.)

=Chapuis et Candèze.= Catalogue des larves des coléoptères, etc. (Mém. Soc. Sci. de Liège, 1853, viii, pp. 351–653, Pls. 1–9, pp. 611, 612.)

=Siebold, Carl Theodor.= Lehrbuch der vergleichenden Anatomie der wirbellosen Thiere, 1848. (Burnett’s transl., Boston, 1854.)

=Burnett, Waldo Irving.= Translation of Siebold’s Anatomy of the Invertebrates, 1854. (Note on the osmeteria of _Papilio asterias_, which he regards as an odoriferous and defensive, rather than tactile, organ, p. 415.)

=Karsten, H.= Bemerkungen über einige shaarfe und brennende Absonderungen verschiedener Raupen. (Müller’s Archiv für Anat. Phys. u. wiss. Med., 1848, pp. 375–382, Taf. 11, 12.) Describes the poison-glands at the base of the spines of Saturnia larvæ.

—— Harnorgane des _Brachinus complanatus_. (Müller’s Archiv, 1848, pp. 367–376, Taf.)

=Laboulbéne, Alexandre.= Note sur les caroncules thoraciques du Malachius. (Annales de la Soc. Ent. de France, 3^e Sér., vi, 1858, pp. 521–528.)

=Saussure, Henri de.= Recherches zoologiques de l’Amerique centrale et du Mexique. (6^e Partie, Études sur les Myriopodes et les Insectes, Paris, 1860.)

=Gerstaecker, C. E. A.= Ueber das vorkommen von ausstülpbaren Hautanhängen am Hinterleibe an Schaben. (Archiv f. Naturgesch., 1861, xxi, pp. 107–115.)

=Liegel, Hermann.= Ueber den Ausstülpungsapparat von Malachius und verwandten Formen. Inaug. Diss., Göttingen, pp. 31, 1 Taf. (n. d., since 1858 and before 1878.)

=Leydig, F.= Zur Anatomie der Insecten. (Archiv f. Anat. Phys. u. wiss. Med., 1859, pp. 33–89, 149–183, Taf. 2–4, pp. 35 and 38.)

—— Ueber bombardier Käfer. (Biolog. Centralbl., x, 1890, pp. 395, 396.)

=Claus, C.= Ueber die Seitendrüsen der Larve von _Chrysomela populi_. (Zeits. f. wissens. Zool., xi, 1861, pp. 309–314, Taf. xxv.)

—— Ueber Schutzwassen der Raupen des Gabelschwanzes. (Würzburger Naturw. Zeitschrift, 1862, iii, xiv; Sitz. am., 28 Juni, 1862.)

=Rogenhofer, Alois.= Drei Schmetterlingsmetamorphosen. (Verhandlungen der k. k. zoolog.-bot. Gesellschaft, Wien, xiii, 1862, pp. 1224–1230.)

=Fitch, Asa.= Eighth report on the noxious and other insects of ... New York. (Trans. N. Y. State Agric. Soc., 1862, xxii, pp. 657–684), p. 677. (Separate.)

=Guenée, Achille.= D’une organe particulier que présente une chenille de Lycæna. (Annales Soc. Ent. de France, Sér. 4, 1867, pp. 665–668, Pl. 13.)

=Landois, L.= Anatomie der Bettwanze, _Cimex lectularius_, mit berücksichtigung verwandter Hemipterengeschlechter. (Zeitsch. f. wissens. Zool., 1868, xvii, pp. 206–224, 218–223, Taf. 11, 12.)

=Studer, Theodor.= Mittheilungen der naturforsch. Gesellschaft in Bern, 1872–1873, No. 792–811, p. 101.

=Candèze, E.= Les moyens d’attaque et de défense chez les insectes. (Bull. Acad. royale de Belgique, 2 Sér., xxxviii, 1874, pp. 787–816.)

=Mayer, Paul.= Anatomie von _Pyrrhocoris apterus_. (Reichert und du Bois-Reymond’s Archiv f. Anat. Phys., etc., 1874, pp. 313–347, 3 Taf.)

=Scudder, Samuel Hubbard.= Odoriferous glands in Phasmidæ. (Psyche, i, pp. 137–140, Jan. 14, 1876; Amer. Nat., x, p. 256, April, 1876.)

—— Prothoracic tubercles in butterfly caterpillars. (Psyche, i, pp. 64, 168, 1876.)

—— Organs found near the anus of the ♀ pupa of Danais, which recall the odoriferous organs mentioned by Burnett, transl. Siebold’s Comp. Anat. as occurring in Argynnis and other genera. (Psyche, iii, p. 278, 1882, p. 453, note 22.)

—— Glands and extensile organs of larvæ of blue butterflies. (Proc. Bos. Soc. Nat. Hist., xxxiii, pp. 357, 358, 1888.)

—— Butterflies of Eastern United States. i-iii, 1889.

—— New light on the formation of the abdominal pouch in Parnassius. (Trans. Ent. Soc. London for 1892, January, 1893, pp. 249–253.)

=Müller, Fritz.= Die Stinkkölbchen der weiblichen Maracujáfalter. (Zeitschr. f. wissens. Zool., 1877, xxx, pp. 167–170, Taf. 9.)

=Plateau, Félix.= Note sur une sécrétion propre aux coléoptères dytiscides. (Ann. Soc. Ent. Belg., 1876, v, xix, pp. 1–10.)

=Edwards, William H.= Notes on _Lycæna pseudargiolus_ and its larval history. (Can. Ent., x, Jan., 1878, pp. 1–14. Fig.)

—— On the larvæ of _Lycæna pseudargiolus_ and attendant ants. (Can. Ent., x, July, 1878, pp. 131–136.)

—— Butterflies of North America, i-iv. Many Pls. Phil., 1868—.

=Voges, Ernst.= Beiträge zur Kenntniss der Juliden. (Zeitsch. f. wissens. Zoologie, xxxi, p. 127, 1878. The scent-glands are retort-shaped bodies, the necks of which open into _foramina repugnatoria_.)

=Rye, E. C.= Secretion of water-beetles. (Ent. Month. Mag., xiv, 1877–1878, pp. 232, 233.)

=Forel, A.= Der Giftapparat und die anal Drüsen der Ameisen. (Zeits. f. wissens. Zool., 1878, xxx, Suppl., pp. 28–68, Taf. 3, 4.)

=Saunders, William.= Notes on the larva of _Lycæna scudderi_. (Can. Ent., x, Jan., 1878, p. 14.)

=Weismann, A.= Ueber Duftschuppen. (Zool. Anzeiger, 26th Aug., 1878, Jahrg., i, pp. 98, 99.)

=Gissler, Carl Friedrich.= On the repugnatorial glands in Eleodes. (Psyche, ii, Feb., 1879, pp. 209, 210.)

—— Odoriferous glands on the 5th abdominal segment in nymph of _Lachnus strobi_. (Fig. 273, p. 804, of Packard’s Report on Forest and Shade Tree Insects, 1890.)

=Brunner von Wattenwyl, K.= Ueber ein neues Organ bei den Acridiodeen. (Verhandl. k. k. Zool. Bot. Gesells. Wien., 1879, xxix; Sitzungsber., pp. 26, 27.)

—— Verhandl. k. k. Zool. Bot. Gesells. Wien. (A peculiar organ on hind femora of Acridiidæ.)

=Rougement, P.= Observations sur l’organe détonnant du _Brachinus crepitans_ Oliv. (Bull. Soc. Sci. Nat. Neuchâtel, 1879, xi, pp. 471–478, Pl.)

=Goossens, Th.= Sur une organe entre la tête et la première paire de pattes de quelques chenilles. (Ann. Soc. Ent. France, ix, p. 4, 1809; Bull., pp. 60, 61.)

—— Des chenilles vésicantes. (Ann. Ent. Soc. France, vi, pp. 461–464, 1887.)

=Coquillett, D. W.= On the early stages of some moths. (Can. Ent., March, 1880, xii, pp. 43–46.)

=Chambers, Victor Tousey.= Notes upon some Tineid larvæ. (Psyche, iii, July, 1880, p. 67. Certain retractile processes “from the sides of certain segments of the larva.”)

—— Further notes on some Tineid larvæ. (Psyche, iii, p. 135, Feb. 12, 1881. Larva of Phyllocnistis has eight pairs of lateral pseudopodia on first eight abdominal segments.)

=French, G. H.= Larvæ of _Cerura occidentalis_ Lint, and _C. borealis_ Bd. (Can. Ent., July, 1881, xiii, pp. 144, 145.)

=Passerini, N.= Sopra i due tubercoli abdominali della larva della _Porthesia chrysorrhœa_. (Bull. Soc. Ent. Ital., 1881, xiii, pp. 293–296.)

=Klemensiewicz, Stanislaus.= Zur näheren Kenntniss der Hautdrüsen bei den Raupen und bei _Malachius_. (Verhandlungen d. Zool. Bot. Gesellsch. Wien., xxxii, pp. 459–474, 1882, 2 Taf.)

=Weber, Max.= Ueber eine Cyanwasserstoffsäure bereitende Drüse. (Archiv für Mikr. Anat., xxi, pp. 468–475, xxiv, 1882.)

=Bertkau, Philip.= Ueber den Stinkapparat von _Lacon murinus_ L. (Archiv f. Naturg., 1882, Jahrg., xlviii, pp. 371–373.)

=Dimmock, George.= Organs, probably defensive in function, in the larva of _Hyperchiria varia_ Walk. (_Saturnia io_ Harris). (Psyche, iii, pp. 352, 353, Aug. 19, 1882. Account of lateral eversible glands on 1st and 7th abdominal segments; they emit neither moisture nor odor.)

—— On some glands which open externally on insects. (Psyche, iii, pp. 387–399, Jan. 15, 1883. Treats of poison-glands, glandular hairs, eversible glands of Cerura, etc.)

=Coleman, N.= Notes on _Orgyia leucostigma_. (Papilio, November-December, 1882, Jan., 1883, ii, pp. 164–166.)

=Müller, F.= Der Anhang am Hinterleibe der _Acræa_-weibchen. (Zool. Anzeiger, 6th Aug., 1883, Jahrg., vi, pp. 415, 416.)

=Dewitz, H.= Ueber das durch die Foramina repugnatoria entleerte Secret bei Glomeris. (Biol. Centralblatt, iv, pp. 202, 203, 1884.)

=Williston, S. A.= Protective secretion of Eleodes ejected from anal gland. (Psyche, iv, p. 168, May, 1884.)

=Poulton, Edward Bagnall.= Notes in 1885 upon lepidopterous larvæ and pupæ, including an account of the loss of weight in the freshly-formed lepidopterous pupæ. (Trans. Ent. Soc., London, June, 1886, pp. 156, 157, 159.)

—— Notes in 1886 upon lepidopterous larvæ, etc. (Trans. Ent. Soc., London, Sept., 1887, pp. 295–301.)

—— Notes in 1887 upon lepidopterous larvæ, etc. (Trans. Ent. Soc., London, 1888, p. 597.)

=Künckel-d’Herculais, J.= La punaise de lit et ses appareils odoriférants. (Comptes rendus, ciii, 1886, pp. 81–83; Annals & Mag. Nat. Hist., 5th Ser., xviii, 1886, pp. 167, 168.)

—— Étude comparée des appareils odorifiques dans les differents groupes d’Hemiptères hétéroptères. (Compt. rend. Acad. Sc., Paris, cxx, pp. 1002–1004.)

=Packard, A. S.= The fluid ejected by notodontian caterpillars. (Amer. Nat., 1886, xx, pp. 811, 812.)

—— An eversible “gland” in the larva of Orgyia. (Amer. Nat., 1886, xx, p. 814.)

—— Fifth Rep. U. S. Ent. Comm. Insects injurious to forest and shade trees, p. 136, 1890.

—— Hints on the evolution of the bristles, spines, and tubercles of certain caterpillars. (Proc. Boston Soc. Nat. Hist., xxiv, 1890, p. 551.)

—— Notes on some points in the external structure and phylogeny of lepidopterous larvæ. (Proc. Bost. Soc. Nat. Hist., xxv, 1890, pp. 83–114.)

—— A study of the transformations and anatomy of _Lagoa crispata_, a bombycine moth. (Proc. Amer. Phil. Soc., Philadelphia, xxxii, 1893, pp. 275–292, 7 Pls.)

—— The eversible repugnatorial scent glands of insects. (Journ., N. Y. Ent. Soc. iii, 1895, pp. 110–127; iv, p. 896; pp. 26–32, 1 Pl.)

=Loman, J. C. C.= Freies Jod als Drüsensecret. (Tijdschr. Neder. Dierk. Ver. Deel 1, 1887, pp. 106–108.)

=Riley, Charles Valentine.= Proc. Ent. Soc., Washington, March 13, 1888, i, pp. 87–89.

—— Notes on the eversible glands of larvæ of _Orgyia_ and _Parorgyia leucopæa_ and _P. clintonii (achatina)_. (See 5th Rep. U. S. Ent. Comm., p. 137.)

=Denham, Ch. S.= The acid secretion of _Notodonta concinna_. (Insect Life, i, p. 147, 1888; hydrochloric acid.)

=Michin, Edward A.= Note on a new organ, and on the structure of the hypodermis, in _Periplaneta orientalis_. (Quart. Journ. Micros. Sc., Dec., 1888, xxiv, 1 Pl.)

—— Further observations on the dorsal gland in the abdomen of Periplaneta and its allies. (Zool. Anz., 27 Jan., 1890, pp. 41–44.)

=Maynard, C. L.= The defensive glands of a species of Phasma, _Anisomorpha buprestoides_. (Contributions to Science, i, April, 1889.)

=Schaeffer, Cæsar.= Beiträge zur Histologie der Insekten. (Zool. Jahrb. Morph. Abth. iii, pp. 611–652, Taf. xxix, xxx, 1889; treats of the ventral glands in prothorax of caterpillars; scales and hairs are secretions from the very greatly enlarged hypodermic cells.)

=Gilson, G.= Les glandes odorifères der _Blaps mortisaga_ et de quelques autres espèces. (La Cellule, v. pp. 1–21, 1 Pl., 1889.)

—— The odoriferous apparatus of _Blaps mortisaga_. (Rep. 58th Meeting Brit. Assoc. Adv. Sc., 1889, pp. 727, 728.)

=Haase, Erich.= Ueber die Stinkdrüsen der Orthoptera. (Sitzgsber. Ges. Naturf. Freunde, Berlin, pp. 57, 58, 1889.)

—— Zur Anatomie der Blattiden. (Zool. Anz., xii Jahrg., pp. 169–172, 1889.)

=Herbst, Curt.= Anatomische Untersuchungen an _Scutigera coleoptrata_. Ein Beitrag zur vergleichenden Anatomie der Articulaten. Dissert., Jena, pp. 36 (Hautdrüsen, Coxal-Organ.); p. 1, 1889.

=Wheeler, William M.= Hydrocyanic acid secreted by _Polydesmus virginiensis_ Drury. (Psyche, v, p. 422.)

—— New glands in the hemipterous embryo. (Amer. Nat., Feb. 1890, p. 187; odorous(?) glands.)

=Jackson, W. Hatchett.= Studies in the morphology of the Lepidoptera, Pt. i. (Trans. Linn. Soc., London, 2 Ser., Zoöl., v, May, 1890.)

=Krauss, Hermann.= Die Duftdrüse der _Aphlebia bivittata_ Brullé (Blattidæ) von Teneriffa. (Zool. Anz., xiii Jahrg., 1890, pp. 584–587, 3 Figs.)

=Fernald, H. T.= Rectal glands in Coleoptera. (Amer. Nat., xxiv, pp. 100, 101, Pls. 4, 5, 1890.)

=Verson, E.= Hautdrüsen system bei Bombyciden (Seidenspinner). (Zool. Anzeiger, 1890, pp. 118–120.)

=Vosseler, Julius.= Die Stinkdrüsen der Forficuliden. (Arch. Mikr. Anat., xxxvi, 1890, pp. 565–578, Taf. 29.)

=Carrière, J.= Die Drüsen am ersten Hinterleibsringe der Insektenembryonen. (Biol. Centralblatt, xi, pp. 110–127, 1891.)

=Borgert, Henry.= Die Hautdrüsen der Tracheaten. Inaugural Diss., Jena, 1891, pp. 1–80.

=Lang, Arnold.= Lehrbuch der vergleichende Anatomie, English Trans. by Henry M. and Matilda Bernard, 1891, pp. 458, 459.

=Kennel, J. von.= Die Verwandtschaftverhältnisse der Arthropoden. (Schriften herausgegeben von der Naturforscher Gesellschaft bei der Universität Dorpat, vi, Dorpat, 1891.)

=Patton, W. H.= Scent-glands in the larva of Limacodes. (Can. Ent., xxiii, Feb. 1891, pp. 42, 43; eight pairs of glands with pores along the edges of the back.)

=Batelli, Andrea.= Di una particolarita nell integumento dell’ _Aphrophora spumaria_. (Monitore Zoöl. Ital. Anno 2, pp. 30–32, 1891. Dermal glands in the hindermost segment.)

=Ash, C. D.= Notes on the larva of _Danima banksii_ Lewin. (Ent. Month. Mag., Sept. 1892, p. 232, Fig.) notodontian larva protrudes from under side of prothoracic segment a Y-shaped, red organ like that of Papilio; no odor or fluid given out.

=Bernard, Henry M.= An endeavor to show that the tracheæ of the Arthropoda arose from setiparous sacs. (Spengel’s Zool., Jahrbuch, 1892, pp. 511–524, 3 Figs.)

=Latter, Oswald.= The secretion of potassium hydroxide by _Dicranura vinula_, and the emergence of the imago from the cocoon. (Trans. Ent. Soc. London, 1892, 287, also xxxii; Prof. Meldola adds that the larva of _D. vinula_ secretes strong, formic acid, and is the only animal known to secrete a strong, caustic alkali.)

—— Further notes on the secretion of potassium hydroxide by _Dicranura vinula_ (imago), and similar phenomena in other Lepidoptera. (Trans. Ent. Soc. London; Nature, 1895, p. 551, March 20, 1895.)

=Zograff, Nicolas.= Note sur l’origine et les parentes des Arthropodes, principalement des Arthropodes trachèates. (Congrès Internationale de Zoologie, 2^e Session à Moscow, Aug. 1892; Part i, Moscow, 1892, pp. 278–302, 1892; cyanogenic glands in Myriopods, p. 287.)

=Swale, H.= Odor of _Olophrum piceum_. (Ent. Month. Mag., v, Jan. 1896, pp. 1, 2.)

=Cuénot, L.= Moyens de défense dans la série animale, Paris, n. d. (1892); the ejection of blood as a means of defence by some Coleoptera. (Comptes rendus, Acad. Sc. France, April 16; Nature, April 26, 1894.)

—— Sur la saignée réflexe et les moyens de défense de quelques insectes. (Arch. Zool. expér. (3), 1897, iv, pp. 655, 679, 680.)

=Holmgren, Emil.= Studier öfverhudens och de körtelartade hudorganens morfologi hos skandinaviska macrolepidopterlarver. (K. Svenska Vetenskaps-Akademiens Handlingar, xxvii, No. 4, Stockholm, 4º 1895, pp. 82, 9 Pls.)

=Lutz, K. G.= Das Blut der Coccinelliden. (Zool. Anzeiger, 1895, pp. 244–255, 1 Fig.)

=Gilson, Gustav.= Studies in insect morphology. (Proc. Linn. Soc. London, March 5, 1896; Nature, p. 500.)

—— On segmentally disposed thoracic glands in the larvæ of the Trichoptera. (Journ. Linn. Soc., London, xxv. 1897.)

=Cholodkowsky, N.= Entomotomische Miscellen, v, Ueber die Spritzapparate der Cimbiciden Larven, pp. 135–143, 2 Taf. Ibid., vi. Ueber das Bluten der Cimbiciden Larven, pp. 352–357, 1 Fig. (Horæ Soc. Ent. Rossicæ, xxx, 1897.)

Also the writings of Darwin, Wallace, Poulton, Weir, Beddard, Butler, Busgen, (pp. 365–367), Girard, Kolbe, Locy.

THE ALLURING OR SCENT-GLANDS

It is difficult to draw the line between repelling and alluring glands. Attention was first definitely called to the alluring odors of Lepidoptera by Fritz Müller, who showed that the males of certain butterflies are rendered attractive to the other sex by secreting odorous oils of the ether series. He pointed out that the seat of the odor is the androconia (see p. 199), while either repellent or pleasant odors are exhaled from abdominal glands.

Those of _Pieris napi_ yield a scent like that of citrons, _Didonis biblis_ gives off three different odors from different parts of the body, besides having a distinctly odorous spot on the hind wings. Both sexes have a sac between the fourth and fifth abdominal segments which exhales a very unpleasant (protective) odor, while the males have on the succeeding segment a pair of glands from which proceeds an agreeable odor like that of the heliotrope. _Callidryas argante_ throws off a musky odor. In _Prepona laertes_ the odor is like that of a bat, in _Dircenna xantho_ it is vanilla-like, the androconia being situated on the front edge of the hind wings. In _Papilio grayi_ the odor is said to be as agreeable and intense as in flowers. Certain sphingids are known to exhale a distinct odor, which Müller has traced to a tuft of hair-like scales at the base of the abdomen, and which fits into a groove in the first segment, so as to be ordinarily invisible.

In the noctuid genus, Patula, the costal half of the hind wing is modified to form a large scent-gland, and in consequence the venation has been modified. The still greater distortion of the veins in the allied genus, Argida, was attributed by the author to its once having possessed a similar scent-gland, now become rudimentary by disuse. (Hampson.)

Peculiar white or orange-colored, hairy, thread-like processes have been found protruding from narrow openings near the tip of the abdomen of Arctian moths (Fig. 367), which throw off, according to J. B. Smith, “an intense odor, somewhat like the smell of laudanum.” We have perceived the same unpleasant odor emanating from the males of _Spilosoma virginica_ and _Arctia virgo_, as well as _Leucarctia acræa_.

We are informed by C. Dury that similar but longer hairy appendages are thrust out by the male of _Haploa clymene_. Many glaucopid moths protrude similar glandular processes. Thus Müller tells us that on seizing a glaucopid female by the wings, nearly the whole body became enveloped in a large cloud of snow-white wool which came out of a sort of pouch on the ventral side of the abdomen.

The male of a glaucopid was seen to dart out a pair of long hollow hairy retractile filaments which in some species exceed the whole body in length. The apparatus secretes a peculiar odor, probably serving to allure the female (Nature), and certain Zygænidæ have on the inner side of the paranal lobes (Afterklappen) glands filled with a sweetly scented fluid. Smith has detected a peculiar brush of hair-like scales in a groove between the dorsal and ventral parts of the basal two segments of the abdomen of _Schinia marginata_ (family Noctuidæ), and when removed it exhaled a laudanum-like smell.

The pupa of _Citheronia regalis_ gives out from the end of the abdomen a scent reminding us of laudanum.

Another mode of disseminating pleasant, alluring odors is that of the males of certain moths, which bear pencils and tufts on their fore or hind legs, and in the case of an Indian butterfly on the greatly elongated palpi. Those on the legs are ordinarily concealed in cavities or furrows in the leg, and may be thrust out and expanded so as to widely diffuse their odor. Such are those of the males of Catocala (Fig. 368), which resemble an artist’s fitch brush. In _Hepialus hecta_, where the arrangements for protecting the tufts are quite abnormal, Bertkau has detected the cells which secrete the odorous fluid. In the male of another Hepialus (_H. humuli_) a peculiar scent proceeds from the curiously aborted and altered hind tibiæ. (Barrett.) In one case, that of a geometrid moth (_Bapata dichroa_ of New Guinea), these pencils occur on all the legs. (Haase). In many species a distinct odor is perceptible when the leg bearing the pencil or tuft is crushed.

These eversible scent-glands have been supposed to be mostly restricted to the Lepidoptera, and to a single known case in the Trichoptera, but similar alluring male glands also occur in the Orthoptera (Locustidæ). H. Garman has described and figured in the cave cricket (_Hadenœcus subterraneus_) “a pair of white fleshy appendages protruding from slits between the terga of the 9th and 10th abdominal somites, the nature of which is not clear,” adding, “the slits through which the organs appear are situated one on each side anterior to and a little within the cerci. When fully protruded, the glands are white, cylindrical, a little tapering, and are about one-eighth of an inch long.” He believes that they are protruded during the period of sexual excitement, and suggests that “the sense of smell is certainly the one best calculated to bring the sexes together in the darkness of caves.” We had previously noticed these organs in alcoholic specimens, but supposed that they were fungous growths. On dissecting and making microscopic sections of them, the gland is, when extended (Fig. 369), seen to be a long, ensiform, sharp, band-like process, with numerous retractor muscular fibres. When at rest each gland is folded about five times, forming a bundle lying on each side of the end of the intestine. The walls are formed of a single layer of epithelium, as seen in Fig. 369, _B_.

In the male of the common wingless cricket, _Ceuthophilus maculatus_, we have discovered what appears to be a pair of scent-glands lying directly over the last abdominal ganglion. They form two large white sacs situated close together, with a short common duct which passes back and opens externally upwards by a transverse slit on the under side of the last segment of the body.

LITERATURE ON ALLURING GLANDS

=Watson, J.= On the microscopical examination of plumules, etc. (Ent. Month. Mag., ii, 1865, p. 1.)

—— On certain scales of some diurnal Lepidoptera. (Mem. Lit. and Phil. Soc. Manchester, Ser. 3, ii, 1868, p. 63.)

—— On the plumules or battledore scales of Lycænidæ. (Mem. Lit. and Phil. Soc. Manchester, Ser. 3, iii, 1869, p. 128.) Further remarks, etc. (Ibid., p. 259.)

=Anthony, J.= Structure of battledore scales. (Month. Microsc. Journ., vii, 1872, p. 250; see also p. 200.)

=Morrison, Herbert Knowles.= On an appendage of the male _Leucarctia acræa_. (Psyche, i, pp. 21–22, October, 1874.)

=Müller, Fritz.= The habits of various insects. (Nature, June 11, 1874, pp. 102–103.)

—— Ueber Haarpinsel, Fitzflecke und ähnliche Gebilde auf den Flügeln männlicher Schmetterlinge. (Jena. Zeitschr. f. Naturw., 1877, xi, pp. 99–114.)

—— Beobachtungen an brasilianischen Schmetterlingen, ii. I. Die Duftschuppen der männlichen Maracujáfalter. (Kosmos, 1877, i, pp. 391–395, Figs. 5, 6.) II. Die Duftschuppen des männchens von _Dione vanillæ_. (Kosmos, ii, 1877, pp. 38–42, 7 Taf.)

—— As maculas sexuaes dos individuos masculinos das especies _Danais erippus_ e _D. gilippus_. (Arch. Mus. Nac. Rio Janeiro, ii, 1877 (1878), pp. 25–29, 1 Pl.)

—— Die Duftschuppen der Schmetterlinge (nach dem “Kosmos” in Ent. Nachr., 1878, pp. 29–32, 109).

—— Wo hat der Moschusduft der Schwärmer seinen Sitz? (Kosmos, ii Jahrg., 1878, pp. 84, 85.)

—— Os orgaos odoriferos dos especias _Epicalia acontius_, Lin. e de _Myscelia orsis_, Dru. (Arch. Mus. Nac. Rio Janeiro, ii, 1879, pp. 31–35.)

—— Os orgaos odoriferos nas pernas de certos Lepidopteres. (Arch. Mus. Nac. Rio Janeiro, ii, 1879, pp. 37–46, 3 Pls.)

—— Os orgaos odoriferos da _Antirrhœa archœa_. (Arch. Mus. Nac. Rio Janeiro, iii, 1878, pp. 1–7, 1 Pl.)

—— A prega costal das Hesperideas. (Arch. Mus. Nac. Rio Janeiro, iii, 1880, pp. 41–50, 2 Pls.)

=Weismann, August.= Ueber Duftschuppen. (Zool. Anzeiger, i, 1878, pp. 98, 99.)

=Arnhart, L.= Sexundäre Geschlechtscharaktere von _Acherontia atropos_. (Verh. d. k. k. zool. bot. Ges. Wien, xxix, 1879, p. 54.)

=Bertkau, Philipp.= Duftapparat an Schmetterlingsbeinen. (Ent. Nachrichten, 1879, Jahrg., pp. 223, 224.)

—— Ueber den Duftapparat von _Hepialus hecta_. (Archiv f. naturg., xlviii Jahrg., 1882, pp. 363–370, Figs.; also in Biol. Centralblatt., ii Jahrg., 1882, pp. 500–502.)

—— Ergänzung (Duftvorrichtungen bei Lepidopteren). (Ent. Nachr., 1880, p. 206.)

—— Entomologische Mizellen. 1. Ueber Duftvorrichtungen einiger Schmetterlinge. (Verh. d. naturhist. Ver. d. preuss. Rheinlande und Westf., 1884, pp. 343–350.)

=Reichenau, W. von.= Der Duftapparat von _Sphinx ligustri_. (Ent. Nachr., 1880, p. 141; also Kosmos, iv Jahrg., 1880, pp. 387–390.)

=Fügner, R.= Duftapparat bei _Sphinx ligustri_. (Ent. Nachr., 1880, p. 166.)

=Lelievre, Ernest.= (Note in Le Naturaliste, June 1, 1880. Both sexes of _Thais polyxena_ emit an odorous exhalation. Notes on exhalation from _Spilosoma fuliginosa._)

=Hall, C. G.= Peculiar odor emitted by _Acherontia atropos_. (Entomologist, London, xvi, p. 14.)

=Åurivillius, Christopher.= Ueber secundäre Geschlechtscharactere nordischer Tagfalter. (Stockholm, 1880, Bihang till K. Svensk. Vet. Akad. Handl., v, pp. 56, 3 Taf.)

—— Des caractères sexuels secundaires chez les papillons diurnes. (Ent. Tidskrift, 1880, pp. 163–166.)

—— Anteckningar om några skandinaviska fjärilarter. (Ent. Tidskr., iv, Årg., 1884, pp. 33–37; Résumé (French), ibid., pp. 55–57.)

=Kirby, W. F.= Fans on the fore legs of _Catocala fraxini_. (Papilio, ii, p. 84, 1882.)

=Bailey, James S.= Femoral tufts or pencils of hair in certain Catocalæ. (Papilio, ii, 1882, pp. 51, 52, 146; also in Stettin Ent. Zeitung, xliii, p. 392.)

=Edwards, Henry.= Fans on the feet of Catocaline moths. (Papilio, ii, p. 146, 1882.)

=Stretch, R. H.= Anal appendages of _Leucarctia acræa_. (Papilio, iii, pp. 41, 42, 1883, 1 Fig.)

=Weed, Clarence M.= Appendages of Leucarctia. (Papilio, iii, 1883, p. 84.)

=Grote, Aug. R.= Appendages of _Leucarctia acræa_. (Papilio, iii, 1883, p. 84.)

=Haase, Erich.= Ueber sexuelle Charactere bei Schmetterlingen. (Zeitschr. f. Ent., Breslau, N. F., 1885, pp. 15–19, 36–44; also Ent. Nachr., xi Jahrg., pp. 332, 333.)

—— Duftapparate indo-australischer Schmetterlinge. (Corresp. Blatt. Ent. Ver. Iris, Dresden, 1886, pp. 92–107, 1 Taf.; ibid., 1887, pp. 159–178; ibid., 1888, pp. 281–336.)

—— Ueber Duftapparate bei Schmetterlingen. (Sitzgsber. Nat. Ges. Iris, Dresden, 1886, pp. 9–10; Abstr. in Journ. R. Micr. Soc., vi, pp. 969–970, 1886.)

—— Der Duftapparate von Acherontia. (Zeitschr. f. Ent., Breslau, N. F., 1887, pp. 5–6.)

—— Dufteinrichtung indischer Schmetterlinge. (Zool. Anzeiger, 1888, pp. 475–481.)

=Dalla Torre, K. W. von.= Die Duftapparate der Schmetterlinge. (Kosmos, 1885, ii, pp. 354–364, 410–423; Abstr. by J. B. Smith in Proc. Ent. Soc., Washington, i, pp. 38, 1888.)

=Smith, John B.= _Cosmosoma omphale._ (Entomologica Americana, i, pp. 181–185, 1886. Describes and figures cavities in under side of 2d–4th abdominal segments of male, filled with a silky substance. This may be for display to attract ♀, as the whole mass must be very conspicuous when protruded. No odor noticed.)

—— Scent organs in some Bombycid moths. (Entomologica Americana, ii, No. 4, pp. 79–80, 1886. Describes and figures long, slender, forked hairy, orange or white, eversible glands, everted from between 7th and 8th segments of abdomen of ♂ of _Leucarctia acræa_, _Pyrrharctia isabella_, _Scepsis fulvicollis_, and _Cosmosoma omphale_.)

—— [Notes on odors and odoriferous structures of various moths and a note by L. O. Howard on odor of Dynastes.] (Proc. Ent. Soc., Washington, i, pp. 40, 55, 56.)

=Müller, W.= Duftorgane der Phryganiden. (Archiv f. Naturgesch., 1887, Jahrg. liii, pp. 95–97.)

=Pollack, W.= Duftapparate der _Hadena atriplicis_ und Litargyria. (xv Jahrb. Westphäl. Prov. Ver. Münster, 1887, p. 16.)

=Patton, W. H.= Scent-glands in the larva of Limacodes. (Can. Ent., 1891, xxiii, pp. 42, 43.)

=Garman, H.= On a singular gland possessed by the male _Hadenœcus subterraneus_. (Psyche, 1891, p. 105, 1 Fig.)

=Barrett, C. G.= Scent of the male _Hepialus humuli_. (Ent. Month. Mag., Ser. 2, iii, 1892, p. 217. Arises from the curiously aborted and altered hind tibiæ.)

Also the writings of Baillif, Duponchel, F. Müller, Scudder (Psyche, iii, p. 278, 1881), Burgess, Keferstein, Alpheraky, Plateau, Marshall and Nicéville, Wood-Mason, White, Hampson.

THE ORGANS OF CIRCULATION

Although Malpighi was the first to discover the heart in the young silkworm, it was not until 1826 that Carus proved that there was a circulation of blood in insects, which he saw flowing along each side of the body, and coursing through the wings, antennæ, and legs of the transparent larva of Ephemera, though three years earlier Herold demonstrated that the dorsal vessel of an insect is a true heart, pulsating and impelling a current of blood towards the head. This discovery was extended by Straus-Dürckheim, who discovered the contractile and valvular structures of the heart. It is noteworthy that both Cuvier and Dufour denied that any circulation, except of air, existed in insects; and so great an anatomist as Lyonet doubted whether the dorsal vessel was a genuine heart, though he pointed out the fact that there are no arteries and veins connected with this vessel. Another French anatomist, Marcel de Serres, thought that the dorsal vessel was merely the secreting organ of the fat-body.

The so-called peritracheal circulation claimed by Blanchard and by Agassiz has been shown by McLeod to be an anatomical impossibility, the view having first been refuted by Joly in 1849.

Except the aorta-like continuation in the thorax and head which divides into two short branches, there are, with slight exceptions (p. 405), no distinct arteries, such as are to be found in the lobster and other Crustacea, and no great collective veins, such as exist in Crustacea and in Limulus. This is probably the result of a reduction by disuse in the circulatory system, since in myriopods (Julidæ and Scolopendridæ) lateral arteries are said to diverge near the ostia.

_a._ The heart

The heart or “dorsal vessel” is a delicate, pulsating tube, situated just under the integument of the back, in the median line of the body, and above the digestive canal. It can be partially seen without dissection in caterpillars. It is covered externally and lined within by membranes which are probably elastic; and between these two membranes extends a system of delicate muscular fibres, which generally have a circular course, but sometimes cross each other. The heart is divided by constrictions into chambers, separated by valvular folds. The internal lining membrane referred to forms the valvular folds separating the chambers. Each of these chambers has, at the anterior end, on each side, a valvular orifice (Fig. 370, ostium, _i_) which can be inwardly closed.

Miall and Denny thus describe the different layers of the wall of the heart of the cockroach:

“There are: (1) a transparent, structureless intima, only visible when thrown into folds; (2) a partial endocardium, of scattered, nucleated cells, which passes into the interventricular valves; (3) a muscular layer, consisting of close-set, annular, and distant, longitudinal fibres. The annular muscles are slightly interrupted at regular and frequent intervals, and are imperfectly joined along the middle line above and below, so as to indicate (what has been independently proved) that the heart arises as two half-tubes, which afterwards join along the middle. Elongate nuclei are to be seen here and there among the muscles. The adventitia (4), or connective tissue layer, is but slightly developed in the adult cockroach.”

Graber says that the heart of insects may be regarded not as an organ _de novo_, but only as the somewhat modified contractile dorsal vessel of the annelids, in which, however, the transverse arteries arising on each side became, with the gradual development of the tracheæ, superfluous and finally abortive. He describes it as a muscular tube composed of very delicate annular fibres, which within and without is covered by a relatively homogeneous, strong, elastic membrane.

The division into separate chambers is effected by means of a folding inwards and forwards of the entire muscular wall. “A portion of each side of the heart is first extended inwards so as very nearly to meet a corresponding portion from the opposite side, and then, being reflected backwards, forms, according to Straus (Consid., etc., p. 356), the interventricular valve which separates each chamber from that which follows it. Posteriorly to this valve, at the anterior part of each chamber, is a transverse opening or slit (Fig. 371, _b_), the _auriculo-ventricular orifice_, through which the blood passes into each chamber, and immediately behind it is a second, but much smaller, _semilunar valve_ (_c_), which, like the first, is directed forwards into the chamber. It is between these two valves on each side that the blood passes into the heart, and is prevented from returning by the closing of the semilunar valve. When the blood is passing into the chamber, the interventricular valve is thrown back against the side of the cavity, but is closed when, by the contraction of the transverse fibres, the diameter of each chamber is narrowed, and the blood is forced along into the next chamber.” (Newport.)

According to Müller, there is but a single pair of ostia in Phasma, and, in the larva of Corethra, the heart is a simple, unjointed tube, not divided into chambers, and Viallanes states that, in the very young larva of Musca, there are no ostia (Kolbe). In the larva of Ptychoptera, Grobben found a short oval heart, with one pair of ostia situated in the 6th abdominal segment; a long aorta proceeds from it, the thoracic portion of which pulsates; from behind the heart arises a pulsating pouch, which connects with the hinder aorta, which does not pulsate, and ends at the base of two tracheal gills. Burmeister was able to find only four pairs of openings in the larva of Calosoma. Newport states that, while Straus figures nine chambers in Melolontha, and, consequently, eight pairs of openings, he has not been able to observe more than seven pairs of openings in _Lucanus cervus_. He has invariably found eight pairs of openings both in the larva and imago of _Sphinx ligustri_, as well as in other Lepidoptera. According to Béla-Dezso, the number of pairs of ostia corresponds to that of the pairs of stigmata.

There also occur, on each side of each chamber, two so-called pear-shaped bodies which are separated from the tubular portion of the heart itself, but, by means of muscular fibres, are united with the chamber and with their valves. These pyriform bodies appear as vesicles or cells with granular contents, besides some nuclei with nucleoli. They are of very small size. According to the measurements of Dogiel, in the larva of _Corethra plumicornis_, they are 0.02 to 0.1 mm. long, and 0.06 to 0.08 mm. broad. He regards these peculiar bodies as apolar nerve-cells of the heart. (Kolbe.)

Besides the venous openings of the heart which open into the pericardial region, Kowalevsky has discovered, in the heart of some Orthoptera (Caloptenus, Locusta, etc.), five pairs of openings by which the cardiac chambers receive the blood of the peri-intestinal region. Graber had divided the cœlom of insects into three regions (pericardial, peri-intestinal, and perineural regions), and hitherto only a union of the heart with the pericardial region by slit-like openings was known. These openings are symmetrically distributed on five abdominal segments; each section of the heart in this region has, therefore, four openings, which are all of a truly venous nature. These openings, called cardio-cœlomic apertures, are visible to the naked eye, being situated on conical papillæ of the walls of the heart. These papillæ pass through the outer diaphragm, and open into the peri-intestinal part of the cœlom, in the Acrydiidæ directly, in the Locustidæ through special canals. The cells of the papillæ are spongy, possessing large nuclei, and similar, as a whole, to glandular cells. (Comptes rendus, cxix, 1894.)

The mechanism by which the ostia are closed consists, according to Graber, of an ∞-shaped muscle passing around the two openings, and which, being interlaced, is sufficient to close the openings. But this is not all. The fore and hinder edge of the ostia project, leaf-like, into the cavity of the heart, and thus form, with the outer walls, two valves which, during the systole, filled with the blood rushing in, not only hermetically close the lateral openings, but also, by the simultaneous closure of the entire chamber by the circular muscles in the middle of the same, the two valves, simultaneously approaching each other, so nearly touch that they form a transverse partition wall in the chamber. But, for the last purpose, _i.e._ for the separation of the chambers from one another, there is a very special contrivance. In the May beetle, we find, besides a valve (Fig. 373, _B_, _e_), opening into the middle of the chambers, a large, stalked cell (_d_), which, in the diastole, _i.e._ in the expansion of the heart, hangs down free on the walls of the heart; but, in the systole or contraction, like a cork, closes the middle of the valve, but does not wholly close the cavity. He has observed, in the larva of Corethra, formal, interventricular valves, which also are not in the middle, but are separated from one another in the interlaced ends. They consist of two longitudinally membranous flaps which move against each other like two valves (Fig. 373, _B_, _b_).

“But what is the necessity for such a complicated mechanism? All the blood from behind passes into the heart, and, for its propulsion a simple muscular tube, whose circular fibres would draw together and contract it, would be thought to be sufficient. But the heart, except in some larvæ, ends posteriorly in a blind sac, and the blood can only pass into it by a series of pairs of lateral openings. Now, as regards the reception and the propulsion of the blood forwards, two modes are conceivable. The simplest way would be that the tubular heart should, along its whole length, contract or expand; that, moreover, the blood should be simultaneously sucked in through all the openings, and that then, also, the contraction, or systole, should take place in every part of the heart at the same moment. But this would, plainly, in so long and thin-walled a vessel, be highly impracticable, since, through such a manipulation, the mass of blood enclosed in the heart would be crowded together rather than really impelled forwards. Only the second case could be admissible, and that is this, that each chamber pulsates, one after another, from behind forwards. But, then, each segmental heart must be separated from the others by a valve. To make the matter wholly clear, we may observe an insect heart pulsating, and this is best seen in one of its middle chambers. This chamber expands (simply by the relaxation of its circular muscles), the ostia, also, consequently open, and a given quantity of blood is drawn in from the pericardial cavity. What now would happen after the succeeding contraction if there were no valves between? The blood would not flow forwards, but seek a way out backwards.

“But, in fact, the valve of the hinder chamber, at this time, closes itself, while, by the simultaneous expansion of the anterior ones, their door opens, and this section of the heart, at the same time, causes a sucking in of the contents of the posterior chamber. This phenomenon is repeated, in the same way, from chamber to chamber, which also acts alternately as ventricle and auricle, or by a sucking and pumping action. One is involuntarily reminded of the ingenious manipulation by which, by the alternate opening and shutting of the flood-gates, a vessel is carried along a canal.

“This wave-like motion of an insect’s heart also has the advantage that, just before a pulse-wave has reached the chambers farthest in front, the hinder ones are already prepared for the production of a second, for, as a matter of fact, often 60, and even 100, and, in very agile insects, 150, waves pass, in a single minute, through the series of chambers, which make it very difficult to follow the flowing of their waves.” (Graber.)

=The propulsatory apparatus.=—But the heart itself is only a part of the entire propulsatorial apparatus to which belongs the following contrivance, the nature of which has been worked out by Graber.

Under the dorsal vessel is stretched a sort of roof-like diaphragm, _i.e._ a membrane, arched like the dorsal wall of the hind-body which is attached, in a peculiar way, to the sides of the body. The best idea can be gained by a cross-section through the entire body (Fig. 374): _H_ is the true dorsal vessel; _S_, the diaphragm. A surface view is seen at 373, _C_, _b_, where it appears as a plate with the edge regularly curved outwards on each side. Its precise mode of working is thus: from each dorsal band of the sides of the abdomen arises a pair of muscles spreading out fan-like, and extending to the heart, so that the fibres of one side pass directly over to those of the other, often splitting apart, or, between the two, extends outwards a perforated, thin web, like an elastic, fibrous sheet (Fig. 373, _A_, _a_), with numerous perforations, forming a diaphragm.

Graber has thus explained the action of the pericardial diaphragm and chamber, as freely translated by Miall and Denny: “When the alary muscles contract, they depress the diaphragm, which is arched upwards when at rest. A rush of blood towards the heart is thereby set up, and the blood streams through the perforated diaphragm into the pericardial chamber. Here it bathes a spongy or cavernous tissue (the fat-cells), which is largely supplied with air-tubes, and having been thus aerated, passes immediately forwards to the heart, entering it at the moment of diastole, which is simultaneous with the sinking of the diaphragm.”

In the cockroach, however, Miall and Denny think that the facts of structure do not altogether justify this explanation: “The fenestræ of the diaphragm are mere openings without valves. The descent of a perforated non-valvular plate can bring no pressure to bear upon the blood, for it is not contended that the alary muscles are powerful enough to change the figure of the abdominal rings.... The diaphragm appears to give mechanical support to the heart, resisting pressure from a distended alimentary canal, while the sheets of fat-cells, in addition to their proper physiological office, may equalize small local pressures, and prevent displacement. The movement of the blood towards the heart must (we think) depend, not upon the alary muscles, but upon the far more powerful muscles of the abdominal wall, and upon the pumping action of the heart itself.”

“The peculiar office,” says Graber, “performed by the heart has already been stated. It is nothing more than a regulator; than an organ for directing the blood in a determinate course in order that this may not wholly stagnate, or only be the plaything of a force acting in another way, as, for example, through that afforded by the body-cavity and the inner digestive canal. At regular intervals a portion of the blood is sucked through the same, and then by means of the anterior supply tube it is pushed onward into the head, whence it passes into the cavities of the tissues. The different conditions of tension under which the mass of blood stands in the different regions of the body then causes a farther circulation. Besides this, the blood passes through separate smaller pumping apparatuses, and through vessel-like modifications of cavities, also through hollow spaces between the muscles, as, for example, in the appendages where a regular backward and forward flow of the blood, especially in the limbs, wings, antennæ, and certain abdominal appendages takes place. Here and there may occasionally occur a narrow place where the flow of blood is obstructed by the accumulation of the blood corpuscles, causing a considerable stagnation.” (Graber.)

=The supraspinal vessel.=—In many insects there is a ventral heart acting on the heart’s blood as an aspirator, or more correctly a ventral sinus lying on the nervous cord, and closed by a pulsating diaphragm. This was discovered by Réaumur in the larva of a fly, and by Graber in the dragon-fly and locusts (Acrydiidæ). A glance at Figs. 375 and 376 will save a long description. The ventral wall forms a furrow, and between its borders (Fig. 377, _e_) extends the diaphragm. During the contraction of the muscles—and this, here, acts from before backwards—the membrane rises up and makes a cavity for the blood, which passes backwards over the nervous cord. The dorsal and ventral sinuses together thus bring about a closed circulation.

It thus appears that the insects are well provided with the means of distribution of their nutritive fluid, and that the blood is kept continually fresh and rich in oxygen. (Graber.)

=The aorta.=—While the heart is mostly situated within the abdomen, it is continued into the thorax and the head as a simple, non-pulsating tube, called the aorta. In Sphinx the aorta, as described by Newport, begins at the anterior part of the 1st abdominal segment, where it bends downwards to pass under the metaphragma and enter the thorax; it then ascends again between the great longitudinal dorsal muscles of the wings, and passes onwards until it arrives at the posterior margin of the pronotum; it then again descends and continues its course along the upper surface of the œsophagus, with which it passes beneath the brain, in front of which and immediately above the pharynx, it divides into two branches, each of which subdivides. Newport, however, overlooked a thoracic enlargement of the aorta called by Burgess the “aortal chamber” (Fig. 310, _a_, _c_).

“In Sphinx and _Vanessa urticæ_, immediately after the aorta has passed beneath the cerebrum, it gives off laterally two large trunks, which are each equal in capacity to about one-third of the main vessel. These pass one on each side of the head, and are divided into three branches which are directed backwards, but have not been traced farther in consequence of their extreme delicacy. Anterior to these trunks are two smaller ones which appear to be given to the parts of the mouth and antennæ, and nearer the median line are two others which are the continuations of the aorta. These pass upwards, and are lost in the integument. The whole of these parts are so exceedingly delicate that we have not, as yet, been able to follow them beyond their origin at the termination of the aorta, but believe them to be continuous, with very delicate, circulatory passages along the course of the tracheal vessels. It is in the head alone that the aorta is divided into branches, since, throughout its whole course from the abdomen, it is one continuous vessel, neither giving off branches, nor possessing lateral muscles, auricular orifices, or separate chambers.” (Newport, art. Insecta, p. 978.)

Dogiel observed in the transparent larva of _Corethra plumicornis_ that the aorta extends only to the hinder border of the brain. Here it divides into two lamellæ, each of which independently extends farther on. One lamella is seen under the brain and under the eye, the other reaches near the eye. The lamellæ are tied to the integument by threads. At the point of division of the aorta is an opening. (Kolbe.)

True blood-vessels appear to exist in the caudal appendages of the May-flies, as the heart appears to divide and pass directly into them (Fig. 378). The last chamber of the heart diminishes in size at the end of the body, and then divides into three delicate tubular vessels which pass into the three caudal appendages, and extend to the end of each one, along the upper side. While the valves of the heart, in all insects, are directed anteriorly because the blood flows from behind, in the larva of the Ephemeridæ the valves of the last chamber of the heart are directed backwards, because from this chamber the blood flows in the opposite direction, _i.e._ into the caudal appendages. During the contraction of the heart, the elongated section of the same in the last abdominal segment receives a part of the mass of blood contained in the last chamber, which is driven by independent contractions into the caudal appendages. These vessels have openings before the end through which the blood enters into the cavity of the appendages, and can also pass back, in order to be taken up by the body cavity. It is possible that these blood-vessels stand in direct relation to respiration. (Zimmermann, Creutzburg, in Kolbe, p. 544.)

=The pericardial cells.=—Along the heart, on both sides, occur the so-called pericardial cells, which differ from the fat-cells, and also the peritracheal cells of Frenzel, and are mostly arranged in linear series, which have a close relation to the circulation of the blood. In the larva of Chironomus, they lie in groups; in that of Culex, they are arranged segmentally. In caterpillars, these pericardial cells are not situated in the region of the heart, but are arranged linearly on the side, and form a network of granulated cells situated between the fat-bodies. Other rows of these cells are situated near the stigmata and the main lateral tracheæ. (Kolbe.)

According to Kowalevsky, the pericardial cells, and the garland-shaped, cellular cord consist of cells, whose function it is to purify the blood, and to remove the foreign or injurious matters mingled with the blood.

=Ampulla-like blood circulation in the head.=—In the head of the cockroach occurs, according to Pawlowa, a contractile vascular sac at the base of each antenna. The cavity has a valvular communication with the blood space below and in front of the brain, and muscle-fibres effect systole and diastole. Each sac is beyond doubt an independently active part of the circulatory system. These organs also occur in Locusta and other Acrydiidæ, and Selvatico has described similar structures in _Bombyx mori_ and certain other Lepidoptera.

=Pulsatile organs of the legs.=—Accessory to the circulation is a special system of pulsatile organs in the three pairs of legs of Nepidæ, generally situated in the tibia just below its articulation with the femur, but in the fore legs of Ranatra, in the clasp-joint or tarsus, just below its articulation with the tibia. First observed by Behn (1835), Locy has studied the organ (Fig. 380) in Corixa, Notonecta, Gerris, besides the Nepidæ. It is a whip-like structure attached at both ends, with fibres extending upward and backward to the integument of the leg, separate from the muscular fibres and does not involve them in its motions, and is not affected by the muscles themselves. “As the blood-corpuscles flow near the pulsating body they move faster, and around the organ itself there is a whirlpool of motion.” The beating of these organs aids the circulation in both directions, and when the motion ceases, the blood-currents in the legs stop; the rate of the pulsating organ is always faster than that of the heart, and the action is automatic.

_b._ The blood

The blood of insects, as in other invertebrates, differs from that of the higher animals in having no red corpuscles. It is a thin fluid, a mixture of blood (serum) and chyle, usually colorless, but sometimes yellowish or reddish, which contains pale amœboid corpuscles corresponding to the white corpuscles (leucocytes) of the vertebrates, though they are relatively less numerous in the blood of insects. The yellow fluid expelled from the joints of certain beetles (Coccinella, Timarcha, and the Meloidæ) is, according to Leydig, only the serum of the blood. In phytophagous insects the blood is colored greenish by the chlorophyll set free during digestion. The blood of _Deilephila euphorbia_ is colored an intense olive-green, and that of _Cossus ligniperda_ is pale yellow. (Urech.) The blood of case-worms (Trichoptera) is greenish. In some insects it is brownish or violet. The serum is the principal bearer of the coloring material, yet Graber has shown that in certain insects the corpuscles are more or less beset with bright yellow or red fat-globules, so as to give the same hue to the blood.

=The leucocytes.=—The corpuscles are usually elongated, oval, or flattened oat-shaped, with a rounded nucleus, or are often amœbiform; and they are occasionally seen undergoing self-division. When about to die the corpuscles become amœbiform or star-shaped. (Cattaneo.) Their number varies with the developmental stage of the insect, and in larvæ increases as they grow, becoming most abundant shortly before pupation. The blood diminishes in quantity in the pupal stage, and becomes still less abundant in the imago. (Landois.) The quantity also varies with the nutrition of the insect, and after a few days’ starvation nearly all the blood is absorbed. Crystals may be obtained by evaporating a drop of the blood without pressure; they form radiating clusters of pointed needles. The freshly drawn blood is slightly alkaline. (Miall and Denny.)

The size of the corpuscles has been ascertained by Graber, who found that the diameter of the circular blood-disks of the leaf-beetle, _Lina populi_, is 0.006 mm.; of _Cetonia aurata_ and _Zabrus gibbus_, 0.008 to 0.01 mm.; and those of certain Orthoptera _(Decticus verrucivorus_, _Ephippiger vitium_ and _Œdipoda cœrulescens_), 0.011 to 0.014 mm. The longest diameter of the elongated corpuscles of _Carabus cancellatus_ is 0.008 mm.; of _Gryllus campestris_, _Locusta viridissima_, _Cossus ligniperda_, _Sphinx ligustri_ (pupa), and others, 0.008 to 0.01 mm.; of _Caloptenus italicus_, _Saturnia pyri_, _Anax formosus_, and others, 0.011 to 0.014 mm.; of _Ephippiger vitium_, _Œdipoda cœrulescens_, _Pezotettix mendax_, _Zabrus gibbus_, _Phryganea_, and others, 0.012 to 0.022 mm.; in _Stenobothrus donatus_ and _variabilis_, 0.012 to 0.035 mm. The largest known are those of _Melolontha vulgaris_, which measure from 0.027 to 0.03 mm.

As regards the nature of the corpuscles, Graber concludes that they are more like the cells of the fat-bodies than genuine cells. That they are not true cells is shown by the fact that after remaining in their normal condition a long time they finally coalesce and form cords. After shrivelling, or after the blood has been subjected to different kinds of treatment, the nucleus is clearly brought out (Fig. 381).

Besides the blood corpuscles there have been detected in the blood round bodies which are regarded as fat-cells. They are circular, and for the most part larger than the blood corpuscles, have a sharp, even, dark outline, and an invariably circular nucleus. (Kolbe.)

The blood of Meloe, besides the amœboid corpuscles, according to Cuénot, contains abundant fibrinogen, which forms a clot; a pigment (uranidine), which is oxidized and precipitated when exposed to the air; a dissolved albuminoid (hæmoxanthine), which has both a respiratory and nutritive function; and, finally, dissolved cantharidine.

The corpuscles arise from tissues which are very similar to the fat-bodies, and which, at given times, separate into cells. The position of these tissues is not always the same in different insects. In caterpillars, they occur in the thorax, near the germs of the wings; in the saw-flies (Lyda), in all parts of the thorax and abdomen; in larval flies (Musca), in the end of the abdomen, just in front of the large terminal stigmata. The place where the blood corpuscles are formed is usually near, or in relation with, the fat-bodies. But while the fat-bodies mostly serve as the material for the formation of the blood-building tissues, in caterpillars the tracheal matrix also, and, in dipterous larvæ, the hypodermis serve this purpose. (Cæsar Schaeffer in Kolbe. See also Wielowiejski, Ueber das Blutgewebe.)

Other substances occur in the blood of insects. Landois (1864) demonstrated the existence of egg albumen, globulin, fibrin, and iron in the blood of caterpillars. Poulton found that the blood of caterpillars often contained chlorophyll and xanthophyll derived from their food plants. A. G. Mayer has recently found that the blood (hæmolymph) of the pupæ of Saturniidæ (_Callosamia promethea_) contains egg albumin, globulin, fibrin, xanthophyll, and orthophosphoric acid, and Oenslager has determined that iron, potassium, and sodium are also present. (Mayer.)

_c._ The circulation of the blood

Every part of the body and its appendages is bathed by the blood, which circulates in the wings of insects freshly emerged from the nymph or pupal state, and even courses through the scales of Lepidoptera, as discovered by Jaeger (Isis, 1837).

In describing the mechanism of the heart we have already considered in a general way the mode of circulation of the blood.

The heart pumping the blood into the aorta, the nutritive fluid passes out and returns along each side of the body; distinct, smaller streams passing into the antennæ, the legs, wings (of certain insects), and into the abdominal appendages when they are present. All this may readily be observed in transparent aquatic insects, such as larval Ephemeræ, dragon-flies, etc., kept alive for the purpose under the microscope in the animalcule box.

Carus, in 1827, first discovered the fact of a complete circulation of the blood, in the larva of Ephemera. He saw the blood issuing in several streams from the end of the aorta in the head and returning in currents which entered the base of the antennæ and limbs in which it formed loops, and then flowing into the abdomen, entered the posterior end of the heart. Wagner (Isis, 1832) confirmed these observations, adding one of his own, that the blood flows backward in two venous currents, one at the sides of the body and intestine, and the other alongside of the heart itself, and that the blood not only entered at the end of the heart, but also at the sides of each segment, at the position of the valves discovered by Straus-Dürckheim.

Newport maintains that the course of the blood is in any part of the body, as well as in the wings, almost invariably in immediate connection with the course of the tracheæ, for the reason that “the currents of blood in the body of an insect are often in the vicinity of the great tracheal vessels, both in their longitudinal and transverse direction across the segments.”

The circulation of the blood in the wings directly after the exuviation of the nymph or pupa skin, and before they become dry, has been proved by several observers. As stated by Newport, the so-called “veins” or “nervures” of the wings consist of tracheæ lying in a hollow cavity, the peritracheal space being situated chiefly under and on each side of the trachea.

Newport gives the following summary of the observations of the early observers, to which we add the observations of Moseley. “A motion of the fluids has been seen by Carus in wings of recently developed Libellulidæ, _Ephemera lutea_ and _E. marginata_, and _Chrysopa perla_; among the Coleoptera, in the elytra and wings of _Lampyris italica_ and _L. splendidula_, _Melolontha solstitialis_ and Dytiscus.” Ehrenberg saw it in Mantis, and Wagner in the young of _Nepa cinerea_ and _Cimex lectularius_. Carus detected a circulation in the pupal wings of some Lepidoptera, and Bowerbank witnessed it in a Noctuid (_Phlogophora meticulosa_); Burmeister observed it in _Eristalis tenax_ and _E. nemorum_, and Mr. Tyrrel in _Musca domestica_, but it has not been observed in the wings of Hymenoptera.

Bowerbank observed that in the lower wing of _Chrysopa perla_ the blood passes from the base of the wing along the costal, post-costal, and externo-medial veins, outwards to the apex of the wing, giving off smaller currents in its course, and that it returns along the anal vein to the thorax. He found that the larger veins, 1⁄408 in. in diameter, contained tracheæ which only measured 1⁄2222 of an inch in diameter; but in others the tracheæ measured 1⁄1340, while the cavity measured only 1⁄500 of an inch. He states, also, that the tracheæ very rarely give off branches while passing along the main veins, and that they lie along the canals in a tortuous course. (Newport, art. Insecta, p. 980.)

Bowerbank, also, in his observations on the circulation in the wings of Chrysopa, “used every endeavor to discover, if possible, whether the blood has proper vessels, or only occupied the internal cavities of the canals; and that he is convinced that the latter is the case, as he could frequently perceive the particles not only surrounding all parts of the tracheæ, and occupying the whole of the internal diameter of the canals, but that it frequently happens that globules experienced a momentary stoppage in their progress, occasioned by their friction against the curved surface of the tracheæ, which sometimes gave them a rotatory motion.”

Moseley found, owing to the large size and number of the corpuscles, that the circulation of the blood in the wings of insects is most easily observed in the cockroach, especially the hind wings. As seen in Moseley’s figure, the blood flows outward from the body through the larger veins (I and II) of the front edge of the wings, which he calls the main arteries of the wings, and more generally returns to the body through the veins in the middle of the wing; the blood also flows out from the body through the inner longitudinal veins (those behind vein IV), and the blood is also seen to flow through some of the small cross-veins. Fig. 383 shows one of the main trunks during active circulation. The corpuscles change their form readily, “the spindle-shaped ones doubling up in order to pass crossways through a narrow aperture.... In the irregularly formed corpuscles, which seem to represent leucocytes amœboid movements were observed.... Corpuscles pass freely above and under the tracheæ, showing that these latter lie free in the vessels.” The hypodermis lining the vessels is best seen in the small transverse veins.

The pulse or heart-beat of insects varies in rapidity in different insects, rising at times of excitement, as Newport noticed in _Anthophora retusa_, to 142 beats in a minute.

When an insect, as, for example, a tineid caterpillar, has been enclosed in a tight box for a day or more, the pulsations of the heart are very languid and slow, but soon, on giving it air, the pulsations will, as we have observed, rise in frequency to about 60 a minute, Herold observed 30 to 40 in a minute in a fully-grown silkworm, and from 46 to 48 in a much younger one. Suckow observed but 30 a minute in a full-grown caterpillar of _Gastropacha pini_, and 18 only in its pupa.

In a series of observations made by Newport on _Sphinx ligustri_ from the fourth day after hatching from the egg until the perfect insect was developed, he found that before the larva cast its first skin the mean number of pulsations, in a state of moderate activity and quietude, was about 82 or 83 a minute; before the second moult 89, while before the third casting it had sunk down to 63; and before its fourth to 45, while, before leaving its fourth stage, and before it had ceased to feed, preparatory to pupating, the pulse was not more than 39. “Thus the number gradually decreases during the growing larva state, but the force of the circulation is very much augmented. Now when the insect is in a state of perfect rest, previously to changing its skin, the number is pretty nearly equal at each period, being about 30. When the insect has passed into the pupa state it sinks down to 22, and subsequently to 10 or 12, and after that, during the period of hibernation, it almost entirely ceases. But when the same insect which we had watched from its earliest condition was developed into the perfect state in May of the following spring, the number of pulsations, after the insect had been for some time excited in flight around the room, amounted to from 110 to 139; and when the same insect was in a state of repose, to from 41 to 50. When, however, the great business of life, the continuation of the species, has been accomplished, or when the insect is exhausted, and perishing through want of food or other causes, the number of pulsations gradually diminishes, until the motions of the heart are almost imperceptible.” Insects, then, he remarks, do not deviate from other animals in regard to their vital phenomena, though it has been wrongly imagined that the nutrient and circulatory functions are less active in the perfect than in the larval condition.

The heart of a larval _Gastrus equi_ taken the day previous from a horse’s stomach beat from 40 to 44 times a minute (Scheiber); while Schröder van der Kolk observed only 30 beats in the same kind of maggot.

In the larva of Corethra, while at rest, the heart contracts from 12 to 16 or 18 times a minute, but when active the number rises to 22. The systole and diastole last from 5 to 6 minutes. (Dogiel.)

Temperature also affects the pulsations, as they increase in frequency with a rise and decrease with a fall in temperature.

=Influence of electricity.=—The influence of electricity on the action of the insect’s heart, from Dogiel’s experiments, is such as to cause an acceleration in the frequency of the beats, while an increase in the strength of the electric currents either diminishes the frequency of the beats or entirely stops the heart’s action. A violent excitation with the induction current causes a systole when the heart’s action has stopped for a long time; and if the excitation lasts uninterruptedly, then the contractions after a while become noticeable, according to the strength of the current. In such a case there are, however, interruptions in the regularity, strength, and order of the contractions. (Kolbe.)

=Effects of poisons on the pulsations.=—Dogiel has also experimented on the influence of poisons in the form of vapor or as liquid solutions on the pulsations of insects, which is much as in vertebrates. The application of carbonic oxide to the larva of Corethra, whose heart one minute previous to the poisoning beat 15 times a minute, accelerated the heart-beats in about 55 minutes to 25 pulsations in a minute. Afterwards there was a retardation in the pulse to the normal beat. Carbonic acid had a similar effect.

The following results obtained by Dogiel are somewhat as tabulated by Kolbe:—

I. Substances which cause the pulsations of the heart to accelerate.

_a._ An induction current of electricity, acting feebly. _b._ Ammonia, acting feebly. _c._ Ethyl ether, acting feebly. _d._ Oxalic acid, acting feebly. _e._ Carbolic acid, acting feebly. _f._ Potassium nitrate, acting feebly. _g._ Aconite, acting feebly.

II. Substances retarding the heart’s action.

_a._ An induction current of electricity, acting energetically. _b._ Ammonia, acting energetically. _c._ Ethyl ether, acting energetically. _d._ Oxalic acid, acting energetically. _e._ Carbolic acid, acting energetically. _f._ Veratrine, acting energetically. _g._ Atropine, acting energetically. _h._ Aconitine, acting energetically. _i._ Potassium nitrate, acting energetically. _g._ Ethyl alcohol. _h._ Chloroform. _i._ Carbonic oxide. _j._ Carbonic acid. _k._ Sulphuretted hydrogen.

III. Substances whose action is indifferent.

_1._ Muscarine. _2._ Curare. _3._ Atropine, acting slowly. _4._ Strychnine.

The above-named substances comprise those which in the vertebrates effect a change in the activity of the motor nerve-ganglia of the heart and the muscular fibres. Hence it follows that the heart of the larval Corethra consists of muscular fibres provided with ganglia, and that the contractions of the muscular fibres are provoked through the agency of the ganglia. But since muscarine, atropine, and curare, whose influence in stopping the heart’s action of vertebrates is known, in insects either have no action or only make the pulsations slower; it seems to follow that the heart of the larval Corethra possesses no similar apparatus for lessening the heart’s action, and this is also confirmed by anatomical studies. On the contrary, aconite acts, as we must from observations conclude, exclusively on the motor centres and the muscles, but not on the apparatus for lessening the heart’s action, which, as has been remarked, is not present in the larval Corethra. (Kolbe _ex_ Dogiel.)

Dewitz has discovered an onward movement of the blood corpuscles, somewhat independent of the general circulation. This independent motion of the blood corpuscles is not only a creeping one like the amœboid motion of the white corpuscles of vertebrates, but they have besides a peculiar swimming movement. Dewitz noticed this in the hind wings of a recently emerged meal-worm beetle (_Tenebrio molitor_), still white and soft, after they had been cut off. The tissues forming the matrix within the wings constitute a network filled with blood. The current of blood within the wing thus cut off may be stopped flowing by a tap on the firmly clamped object-bearer on which the wing is placed, or by drawing it by an apparatus described by the same author, to incite in one way or another the blood corpuscles to swim forwards. When a corpuscle is disposed to move, we see it first stirring restlessly, or wabbling about, in this way changing its form; then it moves forwards, and does not come to a standstill. If it remains still there, after a while, by tapping, it begins again its movements.

“Should one yet doubt the fact of this spontaneous movement of the blood corpuscles, he will surely be convinced of its correctness by observing the so-to-speak reluctantly springing motion of a blood corpuscle in the wing of _Tenebrio molitor_ with the simultaneous change of appearance and shape of the corpuscle.”

This spontaneous or independent motion of the blood corpuscles is also produced by the heating apparatus. As soon as the corpuscles lie still in the severed wing and they are warmed, the corpuscles begin to pass through the meshes of the tissue. When cooled, the motion ceases, but as soon as the temperature rises to a certain grade, the corpuscles again move onwards.

To explain this independent motion Dewitz thinks that they take up and then expel the blood-fluid, and in this way cause their motion. This independent motion is necessitated, in order that the stream of blood may become so regulated, that the blood corpuscles shall not be arrested in their course, but even turn back again out of the farther end of the antennæ and limbs. The chief mechanical power for the blood circulation must go on independently of the propulsatorial apparatus and of the heart. (Kolbe.)

LITERATURE ON THE HEART AND ON THE CIRCULATION OF THE BLOOD

_a._ Anatomy of the organs

=Meckel, J. F.= Ueber das Rückengefäss der Insekten. (Meckel’s Archiv, i, 1815, pp. 469–476.)

=Müller, J. G.= De vasi dorsali Insectorum. Berolini, 1816, pp. 22.

=Serres, P. Marcel de.= Observations sur les usages du vaisseau dorsal ou sur l’influence que le cœur exerce dans l’organisation des animaux articulés, etc. (Ann. du Mus. d’hist. nat., 1818, iv, pp. 149–192, 313–380, 2 Pls.; v, 1819, pp. 59–147, 1 Pl.)

=Herold.= Physiologische Untersuchungen über das Rückengefäss der Insekten. (Schriften d. Gesellsch. z. Beförderung d. Naturk. in Marburg, 1823, i, pp. 41–107.)

=Carus, C. G.= Entdeckung eines einfachen vom Herzen aus beschleunigten Blutkreislaufes in den Larven netzilügliger Insekten. Leipzig, 1827, pp. 40, 3 Taf.

—— Fernere Untersuchungen über Blutlauf in Kerfen. (Acta Acad. Leopold. Carol., 1831, xv, pp. 1–18, 1 Taf.)

=Stadelmayr, L.= Ansichten vom Blutlauf nebst Beobachtungen über das Rückengefäss der Insekten. Diss. München, 1829, pp. 24.

=Berthold.= Beitrage zur Anatomie, Zoologie und Physiologie. Göttingen, 1831.

=Treviranus, G. R.= Ueber das Herz der Insekten, dessen Verbindung mit den Eierstocken und ein Bauchgefäss der Lepidopteren. (Zeitschr. f. d. Physiologie, von F. Tiedemann. G. R. u. L. C. Treviranus, 1832, iv, pp. 181–184, 1 Taf.)

—— Beobachtungen aus der Zootomie und Physiologie. Bremen, 1839.

=Wagner, R.= Beobachtungen über Kreislauf des Blutes und den Bau des Rückengefässes bei den Insekten. (Isis, 1832, iii, p. 30; vii, pp. 320–331, 778–783, Fig.)

=Bowerbank, J. S.= Observations on the circulation of the blood in insects. (Ent. Mag., 1833, i, pp. 239–244, 1 Pl; also in Müller’s Archiv f. Physiolog., 1834, i, pp. 119–120.)

—— Observations on the circulation of the blood and the distribution of the tracheæ in the wing of _Chrysopa perla_. (Ent. Mag., 1837, iv, pp. 179–185.)

=Jaeger.= Ueber die Entdeckung von einer Bewegung in den Schuppen des Schmetterlingsflügel. (Isis, 1837, v, p. 512.)

=Behn, W.= Découverte d’une circulation de fluide nutritif dans les pattes de plusieurs insectes hémipteres. (Ann. Sc. nat., 1835, Sér. 2, iv, pp. 1–12.)

=Newport, G.= Insecta, in Todd’s Cyclopædia of Anatomy and Physiology, 1839. London, pp. 853–994. On the circulation of the blood, p. 976, Figs.

=Duvernoy, G. L.= Résumé sur le fluide nourricier, ses réservoirs et son mouvement dans tout règne animal. (Ann. Sc. nat., 1839, Sér. 2, xii, pp. 300–346.)

=Dufour, L.= Études anatomiques et physiologiques sur une mouche dans le but d’eclairer l’histoire des metamorphoses et de la prétendue circulation dans les insectes. (Ann. Sc. nat. Zool., Sér. 2, 1841, xvi, pp. 5–14.)

—— Note sur la prétendus circulation dans les insectes. (Compt. rend. Acad., Paris, 1844, xix, pp. 188–189.)

—— Études anatomiques et physiologiques sur une mouche, dans le but d’éclaircir l’histoire des metamorphoses et de prétendue circulation des insectes. (Mém. mathémat. des Savants étrangers, Paris, 1846, ix, pp. 545–628, 1 Pl.)

—— Sur la circulation dans les insectes. Bordeaux, 1849, 8º, pp. 40. (Compt. rend. Acad. Sci., Paris, 1849, xxviii, pp. 28–33, 101–104, 163–170.)

—— De la circulation du sang et de la nutrition chez les insectes. Bordeaux, 1851. (Act. Soc. Linn., Bordeaux, 1851, xvii, p. 9.)

—— Études anatomiques et physiologiques et observations sur les larves des Libellules, Appareil circulatoire. (Annal. Sci. nat., Sér. 3, Zool., xvii, 1852, pp. 98–101, 1 Pl.)

=Schröder van der Kolk, J. S. C.= Mémoire sur l’anatomie et physiologie du _Gastrus equi_. (N. Verhandl. Kl. Nederl. Instit., 11, 1845, pp. 1–155, 13 Pl.)

=Nicolet, H.= Note sur la circulation du sang chez les Coléoptères. (Ann. Sc. nat., 1847, Sér. 3, vii, pp. 60–64.)

=Verloren, C.= Mémoire en réponse à la question suivante: éclaircir par des observations nouvelles le phénomène de la circulation dans les insectes, en recherchant si on peut la reconnaître dans les larves de différents ordres de ces animaux. (Mém. couronn. et Mém. d. savants étrang. de l’Acad. Roy. Belgique, xix, 1847.)

=Blanchard, E.= De la circulation dans les insectes. (Ann. Sc. nat., 1848, Sér. 3, ix, pp. 359–398, 5 Pls.)

—— Sur la circulation du sang chez les insectes et sur la nutrition. (Compt. rend. Acad. Sc., Paris, 1849, xxviii, pp. 76–78; 1851, xxxiii, pp. 367–370.)

—— Nouvelles observations sur la circulation du sang et la nutrition chez les insectes. (Ibid., pp. 371–376.)

=Joly, N.= Mémoire sur l’existence supposée d’une circulation péritrachéenne chez les insectes. (Ann. Sc. nat. Zool., Sér. 3, 1849, xii, pp. 306–316.)

=Bassi, C. A.= Rapporto alla sezione di zoologia, anatomia comparata e fisiologia del congresso di Venezia, sul passagio delle materie ingerite nel sistema tracheale degli insetti. (Gazette di Milano, 1847, vi; also Ann. Sc. nat. Zool., Sér. 3, 1851, xv., 362–371.)

=Agassiz, Louis.= On the circulation of the fluids in insects. (Proceed. Amer. Assoc. Adv. Sc., 1849, pp. 140–143; Ann. Sc. nat. Zool., Sér. 3, xv, 1851, pp. 358–362.)

=Leydig, F.= Anatomisches und Histiologisches über die Larve von _Corethra plumicornis_. (Zeitschr. f. wissen. Zool., iii, 1851, pp. 435–451.)

=Wedl, C.= Ueber das Herz von _Menopon pallidum_. (Sitzungsber. der k. Akad. d. Wissensch. Wien., 1855, xvii, pp. 173–180.)

=Scheiber, S. H.= Vergleichende Anatomie und Physiologie der Oestriden Larven. (Sitzungsber. d. k. Akad. d. Wiss. Wien. Math.-naturwiss. Cl., xli, 1860, pp. 409–496, 2 Taf.; The circulatory system, pp. 463–490.)

=Brauer, Fr.= Beitrag zur Kenntnis des Baues und der Funktion der Stigmenplatten der Gastrus-Larven. (Verhdl. d. k. k. zool.-bot. Gesellsch. Wien., xiii, 1863, pp. 133–136.)

=Moseley, H. N.= On the circulation in the wings of _Blatta orientalis_ and other insects, and on a new method of injecting the vessels of insects. (Quart. Jour. of Micr. Science, xi, n. s., pp. 389–395, 1871, 1 P1.)

=Graber, V.= Ueber die Blatkörperchen der Insekten. (Sitzber. Akad. Wien. Math.-naturw. Classe, lxiv, 1871, pp. 9–44, 1 Taf.)

—— Vorläufiger Bericht über den propulsatorischen Apparat der Insekten. (Sitzber. d. k. Ak. d. Wiss. Wien., lxv, 1872, pp. 16, 1 Taf.)

—— Ueber den propulsatorischen Apparat der Insekten. (Archiv f. mikroskop. Anatomie, ix, 1873, pp. 129–196, 3 Taf.)

—— Ueber den pulsierenden Bauchsinus der Insekten. (Archiv f. mikroskop. Anat., xii, 1876, pp. 575–582, 1 Taf.)

=Grobben, Carl.= Über bläschenförmige Sinnesorgane und eine eigenthümliche Herzbildung der Larve von _Ptychoptera contaminata_ L. (Sitzb. k. Akad. Wissensch. Wien., 1875, lxxii, p. 22, 1 Taf.)

=Liebe, Otto.= Ueber die Respiration der Tracheaten, besonders über den Mechanismus derselben und über die Menge der ausgeatmeten Kohlensäure. Inaug.-Diss. Chemnitz, 1872, pp. 28.

=Dogiel, John.= Anatomie und Physiologie des Herzens der Larve von _Corethra plumicornis_. (Mém. Acad. imp. St. Petersbourg, 7 Sér., xxiv, 1877, Nr. 10, pp. 37, 2 Pls.) Separate, Leipzig, Voss.

=Bütschli, O.= Ein Beitrag zur Kenntnis des Stoffwechsels, insbesondere der Respiration bei den Insekten. (Reichert’s und Du Bois-Reymond’s Archiv f. Anatomie u. Physiologie, 1874, pp. 348–361.)

=Béla-Dezso.= Ueber den Zusammenhang des Kreislaufs und der respiratorischen Organe bei den Arthropoden. (Zool. Anzeiger, i Jahrg., 1878, p. 274.)

=Plateau, F.= Communication préliminaire sur les mouvements et l’innervation de l’organe central de la circulation chez les animaux articulés. (Bull. Acad. roy. de Belgique, Sér. 2, xlvi, 1878, pp. 203–212.)

=Jaworovski, Ant.= Ueber die Entwicklung des Rückengefässes und speziell der Muskulatur bei Chironomus und einigen anderen Insekten. (Sitzgsber. d. k. Akad. d. Wissensch. Wien. Math.-naturwiss. Cl., lxxx, 1879, pp. 238–258.)

=Zimmermann, O.= Ueber eine eigentümliche Bildung des Rückengefässes bei einigen Ephemeridenlarven. (Zeitschr. f. wissens. Zool., 1880, xxxiv, pp. 404–406.)

=Burgess, E.= Note on the aorta in lepidopterous insects. (Proc. Bost. Soc. Nat. Hist., xxi, 1881, pp. 153–156, Figs.)

—— Contributions to the anatomy of the milk-weed butterfly, _Danais Archippus_ F. (Anniversary Memoirs Boston Soc. Nat. Hist., 1880, pp. 16, 2 Pls.)

=Vayssière, A.= Recherches sur l’organisation des larves des Éphémérines. (Ann. Sc. nat. Zool., Sér. 6, xiii, 1882, pp. 1–137, 11 Pls.)

=Viallanes, H.= Recherches sur l’histologie des insectes et sur les phénomènes histologiques qui accompagnent le développement post-embryonnaire de ces animaux. (Ann. Sc. nat., Sér. 6, xiv, 1882, pp. 1–348, 18 Pls.)

=Schimkewitsch, W.= Ueber die Identität der Herzbildung bei den wirbel und wirbellosen Tieren. (Zool. Anzeiger, 1885, viii Jahrg., pp. 37–40, Fig.)

—— Noch etwas über die Identität der Herzbildung bei den Metazoen. (Zool. Anzeiger, 1885, pp. 384–386.)

=Creutzburg, N.= Ueber den Kreislauf der Ephemerenlarven. (Zool. Anzeiger, 1885, pp. 246–248.)

=Poletajewa, Olga.= Du cœur des insectes. (Zool. Anzeiger, 1886, ix Jahrg., pp. 13–15.)

=Selvatico, S.= L’aorta nel corsaletto e nel capo della farfalla del bombice del gelso. (Padova, 1887, p. 19, 2 Pls.)

—— Die Aorta im Brustkasten und im Kopfe des Schmetterlings von _Bombyx mori_. (Zool. Anzeiger, 1887, x Jahrg., pp. 562–563.)

=Kowalevsky, A.= Ein Beitrag zur Kenntnis der Excretionsorgane. (Biol. Centralbl., 1889, ix, pp. 33–47, 65–76, 127–128.)

=Tosi, Alessandro.= Osservazioni sulla valvola del cardias in varii generi della famiglia delle Apidi. (Ricerche Lab. Anat. R. Univ. Roma, v, 1895, pp. 5–26, 16 Figs., 3 Pls.)

=Pawlowa, Mary.= Ueber ampullenartige Blutcirculationsorgane im Kopfe verschiedener Orthopteren. (Zool. Anzeiger, xviii Jahrg., 1895, pp. 7–13, 1 Fig.)

Also the writings of Kolbe.

_b._ The blood, blood corpuscles, leucocytes, and blood tissue

=Wagner, R.= Ueber Blutkörperchen bei Regenwürmern, Blutegeln und Dipterenlarven. (Müller’s Archiv f. Anatomie u. Physiologie, 1835, pp. 311–313.)

—— Nachtrage zur vergleichenden Physiologie des Blutes. (Archiv f. Anat. u. Physiologie, 1838.)

=Newport, G.= On the structure and development of the blood. First series. The development of the blood corpuscle in insects and other invertebrata, and its comparison with that of man and the vertebrata. (Abstr. of the paper in Roy. Soc., 1845, v, pp. 544–546: also in Ann. Mag. Nat. Hist., Ser. 3, 1845, iii, pp. 364–367.)

=Landois, H.= Beobachtungen über das Blut der Insekten. (Zeitschr. f. wissens. Zool., xiv, 1864, pp. 55–70, 3 Pl.)

——, =and L. Landois.= Ueber die numerische Entwicklung der histiologischen Elemente d. Insektenkörpers. (Ibid., xv, 1865, pp. 307–327.)

=Rollett, A.= Zur Kenntnis der Verbreitung des Hämatins. (Sitzgsber. d. k. Akad. d. Wiss. Wien., lxiv, 1871.)

=Wielowiejski, H. v.= Ueber das Blutgewebe der Insekten. Eine vorläufige Mitteilung. (Zeitschr. f. wissens. Zool., 1886, xliii, pp. 512–536.)

=MacMunn, C. A.= Researches on myohæmatin and the histohæmatins. (Proc. Roy. Soc. London, 1886, xxxix, pp. 248–252.)

=Peyron, J.= Sur l’atmosphere interne des insectes comparée à celle des feuilles. (Compt. rend. Acad. Sc., Paris, 1886, cii, pp. 1339–1341.)

=Cuénot, L.= Études sur le sang, son rôle et sa formation dans la série animale. Part 2, invertébrés; note préliminaire. (Arch. Zool. Expériment., 1888, Sér. 2, v, pp. xliii-xlvii. See also ibid., Sér. 3, 1897, pp. 655, 679–680.)

=Dewitz, H.= Die selbstandige Fortbewegung der Blutkörperchen der Gliedertiere. (Naturwiss. Rundschau. Braunschweig, 1889, iv Jahrg., pp. 221–222.)

—— Eigenthätige Schwimmbewegung der Blutkörperchen der Gliedertiere. (Zool. Anzeiger, 1889, xii Jahrg., pp. 457–464, Fig.)

=Schäffer, C.= Beitrage zur Histiologie der Insekten. II, Ueber Blutbildungsherde bei Insektenlarven. (Spengel’s Zool. Jahrbücher Abt. f. Anat. u. Ontogenie, iii, 1889, pp. 626–636, 1 Taf.)

=Cattaneo, G.= Sulla morfologia delle cellule ameboidi dei Molluschi e Artropodi. (Boll. Sc. Pavia. Anno 11, 1889, p. 59, 2 Pls.)

=Wagner, W. A.= Ueber die Form der körperlichen Elemente des Blutes bei Arthropoden, Würmern und Echinodermen. (Biolog. Centralblatt, 1890, x, p. 428.)

=Preyer, W.= Zur Physiologie des Protoplasma. II, Die Funktionen des Stoffwechsels. Die Saftströmung. (Patanie’s Naturwiss. Wochenschr., 1891, vi, pp. 1–5.)

=Cholodkowsky, N.= Ueber das Bluten der Cimbiciden-Larven. (Entomologische Miscellen, vi, Horæ Soc. Ent. St. Petersburg, 1897, pp. 352–357, 1 Fig. The fluid thrown out through pores or fissures in the skin is the blood.)

With the writings of Korotaiev, Tichomeroff, Pékarsky, Balbiani, Korotneff, Cuénot, and others.

_c._ The fat-bodies

=Dufour, L.= Recherches anatomiques sur les Carabiques et sur plusieurs autres insectes Coléoptères. Du tissu adipeux splanchnique. (Ann. Sc. nat., viii, 1826, pp. 29–35.)

—— Histoire comparative des métamorphoses et de l’anatomie des _Cetonia aurata_ et _Dorcus parallelepipedus_. Tissu adipeux splanchnique. (Ann. Sc. nat., Zoologie, Sér. 2, 1842, xviii, pp. 178–179.)

=Meyer, H.= Ueber die Entwicklung des Fettkörpers, der Tracheen und der keimbereitenden Geschlechtsteile bei den Lepidopteren. (Zeitschr. f. wissens. Zool., 1849, i, pp. 175–179, 4 Taf.)

=Fabre, J. H.= Étude sur le rôle du tissu adipeux dans la sécretion urinaire chez les insectes. (Ann. Sc. nat., Sér. 4, xix, 1862, pp. 351–382.)

=Leydig, Fr.= Einige Worte über Fettkörper der Arthropoden. (Reichert, u. du Bois-Reymond’s Archiv f. Anat., 1863, pp. 192–203.)

=Lindemann, K.= Zoologische Skizzen. 1. Struktur des Fettkörpers. (Bull. Soc. Imp. d. Natural. Moscou, 1864, pp. 521–526, 1 Pl.)

=Landois, Leonh.= Ueber die Funktion des Fettkörpers. (Zeitschr. f. wissens. Zoologie, xv, 1865, pp. 371–372.)

=Wielowiejski, H. v.= Ueber den Fettkörper von _Corethra plumicornis_ und seine Entwicklung. (Zool. Anzeiger, 1883, vi Jahrg., pp. 318–322.)

—— Ueber das Blutgewebe der Insekten. (Zeitschr. f. wissens. Zool., 1886, xliii, pp. 512–536.)

=Kowalevsky, A. O.= Sur les organes excréteurs chez les arthropodes terrestres. (Congrès internat. Zool., 2^{me} Sess., pp. 196–205, Moscou, 1892.)

THE BLOOD TISSUE

Under this name Wielowiejski has included several important tissues or cellular bodies intimately concerned with the nutrition of the insect. These are:—

1. The blood corpuscles. (See p. 407, leucocytes and phagocytes.) 2. The fat-body proper (_Corpus adiposum_). 3. The pericardial fat-body (pericardial cells). 4. The œnocytes. 5. The garland-shaped cord of muscid larvæ. 6. The subœsophageal body, a peculiar organ found by Wheeler in the embryos and young larvæ of Blatta and Xiphidium. 7. The phosphorescent organs.

_a._ The fat-body

In the body cavity of winged insects and of their larvæ occur yellowish masses of large cells filled with small drops of fat, and forming the “fat-body.” It is of various shapes, more or less lobulated or net-like, and covers or envelops parts of the viscera, also forming a layer under the integument (Fig. 143). The tracheal endings are usually enveloped by the fat-body. It is larger in the larvæ than in the adults, especially in Lepidoptera, in them forming a reserve of nutrition, used during metamorphosis and during the formation and ripening of the eggs and male cells.

Wielowiejski has shown that there is a regular arrangement of the fat-body in the general cavity of the body. For example, in the larva of Chironomus occur the following forms of this tissue. Around the periphery, on each side of the body cavity, is a loose network of lobes with large meshes constituting the peripheral layer or external lobular fat-body; these lobular masses are segmentally arranged.

Within these segmental lobes, on each side of and along the digestive tract, extending along through almost the entire body, is an unbroken strand of this tissue, forming the internal fat-body cords. From the first larval stage, and even before hatching, its cells are so unusually large, being filled with large, clear, mostly colorless fat-drops, that their limits cannot be defined, and their nuclei can only with great difficulty be detected. Only in some large larvæ of Chironomus has Wielowiejski found clearly defined cells; the protoplasm of these cells contain almost no fat-drops.

The fat-body is of mesodermal origin, and as Wheeler insists, is not derived from the œnocytes, as supposed by Graber. Formed from the mesoderm, it is a differentiation of portions of the cœlomic walls, and therefore metameric in origin. That the fat-body gives origin to the blood corpuscles Wheeler is doubtful.

The fat-cells are distinct, spherical, and as a rule possess only one nucleus, though in those of Apis and Melophagus there are two nuclei, and in Musca several. Sometimes the cells contain a substance like the white of an egg, and concretions of uric acid, or these take the place of the fat-drops. The presence of uric acid shows that a very active metabolism goes on in the fat-body. “In some cases it has been proved that the fat-body in the larva is rich in fat and poor in concretions of uric acid, while in the imago it is poor in fat and rich in concretions of uric acid” (Lang).

Leydig, in 1857 (Lehrbuch der Histiologie), spoke of the presence of dark concretions in the fat-body, and afterwards (1864) showed that there was a wide distribution of uric acid salts and concretions. Witlaczil, also, has detected concretions in the fat-body of the Psyllidæ, in larval Cecidomyiidæ, in the larvæ and pupæ of ants, and in the pupa of Musca.

The physiological processes which take place in the fat-bodies are obscure. Graber regarded the whole system of the fat-bodies as “a single, many-lobed lung,” while before him Landois, taking into account the intimate relation existing between the finer tracheal branches and the fat-body, considered that the latter was concerned in respiration. Marchal thinks that the fat-body is a urinary organ, as the urates are formed within the cells of this body.

Moreover, Schäffer maintains that a special kind of fat-body cell has the important function of taking up and giving out nutritious matters during the internal processes of metamorphosis, while he also believes that there is a genetic connection between the fat-body and the blood corpuscles—a view combated by Wheeler.

Kowalevsky finds that the fat-body remains absolutely insensible to the action of the substances which stained the Malpighian tubes (p. 352). So long as the cells are healthy and living they are not stained and do not absorb the colors in question; and this insensibility persists, even when the cells are of a different nature, as those of the fly (adipose and “intercalary” cells).

_b._ The pericardial fat-body or pericardial cells

We have already, on p. 405, called attention to these organs, but they also have an intimate relation to the fat-body.

Kowalevsky (1892) remarks that the disposition of these cells varies much in different insects and even in the same animal. Thus, in the Diptera and the ordinary flies there are found around the lower part of the dorsal vessel 13 pairs of large pericardial cells which lie next to a crowded bed of small cells forming a compact mass around the anterior part of the dorsal vessel. In caterpillars, notably silkworms, from the compact layer of pericardial cells which surround the heart, pass off trunks which are directed towards the lateral walls of the body, also forming close networks around the tracheæ and then passing down into the abdominal cavity of the body of the larva.

In the larvæ of certain Hymenoptera, the trunks which pass off from the pericardial region form a loose cord, a sort of fatty tissue covering the entire body cavity.

This tissue, adds Kowalevsky, entirely differs from œnocytes, or from the so-called glandular body whose formation in Gryllotalpa has been described by Korotaiev, and in _Bombyx mori_ by Tichomiroff. In a recent work wherein has been collected everything known regarding these last-named cells, Pékarsky proves that they are unique in nature and cannot be regarded either as fat-cells, or as pericardial cells, or even as formative leucocytes.

As to the structure of the pericardial cells, Kowalevsky adds that they are always attached to muscular fibres passing off from the heart, and that they lie, so to speak, upon them. In the locusts the muscular fibres supporting the pericardial cells appear distinctly like little staves or sticks. The attachment of the pericardial cells to the muscular fibres has been observed by Cuénot and reproduced by him in his work, but his description somewhat differs from that observed by Kowalevsky in the locust (_Acrydium migratorium_).

As to the nature of the acid excretions which are formed in the pericardial cells, in spite of his attempts to solve the problem, Kowalevsky has been unsuccessful. The only observations in this direction are those of Letellier on the pericardial glands of lamellibranch molluscs, which he found to contain hypouric acid, and it is probable, says Kowalevsky, that the acidity of the pericardial cells in insects is due to the presence of the same acid.

=Leucocytes or phagocytes in connection with the pericardial cells.=—It is thought by Schäffer that the leucocytes or phagocytes may be free or wandering fat-body cells. They play an important part in metamorphosis, while they absorb or feed upon the remains of the larval organs, and thus prove of use in the building up of the organs of the adult insects.

While the faculty of _phagocytosis_ is wanting in the urinary tubes, Balbiani and more recently Cuénot have expressed the opinion that the pericardial cells of insects may have the power of absorbing hard bodies, “acting as a phagocytic gland.” This, however, is called in question by Kowalevsky, from studies made on different insects. On introducing powdered carmine into the body of an insect it has not been absorbed by the pericardial cells, as they have not been colored red. It is the leucocytes which absorb the grains of carmine, and which, after having dissolved them, transmit them to the pericardial cells. Hence, then, the pericardial cells have not the phagocytic power of which Cuénot speaks.

Returning to his own observations on hard bodies introduced into insects, or large globules introduced under the form of a milk emulsion, Kowalevsky has found that these bodies were absorbed in the first place by the free-swimming leucocytes, and in the second place by whole groups or nests of leucocytes situated in different parts of the body, principally on the threads of the adipose body. In the Orthoptera the absorption is immediately effected by means of the cells of the membrane which separates the pericardium from the cavity of the body underneath the heart. The regions where the hard bodies are absorbed in great number coincide with the regions of formation of the blood corpuscles. In his researches on the larvæ of Hyponomeuta and other Lepidoptera, Schäffer describes these regions as forming a sort of island. The nests where the blood globules are formed are the most active centres of phagocytosis.

Balbiani, and also Cuénot, have supposed that the formation of the blood corpuscles takes place in the pericardial cells, but Kowalevsky insists that these cells cannot form the leucocytes, which “are probably formed in different parts of the body, notably in the special nests [_Herde_ of Jäger] situated near the heart, but outside of the pericardial cells.”

In Fig. 384, where the nests of leucocytes (_l_) are shown, it is evident that they are formed where observed, and “could not have come from the pericardial cells, which have their own structure and their special function,” these cells being very large and characteristic.

In Kowalevsky’s preparations of Truxalis, the pericardial cells with deposits of carmine and the groups of leucocytes (Fig. 385, _l_ and _l′_) stained with India ink, we have to deal with elements absolutely different. If the formation of leucocytes was caused by the pericardial cells, these last would be obliged to free themselves from their contents and to modify their essential nature.

_c._ The œnocytes

These cells (Fig. 386), with the exception of the eggs, are the largest in the body, and occur in most if not all winged insects. They were called _œnocytes_ (_oinos_, wine; _kustis_, cyst), by Wielowiejski in allusion to their wine-yellow color. These cells are arranged segmentally (Fig. 387) in clusters, held in place by tracheæ, and are situated mostly on each side of the abdomen, rarely being found in the adjoining parts of the thorax. They are more or less intimately associated with the blood and fat-body. Unlike the fat-body, however, they arise in embryonic life from the ectoderm, either by delamination or by immigration, just behind the tracheal involutions.

The separate cells of each cluster are usually separate, but in rare cases may fuse in pairs or form smaller clusters. In shape they are round or oval, often sending out pseudopodia-like processes, by which they are attached to the tracheal twigs or to each other. “The cytoplasm, which is very abundant, is full of yellowish granules and is sometimes radially situated towards its periphery. The large spherical or oval nucleus contains a densely wound and delicate chromatic filament.” (Wheeler.)

Graber first pointed out the identity of these clusters of cells with certain metameric cell-masses in insect embryos, observed by Tichomiroff in those of the silkworm, and by Korotneff in the embryo mole-cricket.

Although they resemble the blood corpuscles in some insects, they are always much larger, and do not seem to be amœboid, while they are never seen to undergo self-division, or to exhibit any appearance of giving rise to the blood-cells (Wheeler). They have not yet been detected in Thysanura (Synaptera) or in Myriopoda.

_d._ The phosphorescent organs

Phosphorescence is not infrequent in the Protozoa, cœlenterates, worms, and has been observed in the bivalve Pholas, in a few abyssal Crustacea, in myriopods (Geophilus), in an ascidian, Pyrosoma, and in certain deep-sea fishes.

In insects luminosity is mostly confined to a few Coleoptera, and besides the well-known fireflies, an Indian Buprestid (_Buprestis ocelata_) is said to be phosphorescent; also a telephorid larva. Other luminous insects are the Poduran Anurophorus, Fulgora, certain Diptera (_Culex_, _Chironomus_[60] and _Tyreophora_), and an ant (Orya).

The seat of the light is the intensely luminous areas situated either in the head (Fulgora), in the abdomen (Lampyridæ), or in the thorax (in a few Elateridæ of the genus Pyrophorus). The luminous or photogenic organ is regarded by Wielowiejski and also by Emery as morphologically a specialized portion of the fat-body, being a plate consisting of polygonal cells, situated directly under the integument, and supplied with nerves and fine tracheal branches.

In Luciola as well as in other fireflies, including Pyrophorus, the phosphorescent organ or plate consists, as first stated by Kölliker, of two layers lying one over the other, a dorsal one (Fig. 388, _d_) which is opaque, chalky white, and non-photogenic, and a lower one (_v_), the active photogenic layer, which is transparent. Through the upper or opaque layer and on its dorsal surface extend large tracheæ and their horizontal branches, from which arise numerous very fine branches which pass down perpendicularly into the transparent or photogenic layer of the organ. Each tracheal stem, together with its short branches, is enveloped by a cylindrical mass of transparent tissue, so that only the short terminal branches or very fine tracheal capillaries project on the upper part of the cylinder. These finest tracheal capillaries are not in Luciola filled with air, but with a colorless fluid, as was also found by Wielowiejski and others in Lampyris.

These transparent cylinders, with the tracheæ within, forming longitudinal axes, resemble lobules. These lobules are so distributed that they appear on a surface section of this plate as numerous round areas in which circular periphery the tracheal capillaries are arranged with the axially disposed tracheal end-cells. These “tracheal end-cells” are only membranous enlargements at the base of the tracheal capillaries (Wielowiejski). The cylindrical lobules are separated from each other by a substance consisting of abundant large granular cells (parenchym cells) among which project the tracheal capillaries. The cylindrical lobules extend to the hypodermis and come in contact only by their lateral faces with the parenchym.

The structure of the upper opaque chalky white layer of the phosphorescent organ is, compared with that of the photogenic lower portion, very simple. In its loose, pappose, mass are no cellular elements, but when treated with different reagents it is seen to be filled with countless urate granules (guanine) swimming in the fluid it contains, the cell plasma appearing to be dissolved, the cells having lost their cohesion.

In comparing the phosphorescent plate or organ of Luciola with that of Lampyris, the general structure, including the clear cell elements of the cylindrical lobules, which envelop the perpendicular tracheal twigs and their branches, and also the granular parenchymatous cells are alike in both, though the arrangement and distribution of the elements in Luciola is more regular, in Lampyris the tracheal stems being irregularly scattered through the parenchym.

Wielowiejski found in the larval and female Lampyris a higher degree of differentiation than in the male, and Luciola has a more differentiated photogenic organ than Lampyris, as seen in the more regular structure of the lobules.

As regards the light-apparatus of Pyrophorus, or the cucujo, Heinemann shows that, as in the Lampyridæ, it consists of distinct cells, and may be regarded as a glandular structure. It is rich in tracheæ and the other parts already described. In still later researches on a Brazilian Pyrophorus Wielowiejski shows that the phosphorescent plate consists of two layers, the upper usually being filled with crystalline urate concretions, and entirely like those of the Lampyridæ, consisting of distinct polygonal cells, among which are numerous tracheal stems, with tænidia, coursing in different directions, when freshly filled with air, and sending capillaries into the underlying photogenic layer. The latter shows in its structure a striking difference in the cellular arrangement from that of Lampyrids. In the upper or non-photogenic layer are tracheal capillaries which pass down into the underlying cellular plate and which are in the closest possible relations with the single cells—a point overlooked by Heinemann.

=Physiology of the phosphorescence.=—As is well known, the phosphorescence of animals is a scintillating or glowing light emitted by various forms, the greenish light or luminous appearance thus produced being photogenic, _i.e._ without sensible heat.

Langley rates the light of the firefly at an efficiency of 100 per cent, all its radiations lying within the limits of the visible spectrum. “Langley has shown that while only 2.4 per cent of luminous waves are contained in the radiation of a gas-flame, only 10 per cent in that of the electric arc, and only 35 per cent in that of the sun, the radiation of the firefly (_Pyrophorus noctilucus_) consists wholly of visible wave-frequencies.” (Barker’s Physics, p. 385.)

The spectrum of the light of the cucujo was found by Pasteur to be continuous. (C. R. French Acad. Sc. 1864, ii, p. 509.) A later examination by Aubert and Dubois showed that the spectrum of the light examined by the spectroscope is very beautiful, but destitute of dark bands. When, however, the intensity diminishes, the red and orange disappear, and the green and yellow only remain.

Heinemann studied the cucujo at Vera Cruz, Mexico. At night in a dark room it radiates a pale green light which shows a blue tone to the exclusion of any other light. The more gas or lamp light there is present, the more apparent becomes the yellowish green hue, which in clear daylight changes to an almost pure very light yellow with a very slight mixture of green. “In the morning and evening twilight, more constantly and clearly in the former, the cucujo light, at least to my eyes, is an intensely brilliant yellow with a slight mixture of red. In a dark room lighted with a sodium light the yellow tone entirely disappears; on the other hand, the blue strikingly increases.” As regards the spectrum he found that almost exactly half of the blue end is wanting and that the red part is also a little narrower than in the spectrum of the petroleum flame.

Professor C. A. Young states that the spectrum given by our common firefly (_Photinus?_) is perfectly continuous, without trace of lines either bright or dark. “It extends from a little above Fraunhofer’s line C, in the scarlet, to about F in the blue, gradually fading out at the extremities. It is noticeable that precisely this portion of the spectrum is composed of rays which, while they more powerfully than any others affect the organs of vision, produce hardly any thermal or actinic effect. In other words, very little of the energy expended in the flash of the fire is wasted. It is quite different with our artificial methods of illumination. In the case of an ordinary gaslight the best experiments show that not more than one or two per cent of the radiant energy consists of _visible rays_; the rest is either invisible heat or actinism; that is to say, over 98 per cent of the gas is wasted in producing rays that do not help in making objects visible.”

Panceri also remarks that while in the spectroscope the light of some Chætopteri, Beroë, and Pyrosoma exhibit one broad band like that given by monochromatic light, that of Lampyris and Luciola is polychromatic. (Amer. Nat., vii, 1873, p. 314.)

The filtered rays of Lampyris pass (like Röntgen and uranium rays) through aluminium (Muraoka).

The physiology of insect phosphorescence is thus briefly stated by Lang: “The cells of this luminous organ secrete, under the control of the nervous system, a substance which is burnt during the appearance of the light; this combustion takes place by means of the oxygen conveyed to the cells of the luminous body by the tracheæ, which branch profusely in it and break up into capillaries.”

Emery states that the males of Luciola display their light in two ways. When at night time they are active or flying, the light is given out at short and regular intervals, causing the well-known sparkling or scintillating light. If we catch a flying Luciola or pull apart one resting in the day time, or cut off its hind body, it gives out a tolerably strong light, though not nearly reaching the intensity of the light waves of the sparkling light. In this case the light is constant, yet we notice, especially in the wounded insect, that the phosphorescent plate in its whole extent is not luminous, but glows at different places as if phosphorescent clouds passed over it.

It is self-evident that a microscopic observation of the light of the glow-worm or firefly is not possible, but an animal while giving out its light, or a separated abdomen, may readily be placed under the microscope and observed under tolerably high powers. By making the experiment in a rather dark room Emery saw clear shining rings on a dark background. “All the rings are not equally lighted. Comparing this with the results of anatomical investigation, it is seen that the rings of light correspond with the previously described circular tracheal capillaries, _i.e._ the limits between the tracheal-cell cylinder and the parenchym-cells. The parenchym-cells are never stained of a deep brown; this proves that its plasma may be the seat of the light-producing oxidation. Hence this process of oxidation takes place in the upper surface of the parenchym-cells, but outside of their own substance. The parenchym-cells in reality secrete the luminous matter; this is taken up by the tracheal end-cells and burnt or oxidized by means of the oxygen present in the tracheal capillaries. Such a combustion can only take place when the chitinous membrane of the tracheæ is extraordinarily fine and easily penetrable, as is the case in the capillaries of the photogenic plate; therefore the plasma of the tracheal cells only oxidizes at the forking of the terminal tracheal twigs and in the capillaries.” (Emery.)

The color of the light of Luciola is identical in the two sexes, and the intensity is much the same, though that of the female is more restricted. The rhythm of the flashes of light given out by the male is more rapid, and the flashes briefer, while those of the female are longer, more tremulous, and appear at longer intervals.

Emery then asks: What is the use of this luminosity? Is it only to allure the females of Luciola, which are so much rarer than the males? Contrary to the general view that it is an alluring act, he thinks that phosphorescence is a means of defence, or a warning or danger-signal against insectivorous nocturnal animals. If we dissect or crush a Luciola, it gives out a disagreeable cabbage-like smell, and perhaps this is sufficient to render it inedible to bats or other nocturnal animals. An acrid taste they certainly do not possess.

It has long been known that the eggs of fireflies, both Lampyridæ and Pyrophorus, are luminous. Both Newport and more recently Wielowiejski attributes the luminosity not to the contents of the egg, but to the portions of the fat-body cells or fluid covering on the outside of the eggs, due to ruptures of the parts within the body of the female during oviposition. The larvæ at different ages are also luminous.

The position of the luminous organs changes with age. In the larvæ of Pyrophorus before moulting, according to Dubois, the luminous organs are situated only on the ventral side of the head and prothoracic segment. In larvæ of the second stage there are added three shining spots on each of the first eight abdominal segments, and a single luminous spot on the last segment. These spots are arranged in a linear series and thus form three luminous cords. In the adult beetles there is a luminous spot in the middle of the first abdominal sternite, but the greatest amount of light is produced by the two vesicles on the hinder part of the prothorax, the position of which varies according to the species.

LITERATURE ON PHOSPHORESCENCE

=Peters, W.= Ueber das Leuchten der _Lampyris italica_. (Müller’s Archiv f. Anatomie, 1841, pp. 229–233.)

=Kölliker, A.= Die Leuchtorgane von Lampyris, eine vorläufige Mittheilung. (Verhandl. d. phys. medizin. Gesellsch. Würzburg, 1857, viii, pp. 217–224.)

=Schultze, Max.= Ueber den Bau der Leuchtorgane der Männchen von _Lampyris splendidula_. (Sitzber. d. niederrhein. Gesellsch. f. Natur. u. Heilkunde zu Bonn, 1864, Sep. p. 7.)

—— Zur Kenntniss der Leuchtorgane von _Lampyris splendidula_. (Archiv f. mikroskop. Anat., 1865, i, pp. 124–137, 2 Taf.)

=Wielowiejski, H. Ritter von.= Studien über die Lampyriden. (Zeits. f. wissens. Zool., 1882, xxxvii, pp. 354–428, 2 Taf.)

=Emery, Carlo.= Untersuchungen über _Luciola italica_ L. (Zeits. f. wissens. Zool., 1884, xl, pp. 338–355, 1 Taf.)

—— La luce della _Luciola italica_ osservata col microscopio. (Bull. Soc. Ent. Ital. Anno xvii, 1885, pp. 351–355, 1 Taf.)

=Wheeler, William Morton.= Concerning the blood tissue of the Insecta, III; Psyche, vi, 1892, p. 255. (Structure of the light organ of _Photuris pennsylvanica_.)

=Heinemann, C.= Zur Anatomie und Physiologie der Leuchtorgane Mexikanischer Cucujos, Pyrophorus. (Archiv f. mikroskop. Anat., 1886, xxvii, pp. 296–383.)

=Dubois, R.= Contribution à l’étude de la production de la lumière par les êtres vivants. Les Elaterides lumineux. (Bull. Soc. Zool. France, 1886, Année ii, pp. 1–275, 9 Pls.)

=Wielowiejski, H. Ritter von.= Beiträge zur Kenntniss der Leuchtorgane der Insekten. (Zool. Anzeiger, 1889, Jahrg. xii, pp. 594–600.)

=Hudson, G. V.= The habits and life-history of the New Zealand glowworms. (Trans. N. Zealand Inst., xviii, 1890, pp. 43–49, 1 P1.)

=Dubois, R.= Sur le mechanisme de la production de la lumière chez l’_Orya barbarica_ d’Algérie. (Comptes rend. Acad. Sc. France, 1892, cxvii, pp. 184–186. Also in Ann. and Mag. Nat. Hist., 1893 (6), xii, pp. 415–416.)

=Schmidt, P.= Ueber das Leuchten der Zuckmücken (Chironomidæ). (Zool. Jahrb. Morph., Abth. viii, 1894, pp. 191–216, 2 Taf. Also Ann. and Mag. Nat. Hist., xv, pp. 133–141, 1895. The light is due to bacteria. Nature, 1897.)

=Chun, C.= Leuchtorgane und Facettenaugen. (Biol. Centralbl., 1896, pp. 315–320.)

=Muraoka, H.= (On the filtered rays of Lampyris, Wiedemann’s Annalen, Dec., 1896.)

See also the writings of Audouin, Carus, D. Turner, Thompson.

THE RESPIRATORY SYSTEM

While land vertebrates breathe by inhaling the air through the mouth into the lungs, insects respire by internal air-tubes (_tracheæ_), which ramify throughout every part of the body and its appendages. The air enters these tubes through a few openings, called spiracles or _stigmata_, arranged segmentally in the sides of the body. These tracheæ are everywhere bathed by the blood, and thus the latter is constantly aërated or kept fresh; the blood not, as in vertebrates or as in molluscs, seeking the lungs or gills, or any specialized respiratory portion of the body where the oxygen combines with the hæmoglobin, but the respiratory tubes, so to speak, themselves seek out the blood and the blood-tissue in every part of the insect body, penetrating to the tips of the antennæ and of the legs, entering the most delicate tissues, even perhaps passing through the walls of epithelial cells. As Lang remarks, the want of an arterial vascular system is compensated for as well as conditioned by the extremely profuse branching of the tracheæ.

The aquatic larvæ of certain dragon-flies (Agrionidæ), may-flies, case-worms, etc., respire by means of tracheal gills or branchiæ, which are either filamental or leaf-like appendages containing tracheæ. Somewhat similar structures appended to the thorax of pupal aquatic Diptera, as in the mosquito and its allies, enable them to breathe while stationed a little beneath the surface of the water. Other larvæ, as the rat-tail larva of Eristalis, etc., lying at the bottom of shallow pools or in ditches, etc., can breathe by raising slightly above the surface a long appendage with two spiracles at the end, through which the air enters the tracheal system. (See p. 461.)

Although Aristotle, as well as the natural philosophers of the Middle Ages, supposed that insects did not breathe, one can easily see that they do by holding a grasshopper or dragon-fly in one’s hand and observing the rhythmical rise and fall of the upper and lower walls of the abdomen, during which the air enters and passes out of the air-openings or spiracles on each side of the body.

It is plain that insects consume very little air, since caterpillars may be confined in very small, almost air-tight tin boxes, and continue to eat and undergo their transformations without suffering from the confinement. According to H. Müller an insect placed in a small, confined space absorbs all the oxygen. Insects can survive for many hours when placed in an exhausted receiver, or in certain irrespirable gases. “Cockroaches in carbonic acid speedily become insensible, but after twelve hours’ exposure to the pure gas they survive and appear none the worse.” (Miall and Denny, p. 165.) Insects of the swiftest flight breathe most rapidly, their great muscular activity requiring the absorption of an abundance of oxygen.

Warmth, plenty of food, besides muscular activity, increases the demand for oxygen and the quantity of carbonic acid exhaled.

_a._ The tracheæ

It will much simplify our conception of the nature of the air-tubes when we learn that they originate in the embryo as tubular ingrowths of the integument (ectoderm), these branching and finally reaching every part of the interior of the body. They are elastic tubes, and being filled with air are silvery in color, though at their origin near the spiracles they are reddish or violet bluish; or, in the larva of Æschna, reddish brown, this tint being due to a finely granular pigment situated in the peritoneal membrane.

In their essential structure the tracheæ consist of the chitinous intima, which is a continuation of the cuticle of the integument, and of a cellular membrane or outer layer of cells (a continuation of the hypodermis) called the peritoneal membrane, or ectotrachea (Figs. 392, 393).

Leydig discovered that the spiral filaments are not distinct and separate, but intimately connected with the inner membrane (intima), and he detected the outer or peritoneal membrane, which Chun afterwards found to be epithelial in its nature, Minot stating that it is a true pavement epithelium.

Figure 393 represents a longitudinal section of a large trachea of Hydrophilus, showing the peritoneal membrane (_ectotrachea_, _ep_) and the intima or _endotrachea_, divided into the cuticula (_cu_), with the darker colored inner layer, in which are embedded the dark-colored tænidia (_f_).

=Distribution of the tracheæ.=—The distribution of the air-tubes, as Lubbock and also Minot state, depends first upon the shape of the organs, and upon the size of those whose size is variable. Around the large, hollow organs (digestive canal, sexual organs) the tracheæ ramify in all directions, forking so that the branches diverge at a wide angle. In the organs which have muscular walls, like the oviduct, the tracheæ run straight when the walls are distended, but have a sinuous course when the walls are contracted. (Minot.)

“Around the organs of more elongated form the branches of the tracheæ run more longitudinally, as is shown by the air-tubes of the muscles, which also present some peculiarities worthy of especial notice.

“A short, thick trunk arrives at the muscular bundle, and dividing very rapidly, breaks up into a large number of delicate tubes, which penetrate between the muscular fibres, then terminating in tubes of exceeding fineness, which at first sight seem to form a network that might well be called a _rete mirabile_. A closer examination, however, reveals that it is not a real network, but rather an interlacing confusing to the eye. The longitudinal direction of the tracheæ of the muscles presents a striking contrast to the system of divarication represented in Figs. 13 and 14. The course of the tracheæ of the Malpighian tubes is also very curious. There is one large trachea which winds around the tube in a long spiral, giving off numerous small branches which run to the surface of the tube, upon which they form delicate ramifications. Each tube has but a single main trachea, and I think the trachea continues the whole length of the tube, but of this last point I am not quite sure.” (Minot.)

While in the nymphs of Orthoptera the tracheæ very closely resemble those of the adult, in larvæ of insects with a complete metamorphosis the tracheæ differ very much in distribution from those of the adult. The larval tracheæ are also more generalized and more like those of the original type than the tracheæ of perfect insects. (Lubbock.)

In general there are two main tracheæ, one passing along each side of the body, near the digestive canal, connected with its mate by a few transverse anastomosing branches, and sending off a branch to each spiracle, this arrangement being most simple and apparent in the maggots of Diptera. From these two main branches smaller twigs branch off into every part of the body with its appendages, passing among the different organs, often serving as cables to hold them loosely in place; they also penetrate into the component parts of the organ themselves, passing into the fat-bodies, and among the fibres of muscles, where they become finely attenuated and refined like the capillaries of the vascular system of vertebrates. (Figs. 395, 396.)

In the youngest larva of _Corethra plumicornis_ Weismann ascertained the thickness of the longitudinal stem to be 0.0017 mm. That of the finest tracheal endings in the silk-glands of the silkworm was found by Von Wistinghausen to be 0.0016 mm. (Zeits. f. Wiss. Zool. xlix, 1890, p. 575.) Weismann states that in the larvæ of Corethra and Chironomus the tracheal system is only incompletely developed; the tracheæ are not united with each other, and in the youngest larvæ they do not contain air.

Each of the two main tracheæ, as Kolbe states, sends off into each segment of the body three branches.

1. An upper or dorsal branch, which supplies the muscles of the dorsal region.

2. A middle (visceral) branch, whose twigs pass to the digestive canal and back to the organs of reproduction.

3. A lower (ventral) branch, whose twigs are distributed to the ganglia and to the muscles of the ventral region.

In certain Thysanura, as a species of Machilis (Fig. 397), we probably have the primitive condition of the tracheal system, the longitudinal and transverse anastomoses being absent, but in other Thysanura (Japyx, Nicoletia, Lepisma, and a few species of Machilis) they are present.

As Kolbe remarks, whether the fine ends of the tracheæ are closed or open, whether after the analogy of the blood capillaries of vertebrates they anastomose with each other, whether the ends of the air-tubes pass between the cells or penetrate into them, these questions are not fully settled. According to Leydig’s[61] latest views the tracheæ penetrate into the cells and unite with the hyaloplasma. Hence the process of respiration in the last instance takes place in the hyaloplasma. This assumption accords with the fact that in the tracheate Arthropods the terminations of the tracheæ carry the atmospheric air into the space bounded by the cellular network, also to the hyaloplasma filling the spaces. Leydig[62] also thinks that the finest tracheal endings penetrate into the muscular tissue and unite with the primitive muscular fibres.

Kupffer is likewise of the opinion that the fine tracheæ penetrate into the cells, and Lidth de Jeude asserts that they enter the epithelial cells, “each cell containing several branches.” Kölliker, Emery, etc., maintain, however, that the tracheal endings lie between the cells. Wielowiejski,[63] in describing the line tracheæ of the phosphorescent organs, thinks that the tracheal endings (tracheal capillaries) rarely end blindly, but anastomose with one another, forming an irregular network. The latest observer, Gilson (1893), asserts that tracheal twigs penetrate deeply into the epithelial cells of the silk glands of larval Trichoptera as well as of caterpillars, passing through their protoplasm.

A late investigator, C. von Wistinghausen, finds in the tracheæ of the spinning-glands of caterpillars a completely formed network between the terminal branches of two or several tracheal groups. The tracheal tubes of this series of terminal branches pass into this network, which he calls the tracheal capillary end-network (Figs. 398, 400). This last varies in thickness and spreads out under the membrana propria of the glandular mass over the entire surface of the large gland-cells and on a level with the tracheal capillaries. The tracheal endings do not penetrate into the cells, but are separated from the plasma of the cells by a thin membrane. The tracheal capillary end-network appears as a system of fine tubes like the tracheal capillaries, consisting of a peritoneal layer and a chitinous intima (Fig. 400). The walls of these tubes are homogeneous, not porous, though readily permeable by the parenchymatous fluid. The interchange of gases consequently may go on easier and more vigorously in a system of richly anastomosing tubules of the net-like mass of tracheal capillaries, than in tubes ending blindly.

While the diameter of the tracheal capillaries is 0.0016 mm. or 1 µ, that of the tubules composing the tracheal capillary end-network is scarcely measurable, but is less than 1 µ.

These tracheal capillaries also occur on the seminal and other sexual tubes, on the intestine, on the urinary tubes, on the fat-bodies, but are most easily detected on the silk-glands.

The latest researches are those of E. Holmgren, who has studied the branching of the tracheæ in the spinning-glands of caterpillars. He prefers to call the end-cells “transition cells,” as they lead from the tracheal tubes proper to the capillary network. This latter is formed by slender nucleated cells, often with an intracellular lumen, and, according to the author, probably constituting a respiratory epithelium. He finds that both large and small tracheæ may penetrate the gland-cells. (Anat. Anzeiger, xi, 1895, pp. 340–6, 3 figs.; Jour. Roy. Micr. Soc., 1896, p. 182.)

_b._ The spiracles or stigmata

The spiracles are segmentally arranged openings in the sides of the thorax and abdomen, through which the air passes into the air-tubes. In its essential structure a spiracle, or _stigma_, is a slit-like opening surrounded by a chitinous ring, the lips or edges of the opening being membranous and closed by a movable valve of the spiracle attached by its lower edge, which is closed by an occlusor muscle (Fig. 401). The aperture when open forms a narrow oval slit; and in most insects the slit is within guarded by a row of projecting spines or setæ, which form a lattice work or grate to keep out dust, dirt, fluids, etc.

Krancher[64] has described five leading types of stigmata, not, however, taking into account those of the Synaptera.

I. _Stigmata without lips_ (Primitive or generalized stigmata).

_a._ The simplest stigma is an aperture which is kept open by a chitinous ring (Acanthia). The opening may be round or elliptical. There are no lips nor any movement of the edges to be observed. Such air-holes occur in the abdomen of bugs (Hemiptera) and beetles (Coleoptera); within the opening of the stigmata in the same insects is a funnel-like contraction. Also in the Diptera the abdominal stigmata are of the same type.[65] The stigmata of the Pulicidae (Siphonaptera) are more complicated, as the edges of the openings are provided with setæ (Fig. 402).

_b._ The stigma consists of a series of minute single stigmata, which are usually surmounted by a common chitinous ring, and whose tubular continuations unite within in a common trachea, so that the single tubes pass off from the stigma like the fingers on the hand. This form is found in the larvæ and puparia of Diptera.

II. _Stigmata with lips_ (Secondary more specialized stigmata).

_c._ The lips are represented by a single chitinous ring, with sparse spines. One side of the stigma is a little higher, and partly overlaps the other posteriorly; this form is peculiar to the Orthoptera and Libellulidae.

_d._ The lips are roof-like, bent inwards and densely hairy, forming a peculiar kind of felting. The setæ of the lips are in most beetles and many Lepidoptera separate, and more or less branched. In caterpillars, the setæ are so finely branched as to form a loose felt, or sieve-like arrangement.

_e._ The stigmata are round, with a very broad border and a concentric middle portion, the structure being complicated. The concentric middle portion is pouch-like and bears the occlusor muscle. This form occurs in the larvæ of lamellicorn beetles, and can be seen with the naked eye, or with a lens, in Oryctes, Cetonia, and Melolontha (Fig. 403).

_f._ Over the outer opening of the spiracle is an incurved chitinous projection, on one side of which the trachea takes its origin. It is thus in the Hymenoptera.

The remarkable grate-like stigma of the lamellicorn larvæ has the appearance as if the outer closing plate or valve were impenetrable. The earlier observers considered these stigmata to be open, but Meinert regards them as closed; Schiödte, however, has observed by pressing a preserved specimen of a Melolontha larva the alcohol within passing out in drops, through the grate-like plate, and hence he considers this a proof that the stigma is permeable (Kolbe).

More recently (1893) Boas has examined the same structure in the same species of larva as examined by Schiödte, and he finds it to be open only during the process of moulting. He finds that on each side of the larva there are nine short and wide stigmatic branches, each of which is shut off from the exterior by a brown plate; this consists of a reniform sieve-plate, and of a curved bulla which fits into the cavity of the plate. The stigmatic branch, however, is provided with a large external opening, which is homologous with the stigma, but which is usually closed by the plate and bulla, and is only open during the moulting; at first it is circular, but later becomes a cleft. A transverse section shows that the bulla is a simple tegumentary fold, the outer chitinous layer of which has become especially firm. The plate forms a horizontal half-roof, which springs from one side of the tracheal orifice, and is supported by obliquely set bases, which spring from the adjoining part of the inner side of the tracheæ. The plate and bars are purely cuticular structures. (Zool. Anz., 1893; also Journ. Roy. Micr. Soc., p. 54.)

The tracheal system of libellulid nymphs is not closed; on the other hand, in the fully-grown nymphs the anterior stigmata occurring on the dorsal side are large, and the tracheæ arising from them are thick. These stigmata are permeable by the air. In half-grown and still younger stages of Æschna the two anterior thoracic stigmata are undeveloped. In order to breathe, the fully-grown nymph either rises up on the upper side and elevates the end of the body to the surface in order to take the air into the rectum, or it rests with the back of the thorax at the surface in order to breathe through the large stigmata. The young nymphs take in air only through the rectum. The young nymphs of Libellula and its allies, on the other hand, possess large thoracic stigmata, but they prefer to breathe through the rectum. The fully-grown nymphs of Agrion breathe through the thoracic stigmata. (Dewitz, in Kolbe.)

=The position and number of pairs of stigmata.=—The spiracles are usually situated in the soft membrane between the tergites and pleurites, but their exact position varies in different groups. In the Coleoptera they occupy on the thorax a more ventral position, and on the abdomen are placed near the edge of the dorsal side, under the elytra. In the dragon-flies, the first pair is situated much more dorsally than the second and third pairs; the following seven pairs are almost wholly ventral and lie concealed in the membranous fold near the external plate. In the Hemiptera, also, the abdominal stigmata, though entirely free and visible, are situated ventrally.

Primarily, in the embryo a pair of stigmata appear on each segment of the thorax and abdomen, except the 10th and 11th, and even possibly in the head, for a pair of stigmata are said to occur in the head of Podurids (Smynthurus) (Lubbock), though this statement needs confirmation. Scolopendrella, however, is known to possess a pair of cephalic spiracles.

From the foregoing statement it will be seen that while in existing winged insects no more than 10 (in Japyx 11) pairs of stigmata are to be found in any one species, yet that 12 segments of the body, in different groups taken collectively, bear them. The primitive number of pairs of spiracles, therefore, in winged insects, was 12, _i.e._ a pair in each thoracic segment, and a pair in each of the first nine abdominal segments. Insects were originally all holopneustic, and gradually as the type became differentiated into the different orders they became peripneustic or amphipneustic, and, in certain aquatic forms, apneustic. (See pp. 459, 461.)

In the still more primitive, probably wingless, ancestors of insects there was a larger number of stigmata. Hatschek, in 1877, discovered a pair of tracheal invaginations in each of the three posterior head-segments of the embryo of a moth, with stigmatal openings in the 1st and 2d maxillary segments.

Thus early in embryonic life every segment of the body, except those bearing the eyes and the last abdominal, bore a pair of stigmata, so that the primitive insect had at least 15, and perhaps more, pairs of stigmata.

The position of the stigmata is subject to much variation, the result of adaptation to this or that mode of life. Examples are those insects which live in dusty situations or usually more or less concealed in the earth, as in most beetles, and in the Hymenoptera. In such beetles, the stigmata are situated in the thin membrane between the segments; in the Hymenoptera, on the upper edge of the segments. In the Siphonaptera, Pediculina, bed-bug, and similar forms, which breathe an air freer from dust, the spiracles lie free on the outside of the body.

“When the stigmata are free and without any protection on the abdomen, there are other ways by which the entrance of foreign bodies into the tracheæ is prevented. In such cases the body is covered with dense hairs, as in most Diptera and Neuroptera, as well as many Lepidoptera; or there is situated in front of the stigma either a small fissure which is covered over by a number of hairs arising from the edge, as in many Orthoptera; or, as in most insects, a luxurious growth of hairs on the inside of the stigma forms a thick filter for the air. Thus we see that also in this respect each species of insect is completely adapted to its surroundings.” (Krancher.)

=The closing apparatus of the stigma.=—Whether the external opening of the stigma is permanently open or closed, communication with the tracheæ may be cut off at pleasure during respiration by an internal apparatus of elastic chitinous bands and rods and the occlusor muscle.

The parts concerned in this operation are: 1. The closing bow; 2. The closing lever or peg; 3. The closing band; 4. The occlusor muscle (Figs. 405, 406).

“The first three parts are chitinized; they form a ring around the stigmatic opening, and are united to each other by joints. The bow is usually crescentic and as a rule surrounds one-half of the trachea. On the other side is the closing band which, by different contrivances, representing the closing lever or peg, becomes closely pressed against the closing bow. This lever is usually of the shape of a slender chitinous rod, which causes the closure; but it can also bend rectangularly, become converted into a typical lever as in the Lepidoptera, or it may assume the form of two peg-like processes, which press with their base against the closing bow.” (Krancher.)

“The closure of the spiracular opening is effected by the contraction of the muscles, while the opening is due to the elasticity of the chitinous parts. When at rest the spiracle is naturally open, so that the air in the trachea can directly communicate with the external air. Usually one end of the muscle is attached to the closing peg, and the other end to the closing bow. Where, as in Melolontha, the closing apparatus is provided with two levers, then naturally the muscle binds these two together and brings about by powerful contractions a firm closure of the trachea”; but, remarks Krancher, “this is not the only kind; there are numerous modifications. Besides the form just described, the levers assume the form of valves (Sirex), or of a brush (Pulex); or of a ring (larvæ of Diptera) with a circular muscle attached to it; or of a ring which simply becomes compressed (thoracic stigmata of Diptera).”

_c._ Morphology and homologies of the tracheal system

As first shown by Bütschli, the tracheal system is a series of segmentally arranged tubular invaginations of the ectoderm; a pair of stigmata primitively occurring on every segment of the body except perhaps the most anterior, and the last two or last one, a reduction in their number having since taken place, until in the Podurans none have survived. In the supposed ancestor of myriopods and insects, Peripatus, there are tracheæ; but they are very fine, simple, not-branched chitinous tubes which are united into tufts at the base of a flask-shaped depression of the integument, the outer aperture of which depression is regarded as a stigma. In one species (_P. edwardsii_) these tufts and their openings are scattered irregularly over the body; but in another kind (_P. capensis_) some of the stigmata at least show traces of a serial arrangement, being disposed in longitudinal rows—two on each side, one dorsally and one ventrally, those of each row, however, being more numerous than the pairs of legs. (See p. 9 and Fig. 4, _D_.)

It should be observed that in Peripatus, which does not possess urinary tubes, the segmental organs or nephridia are well developed, hence the tracheal tubes coexisting with them cannot be their homologues. We are therefore compelled to regard the tracheal system as of independent origin, arising in the earliest terrestrial air-breathing arthropod, and not indebted for its origin to any structure found in worms, unless perhaps, as both Kennell and Lang suggest, to dermal glands, since, according to Kennell, certain Hirudinea and many Turbellarian worms possess long, mostly unicellular, glands which spread far through the parenchyma of the body. (Kennell.)

Thus Kennell supposes that the ancestors of the Tracheates had spiracles on every segment of the body where the internal organization allowed them to exist. “The reduction of the breathing holes to a smaller number, and their restriction of a pair only to a single segment, was brought about partly by adaptation to a peculiar mode of life,—as insect larvæ especially teach us,—partly also—I may say mechanically—as a result of the obstruction to their development made by the growth or excessive development of other organs.” Among these he reckons the thick, dense cuticula of the integument, the internal fusion of several segments to form body-regions, and the arrangement and great development of the muscles in the head and thorax, etc. (p. 29.)

Kennell has suggested the origin of the tracheæ of Peripatus from the unicellular dermal glands of annelidan ancestors, since he has found glands in certain land-leaches of tropical America, which are provided with enormously long tubular passages united into bundles and opening externally, these tubes appearing to be slightly chitinized. Fig. 407 will show the appearance of a bundle of fine tracheal tubes of Peripatus ending at the bottom of a follicle formed by a deep invagination of the integument, which may be regarded as a primitive spiracle. (See Kennell, Ueber einige Landblutegel des tropical America, Zool. Jahrb. ii, 1886; also Die Verwandtschaftsverhältnisse der Arthropoden, 1891, p. 25.) We may add that Carrière supposes from his study of the embryology of the wall-bee (_Chalicodoma muraria_), published in 1890, that not only the salivary glands, but also the tentorium, are homologues of the tracheæ, while other structures than tracheæ may have evolved from unicellular dermal glands, which are widely distributed. It may in this connection be observed that some authors derive the book-lungs or book-leaf tracheæ of Arachnida from the gills of Limulus; hence if those of Arachnida arose from quite different and more specialized organs than dermal glands, it is not impossible that the tracheæ of Peripatus, Myriopods, and insects arose _de novo_, and then we need not look for any primitive structures in worms from which they arose.

Although Bütschli in 1870 in his embryology of the honey-bee called attention to the “great similarity which the eleven pairs of invaginations in the eleven first trunk-segments in their first indication (_anlage_) have with the spinning-glands, and also with the segmental organs of Annelids,” he did not go further than this, and it is now known that in the 2d maxillary segment open not only spinning-glands, but in the embryo a pair of stigmata.

Paul Mayer, however, regarded the tracheæ and urinary tubes as homodynamous structures, and this view was advocated by Grassi (1885) for the reason that while in the embryo honey-bee there are ten pairs of stigmata, the first thoracic and two last abdominal segments wanting them, the germs of the urinary tubes arise in a corresponding situation on the two last abdominal segments. To this view Emery (Biol. Centralb., 1886, p. 692) objects that in Peripatus the nephridia and tracheæ “have nothing to do with the segmental organs,” as Peripatus besides nephridia possesses both coxal glands and tracheæ.

Both Kennell and Lang derive the coxal glands of Arthropoda from the setiparous or parapodial glands of annelid worms, and the recent endeavor of Bernard to show that the tracheæ arose from setiparous glands seems to be disproved by the fact that in insects as well as in other Arthropoda coxal glands with their outlets exist in the same segments as those bearing stigmata. Reasoning by exclusion, we are led to regard Kennell’s original view as the soundest.

Patten, however, regards the tracheæ as modified ends of nephridia, remarking: “Since in Acilius some of the abdominal tracheæ at first communicate with the cavities of the mesoblastic somites, it is probable that all the tracheæ represent the ectodermic portions of the nephridia.” (Origin of Vertebrates from Arachnids, p. 355.)

It is probable, therefore, that the tracheæ first arose as modifications of dermal glands, as in mites and Peripatus, and that at first they were not provided with tænidia (as in Chilopoda), while in later forms tænidia were developed. In the earliest tracheate forms the stigmata were not segmentally arranged, probably appearing irregularly anywhere in the body, but afterwards in the myriopods and insects became serially arranged.

_d._ The spiral threads or tænidia

It is generally supposed that the so-called “spiral thread” forms a continuous thread from one end of a tracheal branch to the other. This was first shown not to be the case by Platner in 1844. Minot has proved that “there is not a single spiral thread, but several, which run parallel to one another and end after making a few turns around the trachea.”

The tænidia we have found to be in some cases separate, independent, solid rings, though when there is more than one turn the thread necessarily becomes spiral. The tænidia of a main branch stop at the origin of the smaller branches, and a new set begins at the origin of each branch. The tænidia at the origin of the branch do not pass entirely around the inside of the peritoneal membrane; in the axils they are short, separate, spindle-shaped bands (Fig. 409).

At one point in the main trachea of the larva of Datana the tænidia were seen to end singly on one side (at a considerable distance from any branch or axil) at intervals, with a tænidium situated between them, making four or five turns; then there is only one band situated between two ends; this band or thread is succeeded by a set with five turns between the two ends, this set being succeeded by one complete ring situated between two ends; in all cases the ends vary in length, some threads being short and others long, so that they apparently end anywhere along the circumference of the trachea, and this arrangement is seen to apparently extend along the whole length of the trachea. Hence it is seen that as a rule the tænidia vary much in length, and never, as generally supposed, pass continuously from one end to another of a tracheal branch, for there are many spirals in a branch, each making only from one to five turns, most usually four turns. Fig. 408, part of a trachea of _Dyticus marginatus_, shows that at a slight bend in a trachea the tænidia is interrupted, and short, incomplete, wedge-shaped tænidia (_e_) are interpolated; at _A_, _d_ is seen a split in one of the tænidia (compare also MacLeod, Pl. 1, Fig. 9). The threads are quite irregular in width. In the axils of the branches there is, as seen in Fig. 409, a basketwork of independent, short, often spindle-shaped tænidia; these are succeeded by longer ones, until we have threads passing entirely around near the base of each new branch; these being succeeded by others which make from two to five spiral turns.

The shape of the tænidia appears to vary to a great extent. In lepidopterous insects we have observed them to be in their general shape rather flat and slightly concavo-convex, the hollow looking towards the centre of the trachea. Minot’s section (Fig. 393) shows that in Hydrophilus they are cylindrical and solid, and Chun states that those of Stratiomys are round, while in Eristalis they are round, with a ridge projecting into the cavity of the trachea; in Æschna the thread is quadrangular. MacLeod states that sometimes it is cylindrical, in other cases flat, likewise prismatic; Macloskie believes that the spiral threads of the centipede are “fine tubules, externally opening by a fissure along their course.”

Stokes confirms Macloskie’s statements, stating that in the hemipterous _Zaitha fluminea_ “the tænidia are fissured tubules formed within and from chitinized folds of the intima, the convexity of the folds looking towards the lumen of the tracheæ.” In Fig. 414, 1, are represented portions of several tænidia showing the fissure, which is sometimes interrupted; at 2 are seen “the formation of what may be called apertures in a chitinous bridge.” Stokes regards the tænidia as “inwardly directed folds of the membrane.” Near the spiracles the tracheal membrane is externally studded with minute papillæ, as shown at 3, where are represented three broad and incomplete tænidia, with the tapering end, or the beginning, of another. Stokes adds, “Here they are only broad grooves, with no appearance of the narrow fissure of the completed tænidium. At 4 is figured a portion of the internal surface of a large trachea near the external orifice, the tænidia being in an incipient stage, evidently forming more or less of a network, as is usually the case next to the stigma” (compare p. 451, and Fig. 414).

The tracheæ of chilopod myriopods appear to be like those of insects. A number of authors have failed to detect the spiral threads in the Julidæ. As to the Arachnida, several observers, including Menge and Bertkau, have denied the existence of the spiral thread in the spiders with the exception of the Attidæ; and MacLeod finds them “scarcely visible” in Argyroneta.

Besides the tracheæ, the salivary duct is kept permanently distended by tænidia, which, however, are not spiral. They usually form incomplete rings, as in Stomoxys, arranged as shown in Fig. 410.

The labella (proboscis) of flies are supported by incomplete chitinous tubes or “pseudo-tracheæ,” the ends of which form the scraping teeth, this being, according to Dimmock, their primary function. Dimmock describes them as cylindrical channels opening on the surface in zigzag slits. These channels are held open by incomplete rings, one end of which is forked. “These rings are apparently arranged so that one has its fork on one side of the opening of the channel, the next ring the fork on the opposite side of the channel, and so on, in alternation. Their true structure is revealed when flattened out.”

The use of the elastic tænidia is to render the tracheæ elastic, and to keep them permanently open, as is the case with the parallel rings of the trachea of the higher vertebrates. The tracheæ are thus rendered firm and solid, at the least expense of chitinous material. The spiral thread, as MacLeod remarks, “is the realization in nature of what engineers call a form of the greatest resistance.”

The tænidia are wanting in the fine endings of the tracheæ (tracheal capillaries); also in the cockroach, according to Miall and Denny, they are not developed in the large tracheæ close to the spiracles, and the intima or wall of the tube has a tessellated instead of a spiral marking (Fig. 411). The same structure is seen in the Perlidæ (Nemoura, Gerstaecker, Zeit. f. wissen. Zool. xxiv, Taf. xxiii, Figs. 5 and 7); also in Æschna (Hagen, Zool. Anz. 1880, p. 159). In certain fine tracheæ of the eyes of the fly no spiral threads are developed. (Hickson.) The air-sacs or dilated tracheæ are also without tænidia.

While in the living insect the main and smaller tracheæ are filled, with air, it is stated by Von Wistinghausen that the fine capillary ends contain a fluid.

_e._ Origin of the tracheæ and of the “spiral thread”

While we owe to Bütschli the discovery of the mode of origin and morphology of the tracheæ, which as he has shown[66] arise by invaginations of the ectoblast; there being originally a single layer of epiblastic cells concerned in the formation of the tracheæ; we are indebted to Weismann[67] for the discovery of the mode of origin of the “intima,” from the epiblastic layer of cells forming the primitive foundation of the tracheal structure.

Weismann did not observe the earliest steps in the process of formation of the stigma and main trunk of the tracheæ, which Bütschli afterwards clearly described and figured.

Weismann, however, thus describes the mode of development of the intima; after describing the cells destined to form the peritoneal membrane, he says: “The lumen is filled with a clear fluid and already shows a definite border in a slight thickening of the cell-wall next to it.

“Very soon this thickening forms a thin, structureless intima, which passes as a delicate double line along the cells, and shows its dependence on the cells by a sort of adherence to the rounded sides of the cells (Taf. vii, 97 _A_, _a b c_). Throughout the mass, as the intima thickens, the cells lose their independence, their walls pressing together and coalescing, and soon the considerably enlarged hollow cylinder of the intima is surrounded by a homogeneous layer of a tissue, whose origin from cells is recognized only by the regular position of the rounded nuclei (Taf. vii, Fig. 97, _B_).

“Then as soon as the wavy bands of the intima entirely disappear, and it forms a straight, cylindrical tube, a fine pale cross-striation becomes noticeable (vii, 97, _B_, _int_), which forms the well-known ‘spiral thread,’ a structure which, as Leydig has shown, possesses no independence, but arises merely from a partial thickening of the originally homogeneous intima.

“Meyer’s idea that the spiral threads are fissures in the intima produced by the entrance of air is disproved by the fact that the spiral threads are present long before the air enters. Hence the correctness of Leydig’s view, based on the histological structure of the tracheæ, is confirmed by the embryological development, and the old idea of three membranes, which both Meyer and Milne-Edwards maintain, must be given up.”

Weismann also contends that the elastic membrane bearing the “spiral thread” is in no sense a primary membrane, not corresponding histologically to a cellular membrane. On the contrary, the “peritoneal membrane comprises the primary element of the trachea; it is nowhere absent, but envelops the smallest branches, as well as the largest trunks, only varying in thickness, which in the embryo and the young larva of Musca stands in relation to the thickness of the lumen.”

The trachea, then, consists primarily of an epithelial layer, the “peritoneal membrane,” or the invaginated epiblast; from this layer an intima is secreted, just as the skin or cuticle is secreted by the hypodermis. We may call the peritoneal membrane the _ectotrachea_, the intima or inner layer derived from the ectotrachea the _endotrachea_. The so-called “spiral threads” are a thickening of the endotracheal membrane, sometimes arranged in a spiral manner. For these chitinous bands we have proposed the name _tænidia_ (Greek, little bands).

As to the origin of the spiral thread our observations[68] have been made on the caterpillar of a species of Datana, which was placed in alcohol, just before pupation, when the larva was in a semipupal condition, and the larval skin could be readily stripped off. At this time the ectotrachea of the larva had undergone histolysis, nothing remaining but the moulted endotrachea, represented by the tænidia, which lay loosely within the cavity of the trachea. The ectotrachea or peritoneal membrane of the pupa is meanwhile in process of formation; the nuclear origin of the tænidia is now very apparent.

Fig. 412 represents a longitudinal section through a secondary tracheal branch, showing the origin of the chitinous bands, or tænidia. At _t′_ are pieces of six tænidia which have been moulted; _ectr_ indicates the nuclei forming the outer cellular layer, the ectotrachea or peritoneal membrane. These nuclei send long slender prolongations around the inside of the peritoneal membrane; these prolongations, as may be seen by the figure, become the tænidia. The tænidia, being closely approximate, grow together more or less, and a thin endotracheal membrane is thus produced, of which the tænidia are the thickened band-like portions. The endotracheal membrane is thus derived from the ectotrachea, or primitive tracheal membrane, and the so-called “spiral thread” is formed by thickenings of the nuclei composing the secondary layer of nuclei, and which become filled with the chitin secreted by these elongated nuclei. The middle portion of the tænidia, immediately after the moult, is clear and transparent, with obscure minute granules, while the nuclear base of the cell is filled as usual with abundant granules, and contains a distinct nucleolus.

The origin of the tænidia is also well shown by Fig. 413, which is likewise a longitudinal section of a trachea at the point of origin of a branch. The peritracheal membrane or ectotrachea (_ectr_) is composed of large granulated nuclei; and within are the more transparent endotracheal cells; at _t′_ are fragments of the moulted tænidia. The new tænidia are in process of development at _t_; at base they are seen to be granulated nuclei, with often a distinct nucleolus, each sending a long, slender, transparent, pointed process along the inside of the trachea. These unite to form the chitinous bands or spiral threads.

=Internal hair-like bodies.=—In the large tracheæ of Lampyris very fine chitinous bristles project free into the cavity of the tube (Gerstaecher), while according to Leydig there are similar chitinous points in the tracheæ of the Carabid beetle Procrustes. Dugardin had previously (1849) called attention to such hairs, giving a list of the insects in which he observed them. Emery figures a section of the tracheæ of Luciola, “in wendig behaart.”[69] Stokes has described those of _Zaitha fluminea_ (Fig. 414) as “internal chitinous, hair-like bodies arising from the fold of the tænidia and projecting into the lumen of the tubes.” They are hollow, their minute cavity distinctly communicating with that of the tænidium, from which they arise by an enlarged base. They end in an exceedingly fine point which is occasionally bifid or trifid. In Fig. 414, 4, several are shown attached to the wrinkles of the tracheæ near a spiracle, and at 5 is represented a transverse section of a trachea with three hairs projecting into its cavity.[70]

Stokes has also described “certain minute, elliptical bodies in the tænidia, each with an internal, presumably glandular, appendage, to all appearance forming part of the tænidium from which it springs.” These are shown in Fig. 414, at 1, 3, and, more in detail, at 6; those at 7, whose thickness is about 1⁄8000 of an inch, appear as collections of exceedingly minute, rounded apertures in a cushion-like mass. Although not commonly occurring on the tracheal membrane between the tænidia, they may be found there, as at 4.

_f._ The mechanism of respiration and the respiratory movements of insects

By holding a locust in the hand one may observe the ordinary mode of breathing in insects. During this act the portion of the side of the body between the stigmata and the pleurum contracts and expands; the contraction of this region causes the spiracles to open. The general movement is caused by the sternal moving much more decidedly than the tergal portion of the abdomen. When the pleural portion of the abdomen is forced out, the soft pleural membranous region under the fore and hind wings contracts, as does the tympanum, or ear, and the membranous portions at the base of the hind legs. When the tergum or dorsal portion of the abdomen falls, and the pleurum contracts, the spiracles open; their opening is nearly but not always exactly coördinated with the contractions of the pleurum, but as a rule they are. There were 65 contractions in a minute in a locust which had been held between the fingers about ten minutes. It was noticed that when the abdomen expanded, the air-sacs in the first abdominal ring contracted.

For expanding the abdomen no special muscles are required, since it expands by the elasticity of the parts. For contracting its walls there are two sets of muscles, viz., special vertical expiratory muscles serving to compress or flatten the abdomen (Figs. 415–418), and other muscles which draw together or telescope the segments.

It was formerly supposed that when the abdomen contracted the air was expelled from the body and the tracheæ emptied; that, when the abdomen again expanded by its own elasticity, the air-tubes were refilled, and that no other mechanism was needed. But Landois insisted that this was not enough; as Miall and Denny state: “Air must be forced into the furthest recesses of the tracheal system, where the exchange of oxygen and carbonic acid is effected more readily than in tubes lined by a dense intima. But in these fine and intricate passages the resistance to the passage of air is considerable, and the renewal of the air could, to all appearance, hardly be effected at all if the inlets remained open. Landois accordingly searched for some means of closing the outlets, and found an elastic ring or spiral, which surrounds the tracheal tube within the spiracle.” By means of the occlusor muscle this ring compresses the tube, “like a spring clip upon a flexible gas-pipe.” “When the muscle contracts, the passage is closed, and the abdominal muscles can then, it is supposed, bring any needful pressure to bear upon the tracheal tubes, much in the same way as with ourselves, when we close the mouth and nostrils, and then, by forcible contraction of the diaphragm and abdominal walls, distend the cheeks or pharynx.”

Thus an important point in the respiration of tracheate animals, whether insects, myriopods, or arachnids, is, as Landois claimed, the closure of the spiracles, in order that pressure may be brought upon the air in the tubes, so that it may pass onward into the finest terminations.

The injection of air by muscular pressure into a system of very fine tubes may, as Miall and Denny remark, appear extremely difficult or even impossible. Graham (Researches, p. 44) applies the law of diffusion of gases to explain the respiration of insects, but until physical experiments have been made, we may, with Miall and Denny, “be satisfied that an appreciable quantity of air may be made by muscular pressure to flow along even the finer air-passages of an insect.”

As to the respiratory movements of insects, Plateau is the principal authority, and the following account of the process is taken from his elaborate memoir, and from the statements afterwards contributed by him to Miall and Denny’s “The Cockroach.”

Although many observers have superficially described the respiratory movements of various insects, Rathke was the first one to state precise views as to the mechanism of respiration. His posthumous work, treating of the respiratory movements of the movable chitinous plates of the abdomen, and of the respiratory muscles characteristic of all the principal groups, filled an important blank in our knowledge. But, notwithstanding the skill displayed in this research, many questions still remain unanswered which require more exact methods than mere observations with the naked eye or the simple lens.

Plateau, who was followed a year later by Langendorff, conceived the idea of studying, by such graphic methods as are now familiar, the respiratory movements of perfect insects.

“He has made use of two modes of investigation. The first, or graphic method, in the strict sense of the term, consisted in recording, upon a revolving cylinder of smoked paper, the respiratory movements, transmitted by means of very light levers of Bristol board attached to any part of the insect’s exoskeleton. Unfortunately, this plan is only applicable to insects of more than average size. A second method, that of projection, consisted in introducing the insect, carried upon a small support, into a large magic lantern fitted with a good petroleum lamp. When the amplification does not exceed 12 diameters, a sharp profile may be obtained, upon which the actual displacements may be measured, true to the fraction of a millimetre. Placing a sheet of white paper upon the lantern screen, the outlines of the profile are carefully traced in pencil so as to give two superposed figures, representing the phases of inspiration and expiration respectively. By altering the position of the insect so as to obtain profiles of transverse sections, or of the different parts of the body, and, further, by gluing very small paper slips to parts whose movements are hard to observe, the successive positions of the slips being then drawn, complete information is at last obtained of every detail of the respiratory movements; nothing is lost.”

“This method, similar to that employed by the English physiologist, Hutchinson,[71] is valuable, because it enables us, with a little practice, to investigate readily the respiratory movements of very small arthropods, such as flies or lady-birds. It has this advantage over all others, that it leaves no room for errors of interpretation.”

“Not satisfied with mere observation by such means as these, of the respiratory movements of insects, the writer has also studied the muscles concerned, and, in common with other physiologists (Faivre, Barlow, Luchsinger, Dönhoff, and Langendorff), has examined the action of the various nervous centres upon the respiratory organs. The result at which he has arrived may be summarized as follows:—

“1. There is no close relation between the character of the respiratory movements of an insect and its systematic position. Respiratory movements are similar only when the arrangement of the abdominal segments, and especially when the disposition of the attached muscles, are almost identical. Thus, for example, the respiratory movements of the cockroach are different from those of other Orthoptera, resembling those of the heteropterous Hemiptera. Those of the Trichoptera are like those of the aculeate Hymenoptera, while the Locustidæ ally themselves in respect to these movements with the Neuroptera and Lepidoptera.

“2. The respiratory movements of insects, when at rest, are localized in the abdomen. As graphically stated by Graber, in insects the chest is placed at the hinder end of the body. If thoracic respiratory movements exist, they do not depend on the action of special muscles.

“3. In most cases the thoracic segments do not share in the respiratory movements of an insect at rest. The respiratory displacements of the posterior segments of the thorax are, however, less rare than Rathke believed. Plateau has observed them in certain Coleoptera (Staphylinus, Chlorophanus, Corymbites), and they are more feebly manifested in Hydrophilus, Carabus, and Tenebrio. Among the singular exceptions to this rule is the cockroach (_Periplaneta orientalis_), in which the terga of the meso- and metathoracic segments perform movements exactly opposite in direction to those of the abdomen (Fig. 419).

“4. Leaving out of account all details and all exceptions, the respiratory movements of insects may be said to consist of the alternate contraction and recovery of the figure of the abdomen in two dimensions, viz. vertical and transverse. During expiration both diameters are reduced, while during inspiration they revert to their previous amounts. The transverse expiratory contraction is often slight, and may be imperceptible. On the other hand, the vertical expiratory contraction is never absent, and usually marked. In the cockroach (_P. orientalis_) it amounts to one-eighth of the depth of the abdomen (between segments 2 and 3); in _Eristalis tenax_ to one-ninth (at the 2d segment).

“5. Three principal types of respiratory mechanism occur in insects, and these admit of further subdivision:

“_a._ Sterna usually short and very convex, yielding but little. Terga mobile, rising and sinking appreciably. To this class belong all Coleoptera, heteropterous Hemiptera, and Blattina (Fig. 420).

“In the cockroach (Periplaneta), the sterna are slightly raised during expiration (Fig. 421).

“_b._ Terga well developed, overlapping the sterna on the sides of the body, and usually concealing the pleural membrane, which forms a sunken fold. The terga and sterna approach and recede alternately, the sterna being almost always the more mobile. To this type belong Odonata, Diptera, aculeate Hymenoptera, and acrydian Orthoptera (Fig. 422).

“_c._ The pleural membrane, connecting the terga with the sterna, is well developed and exposed on the sides of the body. The terga and sterna approach and recede alternately, while the pleural zone simultaneously becomes depressed, or returns to its original figure. To this type, Plateau assigns the Locustidæ, Lepidoptera, and the true Neuroptera (excluding Trichoptera) (Fig. 423).

“6. Contrary to the opinion once general, changes in length of the abdomen, involving protrusion of the segments and subsequent retraction, are rare in the normal respiration of insects. Such longitudinal movements extend throughout one entire group only, viz. the aculeate Hymenoptera. Isolated examples occur, however, in other zoölogical groups.

“7. Among insects, such as large beetles, Locustidæ, dragon-flies, etc., sufficiently powerful to give good graphic tracings, it can be shown that the inspiratory movement is slower than the expiratory, and that the latter is often sudden.

“8. In most insects, contrary to what obtains in mammals, only the expiratory movement is active; inspiration is passive, and effected by the elasticity of the body-wall.

“9. Most insects possess expiratory muscles only. Certain Diptera (_Calliphora vomitoria_ and _Eristalis tenax_) afford the simplest arrangement of the expiratory muscles. In these types, they form a muscular sheet of vertical fibres, connecting the terga with the sterna, and underlying the soft, elastic membrane which unites the hard parts of the somites. One of the most frequent complications arises by the differentiations of this sheet of vertical fibres into distinct muscles, repeated in every segment, and becoming more and more separated as the sterna increase in length. Special inspiratory muscles occur in Hymenoptera, Acridiidæ, and Trichoptera.

“10. The abdominal, respiratory movements of insects are wholly reflex. Like other physiologists who have examined this side of the question, Plateau finds that the respiratory movements persist in a decapitated insect, as also after destruction of the cerebral ganglia or œsophageal connectives; further, that in insects whose nervous system is not highly concentrated (_e.g._ Acridiidæ and dragon-flies), the respiratory movements persist in the completely detached abdomen; while all external influences which promote an increased respiratory activity in the uninjured animal, have precisely the same action upon insects in which the anterior, nervous centres have been removed, upon the detached abdomen, and even upon isolated sections of the abdomen.

“The view formerly advocated by Faivre, that the metathoracic ganglia play the part of special, respiratory centres, must be entirely abandoned. All carefully performed experiments on the nervous system of Arthropoda have shown that each ganglion of the ventral chain is a motor centre, and, in insects, a respiratory centre, for the somite to which it belongs. This is what Barlow calls the ‘self-sufficiency’ of the ganglia.” (Miall and Denny.)

Plateau has made similar observations upon the respiration of spiders and scorpions; but, to his great surprise, he was unable, either by direct observation, or by the graphic method, or by projection, to discover the slightest respiratory movement of the exterior of the body. This can only be explained by supposing that inspiration and expiration in pulmonate Arachnida are “intrapulmonary,” and affect only the proper, respiratory organs. The fact is less surprising because of the wide zoölogical separation between Arachnida and insects.

_g._ The air-sacs

In flying insects the tracheæ are in certain parts of the body enlarged into sacs of various sizes. These air-sacs were first observed by Swammerdam in a beetle (Geotrupes) and afterwards by Sir John Hunter in the bee, Sprengel subsequently discovering them in other insects. Those of the cockroach were described and illustrated in a very elaborate and detailed way by Straus-Dürckheim (Figs. 424 and 425). These vesicles are without tænidia. In the locust (_M. femur-rubrum_) there is a pair of very large vesicles in the prothorax (Fig. 396). The five pairs of large abdominal air-sacs arise, independently of the main tracheæ, directly from branches originating from the spiracles. All these large sacs are superficial, lying directly beneath the hypodermis, while the smaller ones are buried among the muscles. We have detected 53 of these vesicles in the head.

In the honey-bee (Fig. 426) and humble bee (Fig. 427) as well as the flies there are two enormous air-sacs at the base of the abdomen. In larval and wingless insects these sacs are entirely absent.

=The use of the air-sacs.=—It was supposed by Hunter as well as by Newport, and the view has been generally held, that the use of these sacs is to lighten the weight, _i.e._ lessen the specific gravity of the body during flight. It has, however, been suggested to us by A. A. Packard that this view from the standpoint of physics is incorrect. It is evident that the wings have to support just as much weight when the insect is flying, whether the tracheæ and vesicles are filled with air or not, the body of the insect during flight not being lightened by the air in the sacs. The use of these numerous sacs, some of them very spacious, is to afford a greater supply of air or oxygen than that contained in the air-tubes alone, and thus to afford a greater breathing capacity. The sacs are largest in dragon-flies, moths, flies, and bees, which are swift of flight. When we compare the active movements of these insects on the wing with those of a caterpillar or maggot, it will be seen that the far greater muscular exertions of the volant insect create a demand for a sudden and abundant supply of air to correspond to the increased rapidity of respiration; and the enlargements of the air-tubes, rapidly filled with air at each inspiration, render it possible to supply the demand.

The case is thus seen to be very different from that of those fishes which, having a swimming-bladder, can in the water change the specific gravity of their bodies. The case of insects is almost exactly paralleled by that of birds, where, as stated by Wiedersheim, the air-sacs appear to form integral parts of the respiratory apparatus: “a greater amount of air can by their means pass in and out during inspiration and expiration, especially through the larger bronchi, and consequently there is less necessity for the expansion of the lung parenchyma.” In other words, the supply of air in these sacs, as in insects, increases the breathing capacity of the bird during flight. Wiedersheim’s retention of the old idea that the specific gravity of the body is lessened (p. 262) seems, however, to be incorrect, as the weight of the bird’s body is not diminished by the air contained in the sacs.

_h._ The closed or partly closed tracheal system

There are two chief morphological tracheal systems: 1. The open or normal and primitive (_holopneustic_) type, and 2. The closed, or secondary and adaptive, _i.e._ _apneustic_, type. The open system is characterized by the presence of the stigmata. Through them the air directly enters into the tracheal tubes, whose delicate walls allow the exchange of gases in the blood. This type occurs in all sexually mature individuals, and also in the greater number of larvæ.

The closed or apneustic tracheal system is distinguished either by the want of stigmata, or, if present, they are not open, and do not function, so that the tracheæ cannot communicate with the air. In such cases the direct oxygenation of the blood is effected through the delicate integument, especially over the surface of the body in general, or in certain specialized places where the gill-like expansions of the skin are rich in tracheæ; such outgrowths, generally tubular or leaf-like, are called by Palmén _tracheal gills_.

This closed form of the tracheal system only occurs in the larval stage of aquatic or parasitic insects, as in the Plectoptera (Ephemeridæ), Perlidæ, Odonata, and Trichoptera, besides single genera of other orders, _i.e._ among Coleoptera, Gyrinus, Pelobius, Cnemidotus, and the young larva of Elmis; in the aquatic caterpillar of Paraponyx; in certain Diptera (Corethra, Chironomus, etc.), and some of the parasitic Hymenoptera (Microgaster).

Palmén has discovered that in the nymphs of Ephemeridæ, Perlidæ, Odonata, and the larvæ of most Trichoptera the tracheal branch (stigmatal branch) sent from the longitudinal trachea to where the thoracic stigmata would be situated if present, or where their vestiges only exist, are aborted, becoming simple solid cords not filled with air (Fig. 436, _vf_, and 447, _f_, funiculus or stigmatic cord). In the imago, however, they resume their function, connecting with the open functional stigmata. In Corethra, in its earliest stages, the entire tracheal system is, like the stigmatic branch, a system of solid cords and empty of air. (Palmén.)

Embryology shows that these stigmatal branches are well developed, and are formed at the same time as the stigmata. It was also shown by Dewitz, in a posthumous paper (1890), that in the young larval stage of the Odonata and Ephemeridæ the tracheal system is at first an open one, and in some of the families (Libellulidæ, Agrionidæ, and Ephemeridæ) thoracic stigmata are seen at a very early stage. From numerous experiments Dewitz concludes that in the young stages of Odonata and Ephemeridæ there is an open tracheal system; certainly in very young nymphs the thoracic spiracles allow the air to pass out. Fully grown nymphs of Æschnidæ, Libellulidæ, and Agrionidæ are capable not only of forcing the air out, but also, like the perfect insect, of inhaling it. Moreover, he proved that the gills of Ephemeridæ and Agrionidæ are not indispensable for the maintenance of life, as the insects can live without them, breathing either through the skin or by the rectum, or in both ways. It would seem that while in freshly hatched or very young larvæ of aquatic insects of different orders the skin is so delicate as to allow of dermal respiration, in after life, when the skin becomes thicker and denser, these expansions (gills), provided with a very thin and delicate skin, of a necessity grow out from the walls of the body.

It thus appears that the closure and total or partial abolition of the stigmata are in adaptation to aquatic life, and that such insects have descended from terrestrial air-breathing winged forms. This is an important argument against the view that the wings are modified tracheal gills.

In this connection may be noticed the closure of the 2d and 3d thoracic stigmata in holopneustic insects. We have found on laying open the body of a Sphinx larva that a large number of tracheal branches are seen to arise from the prothoracic and from the first pair of abdominal stigmata. Now between these points there are no spiracles or any external signs of them, there being in Lepidoptera no mesothoracic or metathoracic spiracles. Yet the main lateral trachea between the prothoracic and first abdominal segments deviates from its course and bends down to send off a small shrivelled stigmatal branch or cord to a place where, did a spiracle exist, we should look for it. In the larva of _Platysamia cecropia_, a similar vestigial stigmata branch is present.

In the larva of Corydalus, also, a trachea as large as the main longitudinal one takes its origin and passes directly under the main trachea. Now both tracheæ send a stigmatal branch opposite to where the mesothoracic stigma should be, if present, _i.e._ on the hind edge of the segment.

Verson, moreover, has found in the freshly hatched silkworm vestiges of meso- and metathoracic stigmata, each consisting of a circle of high hypodermal cells radially arranged around a common centre. The stigmatal branch is long, but shrivelled; its peritoneum is widened out into several berry-like saccules filled with cell-elements. In profile these rudimentary stigmata appear as a series of high hypodermal cells, which form the basis of a short blind tube.

After the second moult there begins a peculiar transformation of the rudimentary stigmata. The stigmatal branch connected with them sends off at various points thick tufts of capillary tracheæ which press against the base of the blind tube. Gradually lengthening, they form a fold which continues to increase in length. The numerous tufts of tracheal capillaries extend beyond the inner surface of the two layers of which the developing wing consists, the berry-like saccules are drawn into the wing and converted into more or less thick tubes, which finally form the “veins.” It is clear, therefore, says Verson, as Landois claimed, that the wings of Lepidoptera must be regarded as in the fullest sense organs of respiration. (Zool. Anz., 1890, p. 116.)

The number of pairs of stigmata varies, especially in maggots or larval Diptera, in adaptation to their varied modes of life. The larvæ of most flies (Muscidæ) have a pair of peculiarly shaped processes on the prothoracic segment bearing spiracular openings, and two anal spiracles, while in _Ctenophora atrata_ L. only the anal pair are present. In the rat-tailed maggots (Eristalis) the long caudal process ends in two stigmata forming a respiratory tube, which can be thrust out of the water for the reception of air. In the larval mosquito (Fig. 433) and its ally, Mochlonyx, a short thick dorsal tube arises from the penultimate segment of the body, in which the two main tracheæ end, opening outward by a single spiracular aperture. Other dipterous larvæ (Simulium, Tanypus, and Ceratopogon) possess no spiracles, the tracheal system being a closed one.

The larvæ of most water beetles (Dyticidæ, Hydrophilidæ) possess but two spiracles, which, as in maggots, are situated at the end of the body. The aquatic larva of Amphizoa, according to Hubbard, breathes much as in the Dyticidæ, by means of two large valvular spiracles placed close together at the end of the body; “closed or rudimentary stigmata also occur on the mesothorax and on abdominal segments one to seven inclusive.”

Hubbard adds: “The larva of Pelobius is wholly aquatic and breathes by branchiæ, but the obsolete stigmata are indicated precisely as in Amphizoa, with the exception of the last pair, which in Amphizoa are open spiracles, but in Pelobius are suppressed; the terminal eight segments being prolonged in a swimming stylet.”

From a review of the distribution of spiracles, and their atrophy, partial or total, it will be seen that there are intermediate stages between the open (holopneustic) and closed (apneustic) systems. These, following Schiner, Brauer, and Palmén, may be defined thus:

1. _Metapneustic type._—The larvæ possess only a single pair of open stigmata situated at the end of the body. (The dipterous Eristalis, Tipula, Culex, Ptychoptera, Bittacomorpha (Plate I.) with certain Tachinidæ, and in Coleoptera, the larvæ of Dyticus, and allies of Hydrophilus and Cyphon.)

2. _Propneustic type._—The pupæ of Corethra, Culex, etc., in which only the most anterior pair of spiracles are open.

3. _Amphipneustic type._—Larvæ with a pair of open spiracles situated at each end of the body, the intermediate spiracles being closed. (Most dipterous larvæ, Musca, after the first moult, Œstridæ, Asilidæ, and Syrphus.)

4. _Peripneustic type_; with prothoracic and abdominal spiracles, the mesothoracic pair atrophied or closed. (The larvæ of Neuroptera, Mecoptera, Trichoptera, Lepidoptera, of most Coleoptera,[72] of most Diptera, and of most of the Hymenoptera.[73])

These differences in the number of functional spiracles are in direct relation with the surroundings of the insects, the physical conditions of existence evidently determining the position of the active functional open spiracles and the closure of those useless to the organism.

_i._ The rectal tracheal gills, and rectal respiration of larval Odonata and other insects

The remarkable mode of respiration by tracheal gills situated within the intestine of the nymphs of dragon-flies was first described by Swammerdam and afterwards by Réaumur. The most complete and best illustrated modern account is that of Oustalet. In these insects the large rectum is lined with six double longitudinal ridges, in Æschna bearing numerous delicate tubes or papillæ, each of which contains very numerous (by estimate 24,000) tracheal branches (Fig. 431); while in Libellula the gills are lamellate (Fig. 432). The tracheæ arise both from the main dorsal and visceral longitudinal trunks, which give rise to secondary branches passing into the walls of the rectum and sending into the branchial papillæ fine twigs, which, extending to the distal end of the papilla or lamella, recurve and anastomose with the efferent twigs.

The anal opening is externally protected by the suranal and lateral triangular chitinous plates, three to five in all. When open, the water passes into the rectum and bathes the rectal gills, where it may be forcibly expelled as if shot out from a syringe, thus propelling the insect forward. In Libellula the anus affords direct access to the intestinal cavity, but in Æschna Oustalet describes “a sort of vestibule separated from the rectum by a circular valvule.” He also states that the inspiration and the repulsion of water is produced at irregular intervals, and rather by the movements of the dorsal and sternal arches of the abdomen than by the contractions of the rectum, since the walls of this organ are less muscular than is supposed.

The nymph of Calopteryx (and probably of all the group Calopteryginæ) possesses rectal gills besides external caudal tracheal gills. There are three double rectal longitudinal folds or ridges, interpenetrated by tracheal twigs. (Dufour, denied by Poletaiew, but confirmed by Hagen.)

Dewitz claims that the caudal gills of the Agrionidæ are not their sole means of respiration, since he cut off the caudal tracheal gills of an Agrionid nymph, which continued to live for a week. Hence he thinks that there may be a rectal respiration, since under the microscope he saw a stream of water pass in and out of the end of the intestine.

Dewitz’ experiments prove that in young Ephemerids there may be besides branchial, both rectal and skin respiration. He saw under the microscope the anus for a while opened and then closed, causing the rectum to move; powdered carmine mixed with water was drawn into and then expelled from the rectum. There was, however, no enlargement and contraction of the abdomen as in the rectal respiration of Æschna. (Zool. Anz. 1890, p. 500.)

Eaton states that there is a rectal respiration in the nymphs of may-flies, and Palmén observed in young larvæ of Bætis and Cloëon that the rectum took in “by gulps” water colored by carmine and expelled the whole of it at once, in order to fill it again in the same way. “This rectal respiration therefore corresponds to that of Libellulid larvæ.”

Besides breathing by spiracles, by tracheal gills, as well as through the integument, the larva of Culex has been observed by Raschke to have a rectal respiration. At the anterior end of the rectum arises a countless number of fine tracheæ, which pass through the walls and, subdividing, end in numberless very fine twigs in the papilla-like folds situated within the rectum. The supply of tracheal twigs is greatest where the papillæ are largest. (Figs. 433, 434.)

_j._ Tracheal gills of the larvæ of insects

In many aquatic insects respiration is carried on by tracheal gills. These are delicate, hollow, leaf-like or tubular outgrowths of the integument usually attached to the sides or end of the hind-body, and containing a trachea which usually sends off numerous minute branches, so that the exchange of gases readily takes place in them.

Palmén has shown that these tracheal gills, as he calls them, are not developed on the same segments as the stigmata, and that the two structures have no genetic connection with each other. It is evident that these gills are secondary, adaptive organs.

In some cases (see p. 475) the tracheæ are wanting, but as such gills are filled with blood, the air contained in the water must pass in through their delicate walls.

In the Plectoptera (Ephemeridæ) the tracheal gills are either foliaceous or filamentous; when foliaceous they form simple or double leaves, with or without branches, or with a fringe of tubules, or under the leaf-like cover-bearing tufts of filaments. They are situated on the (usually) basal seven abdominal segments, at their hinder edge (Figs. 435, 436). In Oligoneuria and Jolia a pair occurs on the under side of the head, attached to the maxillæ, while in Jolia there is a pair on the under side of the first thoracic segment at the insertion of each of the legs. In certain genera (Heptagenia, Oligoneuria, and Jolia), they are in the form of a flat cover, under which lies a tuft of respiratory tubes, or (Ephemerella) a small bifid cluster of very delicate leaves (Fig. 437, _A_). In Cœnis and Tricorythus the tracheal gills of the second pair are modified to form plates covering all the succeeding pairs, those of the first pair being nearly atrophied and well-nigh functionless. (Fig. 437, _B_.)

Finally, in the highly modified forms Bætisca and Prosopistoma the tracheal gills are entirely concealed and protected by mesothoracic projections so as to form a true respiratory chamber, to which the water has access either by an opening behind, as in Bætisca, or by three openings, two ventral and one dorsal (Fig. 441), as in Prosopistoma.

The slender cylindrical tracheal gills of Heptagenia in the third or fourth nymphal stage are 2–jointed, and the first abdominal pair in Cænis are said by Palmén to be finger-shaped and 2–jointed. In _Polymitarcys virgo_ the gills do not appear until the eighth or tenth day after hatching.

Dewitz found that young nymphs of Ephemerids will well endure the amputation of their gills, while fully grown ones die. Amputation of the lateral gills hastens ecdysis. After the change of skin, the gills are smaller than before, and at first contain no tracheæ, but in a few weeks they develop as completely as in normal individuals. The caudal gills were also renewed.

In the nymphs of Perlidæ the tracheal gills are usually present, and are either foliaceous (Nemoura) or more commonly filamentous in shape (Fig. 442). They are situated either on the prosternum (Nemoura and Pteronarcys), or on each side of the thorax, or on the sides of the abdomen, or are restricted to a tuft on each side of the anus at the base of the caudal stylets (Pteronarcys and Perla). Unlike the Ephemeridæ the gills persist in certain genera throughout life.

The larvæ of the aquatic Neuroptera, Sisyra, Sialis, and Corydalus possess lateral pointed bristle-like tracheal gills, which in Sisyra are 2–jointed; those of Sialis are, in the living larva, curved upwards and backwards (Fig. 444). Corydalus is also provided with a ventral tuft of delicate filamentous gills, which, however, according to Riley, do not appear until after the first moult.

While the nymphs of Agrionidæ (which have rectal gills) respire chiefly by the large caudal foliaceous gills (Fig. 445), there are, according to Hagen, two genera of the Calopteryginæ (Euphæa, Fig. 445, and Anisopleura) whose nymphs possess seven pairs of external lateral tracheal gills, in shape like those of Sialis, besides three caudal and three rectal tracheal gills.[74]

Hagen has also detected in the under side of the 5th abdominal segment of Epitheca and Libellula a pair of sacs of the shape of a Phrygian bonnet, each of which contains a smaller sac lined with epithelium,—as in Æschna they occur in the 5th and 6th, and in Gomphus in the 4th, 5th, and 6th segments. This serial arrangement appears to confirm Hagen’s suggestion that they are survivals of abdominal gills, which in Euphæa are completely evaginated.

In the Trichoptera, all of which, except Enoicyla, are apneustic, and most of which have tracheal gills, the latter are filamentous, and arise either from the dorsal and ventral sides of the abdominal segment, or they grow out from the sides; while in certain genera (Neuronia, Phryganea, etc.) the gills are represented by conical hooks on the sides of the 1st abdominal segment, which are evidently respiratory, as they contain numerous tracheæ. The tracheal gills are either single or more rarely form tufts (Figs. 447, 448).

In Hydropsyche (Fig. 448) the tracheal gills persist throughout life, while in other genera they only last through the pupal stage. When first hatched, the larva of Phryganea lacks gills. The larvæ of most of the Hydropsychidæ, Rhyacophilidæ, and Hydroptilidæ have no gills, though they appear well developed in the pupal stage. (Klapálek.)

The only lepidopterous larva known to be provided with tracheal gills is that of the pyralid genus Paraponyx. Its thread-like gills, arranged in tufts of three or four, arise from a common tubercle situated on the sides of nearly all the segments. Wood-Mason describes the East Indian _P. oryzalis_ as “covered with a perfect forest of soft and delicate white filaments,” arranged in tufts disposed in four longitudinal rows. “The stigmata of the 2d, 3d, and 4th abdominal somites only are clearly discernible.” The caterpillar crawls “free and uncovered” over the submerged leaves of the rice plant “in the very midst of the water.” In a Brazilian species of Paraponyx described as _Cataclysta pyropalis_, by W. Müller, the tufts are reduced to simple unbranched filaments, and the case is more complex than in the European species (Fig. 449).

Of coleopterous larvæ breathing by tracheal gills there are but few. The larva of Gyrinus (Fig. 454) respires by 10 pairs of slender, hairy abdominal gills similar to those of Corydalus, and the stigmata are entirely wanting. Somewhat similar are the tracheal gills of _Hydrocharis caraboides_. Hydrobius has shorter setose gills, our American species having seven pairs of short setose gills. It has two spiracles at the end of the body, through which the air is taken by thrusting the body out of the water. The larvæ of two other aquatic coleopterous genera, Pelobius and Cnemidotus, also have gills; those of the former situated at the base of the coxæ, and brush-like, but containing no tracheæ, though filled with blood, while those of Cnemidotus are very long, bristle-like, jointed, and arising from the dorsal side of the thoracic and abdominal segments. The stigmata are wanting. (Schiödte.)

The larva of the dipterous genus Tanypus respires by two caudal papilliform processes, in each of which a trachea ramifies.

Certain larvæ with both stigmata and tracheal gills are enabled either to live in or out of water or on the surface, as in the case of certain beetles (Cyphonidæ, Elmidæ, Hydrophilidæ, Fig. 452), or the larval mosquito and Psychodes (Fig. 455); also the nymphs of dragon-flies.

The larvæ of the Cyphonidæ (Helodes, Cyphon, Hydrocyphon) possess but a single pair of stigmata, situated in the penultimate abdominal segment, while at the end of the abdomen are delicate tracheal gills. The two main tracheal trunks are much swollen. When on the surface of the water the larva breathes through the stigmata situated near the end of the abdomen; when floating in the water, the larva, like that of Gyrinus, carries along at the end of its body a bubble of air. The gills are only of use, as Rolph thinks, when the insect is compelled to remain a long time under water.

The larva of our native _Prionocyphon discoideus_ (Say) is described by Walsh as “vibrating vigorously up and down a pencil of hairs proceeding from a horizontal slit in the tail”; this pencil is composed “of three pairs of filaments, each beautifully bipectinate. I presume it is used to extract air from the water.” When the larva is at the surface the pencil of hairs touches the surface of the water, and occasionally a bubble of air is discharged from the tail. “The general habit is to crawl on decayed wood beneath the surface, occasionally swimming to the surface, probably for a fresh supply of air.” (Proc. Ent. Soc. Phil., i, p. 117.)

The larvæ of the small water beetles of the family Elmidæ (Elmis, Potamophilus, Macronychus, and Psephenus) have similar habits. That of Elmis has ten dorsally situated pairs of spiracles, and on the end of the body bushy gills which are protruded at pleasure. The young larva is without spiracles, its tracheal system being closed. Macronychus and Potamophilus have similar habits. In the larva of the latter genus, which has nine pairs of spiracles, there are at the end of the body on each side three tufts of thread-like gills which are connected with the two main horizontal tracheæ, while the branches of the abdominal tracheæ are dilated into numerous (64) bladder-like sacs. The larva usually breathes through the caudal gills. When the water is low or dried up, the air is inhaled directly through the spiracles. (Kolbe.)

The larva of _Psephenus lecontei_, by its broad hemispherical body, is adapted to adhere to the smooth surface of rounded stones, in which situation we have found it. Although it is said by Rolph to have two pairs of spiracles, one pair on the mesothoracic and the other on the 1st abdominal segment, it probably rarely rises to the surface to breathe the air direct.

It possesses five pairs of gills on the under side of the 2d to the 6th abdominal segments. Each gill has finger-shaped processes on its hinder edge, which are “from their constant motion evidently connected with respiration.” Tracheæ may be seen, according to H. J. Clark, entering the gills, and “the circulation of water among the branchiæ is kept up by the flapping of the tail-pieces.” The larva of _Helichus fastigiatus_ is said by Leconte to be “very nearly allied, while the remotely allied _Stenelmis crenatus_ has no external branchiæ.”[75]

The larva of the mosquito also has two modes of respiration, breathing either at the surface of the water through the two spiracles situated on the projection (siphon) at the hinder end of the body which is thrust out into the air; or when at the bottom respiring by tracheal gills. The pupa also has a double mode of respiration, either taking in air at the surface by the two thoracic horns with stigmatic openings, or when submerged using its tracheal gills.

Besides its long caudal tracheal air-tubes, the larval Eristalis is said by Chun to thrust out from the anus a number (20) of short tracheal filaments which float about in the water and serve to absorb the air.

An aquatic Brazilian larva of the family Psychodidæ has been found by Fritz Müller to take down under the water a large bubble of air (Fig. 455, _C_), the main tracheal trunk ending each in an opening at the end of the body (_A_, _B_); besides this, while at the bottom it breathes by three digitiform tracheal gills; another species having two pairs (_C_, _a_).

The remarkable larvæ of the Blepharoceridæ (represented in the United States by _Blepharocera capitata_), which live permanently in swift streams, attached by median suckers to stones, are apneustic, and breathe solely by leaf-like tracheal gills (Fig. 456, _br_) attached to the under side of the second to sixth abdominal segments. Those of the European Liponeura are said by Wierzejski to be branched, tree-like. Also immediately in front of the anus and behind the last sucker are four membranous sacs provided with tracheæ, but which are not capable of being withdrawn. These are said by Müller to be the same as what Dewitz states to serve as gills, and by Wierzejski they are homologized with the four anal gills of Chironomus.

The double mode of respiration in the larva of the horse bot-fly has been described by Scheiber. On the hinder end of the body are the stigmatic plates, which contain two lateral gill-plates and the middle stigmatal leaf. Besides this there is a pair of slightly developed prothoracic spiracles. The embryo and also freshly hatched larva of _Gastrophilus equi_ do not possess these gill-plates, but on the end of the body are, according to Joli, two long thread-like gills. The freshly hatched larva of the allied _Cephenomyia rufibarbis_ bears two caudal projections. (Kolbe.) As in shrimps and other Crustacea the gills are kept in constant motion, the water being driven over them by the rapid movements of the telson, so in the larval may-flies, and in the case-worm (Macronema), the gills move more or less rapidly. In case-worms as well as larval Perlidæ, Sialidæ, Paraponyx, and Hydrophilidæ the abdominal region is constantly moved to promote respiration. (Kolbe.)

=Blood-gills.=—Fritz Müller describes in trichopterous larvæ certain delicate anal tubular processes into which the blood flows, and which do not as a rule contain tracheæ, though occasionally very fine tracheal branches. Müller compares them with the gills of crabs and of shrimps. They are eversible finger-like tubules. They are used when the tracheal gills are temporarily not available. Their number varies even in the same genus. There are six in certain Rhyacophilidæ; five in different Hydropsychidæ; in Macronema there are four, and they are green when filled with the green blood of that insect, the tracheal gills being whitish. In the freshly hatched larva, while the tracheal gills are present, no anal blood-gills are visible. Similar blood-gills also occur in the pupæ of certain caddis-flies. (Pictet.)

Similar anal gills filled with blood occur in the larvæ of the fireflies (Lampyris, etc.), and perhaps, Kolbe thinks, serve for respiration, though other authors believe them to be adhesive organs.

The larva of Pelobius has true blood-gills. (Schiödte. See p. 461.)

The eversible ventral segmental sacs of Scolopendrella, Campodea, and Machilis, as well as the ventral tube (collophore) of Podura, Smynthurus, etc., may, as Oudemans and Haase have suggested, serve a respiratory purpose, though they lack tracheæ, and differ from blood-gills in containing no gases; yet the blood is forced into them, causing their eversion. Oudemans observed that Machilis everted its sacs when the vessel in which it was put was filled with warm, damp air. The sacs are only thrust out when the creature is completely at rest.

Structures referable to blood-gills also occur temporarily in the embryo of Orthoptera; Rathke observed them in the mole-cricket; Ayres observed them in _Œcanthus niveus_, where they form two stalked broad oval appendages on the first abdominal appendages, which he regarded as gills. Patten observed them in _Phyllodromia germanica_, as pear-shaped structures occurring in the same situation, but regarded them as sense-organs, as did Cholodkowsky. Graber found these structures in the embryo of the May-beetle, which looked like the other embryonic limbs, but survived after the disappearance of the latter, being longer and broader and unjointed. These disappeared shortly before birth. In Hydrophilus they remain, Graber states, after birth. Nussbaum has seen them in Meloë.

Finally, Wheeler has discussed at length these embryonic organs, which he regards as glandular structures, and calls _pleuropodia_, their primitive function having been that of limbs. He has detected them in the embryo of _Periplaneta orientalis_, _Mantis carolina_, _Xiphidium ensiferum_ (Fig. 387); also in the Hemiptera (_Cicada septemdecim_, _Zaitha fluminea_), and in _Sialis infumata_. He discards the view that they were once gills or sense-organs, and concludes that they were glands. But, as we have suggested, their function once that of gills, and still respiratory in Synaptera, has perhaps become in the winged insects glandular and repugnatorial. Instead, then, of being modified abdominal limbs afterwards serving as glands, as Wheeler claims, we are inclined to believe that they functioned as blood-gills.

_k._ Tracheal gills of adult insects

Tracheal gills are known to be retained by a few insects in the imago stage, the nymphs in all stages breathing by them. The most notable example is the perlid genus Pteronarcys, in which, as Newport states, there are eight sets, comprising 13 pairs of branchial tufts distributed over the under surface of the thoracic and first two abdominal segments.

The first set, consisting of three pairs of tufts, partly encircling the neck like a ruff, arises from the soft membrane connecting the head and prosternum. The thoracic tufts originate between and behind the coxæ, as well as on the front margin of the meso- and metathoracic segments. The number of filaments in each tuft varies from about 20 to 50 or more, the densest tufts being those of the two hinder thoracic segments. Each filament is usually simple, though in a few cases they are branched (Fig. 457, _A_).

The adult Pteronarcys is nocturnal, flying only at dewfall or in the night, and Mr. Barnston observed it when on the wing, “constantly dipping on the surface of the water”; by day it hides “in crevices of rocks which are constantly wetted by the spray of falling water, under stones and in other damp places.” It may thus be compared with the Amphibians, Necturus and Proteus, whose gills are retained in adult life. A similar large Chilian Perlid (_Diamphipnoa lichenalis_ Gerst.) differs in completely lacking the thoracic gills, though there are four pairs on the abdomen, _i.e._ a pair on each of the first four segments. In this form the number of individual filaments in the largest tufts may amount to about 200.

Another Perlid (_Dictyopteryx signata_) is said by Hagen to have two pairs of gill-tufts on the under side of the head; the first pair situated on the base of the submentum, the second on the membrane connecting the head and prosternum.

Kolbe states that in the imagines of _Perla marginata_ and _P. cephalotes_ on the hinder edge of the thoracic stigmata arise three very small chitinous plates, which, on their under side and on the edges are beset with numerous short white filaments. These completely correspond to the filaments of the tuft-like larval gills. Persistent anal gills also occur in the imagines of Perla.

In _Nemoura lateralis_ and _cinerea_ the tracheal gills are differently disposed. On each side of the anterior edge of the prosternum arise delicate tightly twisted filaments, like those of the larva. (Einführung, p. 536.)

Hagen also states that in the dragon-fly, Euphæa, the gills of the nymphs are retained in the imago, and Palmén remarks that in Æschna the rectal gills of the nymph persist in the imago, though not used for respiration.

Palmén gives an instance of a caddis-fly (Hydropsyche, Fig. 448) retaining its gills through the imago stage, but they are unfit for respiration, as they are minute and shrunken.

A walking-stick (_Prisopus flabelliformis_) found in the mountains of Brazil has the remarkable habit, according to Murray, of spending “the whole of the day under water, in a stream or rivulet, fixed firmly to a stone in the rapid part of the stream,” with its head turned up stream; but leaving the water at dark. The under side of the body, including the head, is hollowed so that the creature may adhere, sucker-like, to smooth stones; the claws, claspers, and flaps on the legs aid in retaining its hold, while the outer margin of the legs is dentate and thickly fringed with hair to repel the water.

Another form, closely related to Prisopus, from Borneo (_Cotylosoma dipneusticum_) is said by Wood-Mason to be even more profoundly modified for an aquatic life, since it has not only spiracles, but also, as he claims, tracheal gills. From each side of the body, in fact along the lower margins of the sides of the metathorax, there stand straight out five equal, small, but conspicuous ciliated oval plates, “which, when the insect is submerged and its stigmata are closed, doubtless serve for respiration.” The author did not note the actual presence of tracheæ in these plates.

LITERATURE ON THE ORGANS AND PHYSIOLOGY OF RESPIRATION

_a._ On the tracheal system in general

=Lyonet, P.= Traité anatomique de la chenille qui ronge le bois du saule. (La Haye, 1760; 2d edit., La Haye, 1762, pp. 616, 18 Pls.)

=Treviranus, G. R.= Beiträge zur Anatomie und Physiologie der Tiere und Pflanzen, 1816.

—— Das organische Leben. Bremen, 1831.

=Rengger, J. R.= Physiologische Untersuchungen über die tierische Haushaltung der Insekten. Tübingen, 1817. (Germar’s Mag. f. Ent., 1818, iii, pp. 410–413.)

=Dufour, L.= Recherches anatomiques sur les Carabiques et sur plusieurs autres insectes Coléoptères. Organes de la respiration. (Ann. Sc. nat., viii, 1826, pp. 19–27, 2 Pls.)

—— Études anatomiques et physiologiques sur les insectes Diptères de la famille des Pupipares. Appareil respiratoire. (Ann. Sc. nat. Zool., Sér. 3, iii, 1845, pp. 56–64, 1 Pl.)

—— Description et anatomie d’une larve à branchies externes d’Hydropsche. (Ann. Sc. nat, Zool., Sér 3, 1847, viii, pp. 341–354.)

—— Études anatomiques et physiologiques, et observations sur les larves des Libellules. Appareil respiratoire. (Ann. Sc. nat. Zool., Sér. 3, xvii, 1852, pp. 76–97, 3 Pls.)

—— Recherches anatomiques sur les Hyménoptères de la famille des Urocerates. Appareil respiratoire. (Ann. Sc. nat. Zool., Sér. 4, i, 1854, pp. 203–209, 1 Pl.; see also p. 344.)

=Burmeister, H.= Handbuch der Entomologie, i, 1832, pp. 169–194, 416–436.

=Kirby, W., and W. Spence.= Introduction to entomology, 1833, iv, pp. 35–81.

=Bowerbank, J. S.= Observations on the circulation of blood and the distribution of the tracheæ in the wing of _Chrysopa perla_. (Ent. Mag., iv, 1837, pp. 179–185.)

=Platner, E. A.= Mitteilungen über die Respirationsorgane in der Haut bei der Seidenraupe. (Müller’s Archiv f. Physiol., 1844, pp. 38–49.)

=Filippi, F. de.= Alcuni osservazioni anatomico-fisiologische sugl’ Insetti in generale, ed in particulare sul Bombice del Gelso. (Ann. R. Acad. d’ Agricoltura di Torino, 1850, ii, p. 25, 1 Pl.; Transl. by C. A. Dohrn, Stettin, Ent. Zeit., 1852, xiii, pp. 258–267; xiv, pp. 124–132, 1 Pl.)

=Newport, G.= On the formation and the use of the air-sacs and dilated tracheæ in insects. (Trans. Linn. Soc. London, 1851, xx, pp. 419–423.)

=Lubbock, J.= Distribution of tracheæ in insects. (Trans. Linn. Soc. London, 1860, xxiii, pp. 23–50.)

=Landois, L.= Anatomie des _Phthirius inguinalis_ Leach. (Zeitschr. f. wissens. Zool., xiv, 1864, pp. 1–26, 5 Taf.)

—— Anatomie des _Pediculus vestimenti_ Nitzsch. (Ibid., xv, 1865, pp. 32–55, 3 Taf.)

—— Anatomie des Hundeflohs (_Pulex canis_). (Nova Acta Leop.-Carol. Akad. der Naturf., Dresden, 1866, xxxiii, 1867, pp. 67, 7 Taf.)

—— Anatomie der Bettwanze (_Cimex lectularius_ L.). (Ibid., xviii, 1868, pp. 206–224, xix, 1869, pp. 206–233, 4 Taf.)

=Meinert, Fr.= Campodeæ: en familie af Thysanurernes orden. (Naturhistorisk Tidsskr., 3 Raek., iii, 1864–65, pp. 400–440, 1 Pl.)

=Reinhardt, H.= Zur Entwicklungsgeschichte des Tracheensystems der Hymenopteren mit besonderer Bezeichung auf dessen morphologische Bedeutung. (Berlin. Ent. Zeitschr., 1865, ix, pp. 187–218, 2 Taf.)

=Gerstaecker, A.= Bronn’s Klassen und Ordnungen des Tierreichs, v, 1866–1879. Organs of respiration, pp. 119–131.

=Pouchet, G.= Développement du système trachéen de l’Anophèle (_Corethra plumicornis_). (Archiv zool. expérimentale, i, 1872, pp. 217–232, 1 Fig.)

=Graber, V.= Ueber eine Art fibrilloiden Bindegewebes der Insektenhaut und seine lokale Bedeutung als Trachealsuspensorium. (Archiv f. Mikroskop. Anat. x, 1874, pp. 124–144, 1 Taf.)

—— Die Insekten; München, 1877. Organs of respiration, pp. 346–369.

=Packard, A. S.= On the distribution and primitive number of spiracles in insects. (Amer. Naturalist, viii, 1874, pp. 531–534.)

—— On the nature and origin of the so-called “spiral thread” of tracheæ. (Amer. Naturalist, xx, 1886, pp. 438–442, 2 Figs., p. 558.)

=Wolff, O. J. B.= Das Riechorgan der Biene nebst einer Beschreibung des Respirationswerkes der Hymenopteren, des Saugrüssels und Geschmacksorganes der Blumenwespen. (Nova Acta d. kais. Leop-Carol. Akad. der Naturf., xxxviii, 1876, pp. 1–251, 8 Taf.)

=Palmén, J. A.= Zur Morphologie des Tracheensystems. Leipzig, 1877, pp. 140, 2 Taf.

=Moseley, H. N.= Origin of tracheæ in Arthropoda. (Nature, xvii, 1878, p. 340.)

=Poletajew, Olga.= Quelques mots sur les organes respiratoires des larves des Odonates. (Horæ Soc. Ent. Ross., xv, 1880, pp. 436–452, 2 Pls.)

=Viallanes, H.= Sur l’appareil respiratoire de quelques larves de Diptères. (Compt. rend. Acad. Sc., Paris, 1880, pp. 1180–1182.)

=MacLeod, J.= La structure des trachées et la circulation peritrachéenne. Bruxelles, 1880, pp. 70, 4 Pls.

=Hagen, H. A.= Beitrag zur Kenntnis des Tracheensystems der Libellenlarven. (Zool. Anzeiger, 1880, pp. 157–162.)

—— Einwürfe gegen Palmens Ansicht von der Entstehung des geschlossenen Tracheensystems. (Ibid., 1881, pp. 404–406.)

=Macloskie, G.= The structure of the tracheæ of insects. (Amer. Naturalist, 1884, xviii, pp. 567–573, Fig.)

=Haase, E.= Das Respirationssystem der Chilopoden und Symphylen (Scolopendrellen) vergleichen mit dem der Hexapoden. (Zeitschr. f. Ent. N. F., ix, Breslau, 1884.)

—— Das Respirationssystem der Symphylen und Chilopoden. (Zool. Beiträge, von A. Schneider, i, 1884, pp. 65–95, 3 Taf.; Zool. Anzeiger, 1883, pp. 15–17.)

=Grassi, B.= I progenitori degli Insetti e dei Miriapodi. L’Japyx e la Campodea. (Atti d. Accad. Gioenia d. Sc. Nat. Catania, 1885, Sér. 3, xix, pp. 83, 5 Pls.)

—— I progenitori dei Miriapodi e degli Insetti. Anatomia comparata dei Tisanuri. (Reale Accad. d. Lincei di Roma, Anno 284, 1887.)

=Meinert, Fr.= De eucephale Myggelarver. Sur les larves eucephales des Diptères. (Vidensk. Selsk. Skrifter., 6 Raekke, naturvid. og mathem. Afd. Kjöbenhavn, 1886, iv, pp. 369–493, 4 Pls.)

=Cajal, S. R.= Coloration par la méthode de Golgi des terminaisons des trachées et des nerfs dans les muscles des ailes des insectes. (Zeitschr. f. wiss. Microscopie, 1890, vii, pp. 332–342, 1 Pl.)

=Wistinghausen, C. v.= Ueber Tracheenendigungen in den Sericterien der Raupen. (Zeitschr. wissensch. Zool., xlix, 1890, pp. 565–582, 1 Taf.)

=Stokes, Alfred C.= The structure of insect tracheæ, etc. (Science, 1893, pp. 44–46, 7 Figs.)

=Sadones, J.= L’appareil digestif et respiratoire larvaire des Odonates. (La Cellule, xi, 1895, pp. 271–325, 3 Pls.)

=Holmgren, Emil.= Über das respiratorische Epithel der Tracheen bei Raupen. (Festschrift Lilljeborg. Upsala, 1896, pp. 76–79, 2 Taf.) See also p. 437.

Also Gegenbaur’s Comparative Anatomy, Engl. Trans.

_b._ On the Stigmata

=Loewe, C. L. W.= De partibus quibus insecta spiritus ducunt. Diss. inaug. Halæ, 1814, pp. 28.

=Sprengel, C.= Commentarius de partibus quibus Insecta spiritus ducunt. Lipsiæ, 1815, pp. 38, 3 Taf.

=Dufour, L.= Recherches anatomiques sur l’Hippobosque des chevaux. (Ann. Sc. nat., 1825, vi, pp. 299–322, 1 Pl.)

—— Nouvelles observations sur la situation des stigmates thoraciques dans les larves des Bupresticides. (Ann. Soc. Ent. France, Sér. 2, 1844, ii, p. 203.)

=Landois, H.= Der Tracheenverschluss bei _Tenebrio molitor_. (Reichert u. Dubois-Reymond’s Archiv f. Anat., 1866, pp. 391–397, 1 Taf.)

—— =und W. Thelen.= Der Tracheenverschluss bei den Insekten. (Zeitschr. wissensch. Zool., xvii, 1867, pp. 187–214, 1 Taf.)

—— Der Stigmenverschluss bei den Lepidopteren. (Reichert u. Dubois-Reymond’s Archiv f. Anat., 1886, pp. 41–49, 1 Taf.)

=Hagen, H. A.= Beitrag zur Kenntnis des Tracheensystems der Libellen-Larven. (Zool. Anzeiger, 1880, pp. 157–162.)

—— Einwürfe gegen Palmén’s Ansicht von der Entstehung des geschlossenen Tracheensystems. (Ibid., 1881, pp. 404–406.)

=Krancher, O.= Der Bau der Stigmen bei den Insekten. (Zeitschr. wissensch. Zool., xxxv, 1881, pp. 505–574, 2 Taf.; Zool. Anz., 1880, pp. 584–588.)

=Meinert, Fr.= Spirakelpladen hos Scarabæ-Larverne. (Vid. Meddel. Nat. For. Kjöbenhavn (4), Aarg. iii, 1882, pp. 289–292.)

—— Noget mere om Spiracula cribraria og Os clausum. En Replik. (Ibid. (4), Aarg. v, 1884, pp. 68–91, Fig.)

=Schiödte, J. G.= Spiracula cribraria—os clausum: lidt om naturvidenskabelig Methode og Kritik. (Nat. Tidsskrift (3), xiii, 1883, pp. 427–473; also Jahresber. Neapel, 1883, p. 105.)

=Verson, E.= Il meccanismo di chiusura negli stimmati di Bombix mori. (Atti Istit. Veneto. Sc., 1887, p. 9, Pl.)

—— Der Bau der Stigmen von Bombyx mori. (Zool. Anzeiger, 1887, x Jahrg., pp. 561, 562.) See also Zool. Anzeiger, 1890, p. 116.

=Haase, E.= Die Stigmen der Scolopendriden. (Zool. Anzeiger, 1887, x Jahrg., pp. 140–142.)

—— Holopneustie bei Käfern. (Biolog. Centralbl., 1887, vii, pp. 50–53.)

=Carlet, G.= Note sur un nouveau mode de fermeture des trachées, “fermeture operculaire” chez les insectes. (Comp. rend. Acad. Sci. Paris, 1888, cvii, pp. 755–757.)

=De Meijere, J. C. H.= Über zusammengesetzte Stigmen bei Dipterenlarven [etc.]. (Tijd. Ent., xxxviii, 1895, pp. 65–100, 33 Figs.)

Also the other writings of Palmén, Dufour, Dewitz, Boas, Verson.

_c._ On tracheal gills and tracheal respiration

=Pictet, F. J.= Mémoires sur les larves des Némoures. (Annal. Sc. nat., 1832, xxvi, pp. 369–391, 2 Pls.)

—— Recherches pour servir à l’histoire et à l’anatomie des Phryganides. Genève, pp. 235, 20 Pls.

—— Histoire naturelle générale et particulière, des insectes Neuroptères. I, Monographie: Famille des Perlides, Genève, 1841, 1842, pp. 423, 53 Pls.

—— Histoire naturelle générale et particulière des insectes Neuroptères. II, Monographie: Famille des Ephémérines. Genève, 1843–1845, pp. 300, 47 Pls.

=Dufour, L.= Recherches anatomiques et considerations entomologiques sur les insectes Coléoptères des genres Macronychus et Elmis. (Ann. Sc. nat. Zool., Sér. 2, 1835, iii, pp. 151–174, 1 Pl.)

—— Description et anatomie d’une larve à branchies externes d’Hydropsyche. (Ibid., Sér. 3, 1847, viii, pp. 341–354, Fig.)

—— Recherches anatomiques sur la larve à branchies extérieures du _Sialis lutarius_. (Ibid., Sér. 3, 1848, ix, pp. 91–99, Fig.)

—— De diverses modes de respiration aquatique chez les insectes. (Compt. rend. Acad. d. Sc. Paris, 1849, xxix, pp. 763–770; Ann. and Mag. Nat. Hist., Sér. 2, 1850, vi, pp. 112–118.)

—— Études sur la larve du Potamophilus. (Ann. Sc. nat., Sér. 4, xvii, 1862, pp. 162–173, 1 Pl., Bericht v. Gerstaecker f. 1862, pp. 16, 17.)

=Grube, A. E.= Beschreibung einer auffallenden an Süsswasser-schwammen lebenden Larve (Sisyra). (Wiegmanns Archiv f. Naturgesch., 1843, ix, pp. 331–337, Fig.)

=Schröder van der Kolk, J. L. G.= Mémoire sur l’anatomie et physiologie de _Gastrus equi_. (Nieuwe Verhandl. d. K. Nederl. Instit. Amsterdam, 1845, ix, pp. 1–155, 13 Pls.; Erichson’s Bericht. f. 1845, p. 109.)

=Scheiber, S. H.= Vergleichende Anatomie und Physiologie der Œstriden-Larven. Respirationssystem. (Sitzungsber. Akad. Wissensch. Wien. Math.-naturw. Cl., 1862, xlv, pp. 7–39.)

=Lubbock, J.= On the development of _Chloëon dimidiatum_. (Trans. Linn. Soc. London, I, 1868, xxiv, pp. 61–78, 2 Pls.; II, 1866, xxv, pp. 477–492.)

=Oustalet, E.= Note sur la respiration chez les nymphes des Libellules. (Ann. Sc. nat. Zool., Sér. 5, xi, 1869, pp. 370–386, 3 Pls.)

=Rolph, W. H.= Beitrag zur Kenntnis eininger Insektenlarven. 1 Taf. Inaug. Dissertat. Bonn, 1873.

=Chun, C.= Ueber den Bau, die Entwicklung und physiologische Bedeutung der Rektaldrüsen bei den Insekten. Frankfurt a. M., 1875.

=Haller, G.= Die Stechmückenlarve. Kleinere Bruchstücke zur vergleichenden Anatomie der Arthropoden. I. Ueber das Atmungsorgan der Stechmückenlarven. (Archiv f. Naturgesch., xliv, 1878, pp. 91–96, 1 Taf.)

=Vayssière, A.= Recherches sur l’organisation des larves des Ephémérines. (Ann. d. Sc. nat. Zool., Sér. 6, xiii, 1882, pp. 1–137, 11 Pls.)

—— Monographie zoologique et anatomique du genre Prosopistoma Latr. (Ibid., Sér. 7, ix, 1890, pp. 19–87, 4 Pls.)

=Eaton, A. E.= Notes on some species of Cloëon. (Ann. Mag. Nat. Hist, Ser. 3, xviii, 1866, pp. 145–148.)

—— A revisional monograph of recent Ephemeridæ or may-flies. (Trans. Linn. Soc. London, 1883–1887, Ser. 2, iii, 63 Pls.)

=Müller, Wilh.= Ueber einige im Wasser lebende Schmetterlingsraupen Brasiliens. (Archiv f. Naturgesch., 1884, i Jahrg., pp. 194–212, 1 Taf.)

=Vogler.= Die Tracheenkiemen der Simulien-Puppen. (Mitteil. Schweiz Ent. Gesellsch., 1887, vii, pp. 277–282.)

=Raschke, E. W.= Die Larve von _Culex nemorosus_. Ein Beitrag zur Kenntnis der Insekten-Anatomie und Histiologie. (Archiv für Naturgesch., 1887, liii Jahrg., pp. 133–163, 2 Taf.; Zool. Anz., 1887, x Jahrg., pp. 18, 19.)

=Klapálek, Fr.= Untersuchungen über die Fauna der Gewässer Böhmens. I. Metamorphose der Trichopteren. (Archiv f. naturwissensch. Landesdurchforschung von Böhmen, Prag, 1888, vi, No. 5, pp. 63; No. 6, 1893, pp. 145, Figs.)

=Müller, Fritz.= Larven von Mücken und Haarflüglern mit zweierlei abwechselnd thätigen Atemwerkzeugen. (Ent. Nachr., 1888, xiv Jahrg., pp. 273–277; also Zool. Anzeiger, iv, 1881, pp. 499–502.)

=Kolbe, H. J.= Ueber den Kranzförmigen Laich einer Phryganea. (Sitzungsber. d. Gesellsch. naturforsch. Freunde in Berlin, 1888, pp. 22–26.)

=Haase, Erich.= Die Abdominalanhänge der Insekten mit Berücksichtigung der Myriopoden. (Morpholog. Jahrbuch, 1889, xv, pp. 331–435, 2 Taf.)

=Dewitz, H.= Einiger Beobachtungen, betreffend das geschlossene Tracheensystem bei Insectenlarven. (Zool. Anzeiger, xiii, 1890, pp. 500–504, 525–531.)

=Miall, L. C.= Some difficulties in the life of aquatic insects. (Nature, xliv, London, 1891, pp. 456–462.)

—— Natural History of aquatic insects, 1895, 116 Figs., pp. 1–395.

=Weltner, W.= (Note on Sisyra.) (Ent. Nachr., p. 145, 1894.)

Also papers by Hagen, Dewitz, Williams, Tömösváry (1884).

_d._ Literature on rectal respiration

=Suckow, F. W. L.= Respiration der Insekten, insbesondere über die Darmrespiration der _Æschna grandis_. (Zeitschrift f. d. organ. Physik, von Heusinger, 1828, ii, pp. 24–29, 4 Taf.)

=Dufour, L.= Sur la respiration branchiale des larves des grandes Libellules comparée à celle des poissons. (Compt. rend. de l’Acad. Sc. Paris, 1848, xxvi, pp. 301–303.)

—— Études anatomiques et physiologiques et observations sur les larves des Libellules. (Ann. Sc. nat. Zool., Sér. 3, 1852, xvii, pp. 76–97, 3 Pls.)

=Gilson, G. and J. Sadones.= Larval gills of Odonates. (Journ. Linn. Soc. London, 1897.)

_e._ Physiology of Respiration

=Bonnet, Ch.= Recherches sur la respiration des chenilles. (Mém. Math. des Savants Étrangers, Paris, 1768, v, pp. 276–303.)

=Treviranus, G. R.= Biologie, oder Philosophie der lebenden Natur, für Naturforscher und Aerzte. 6 Bände, Göttingen, 1802–1822. (Atmung, in Bd. iv.)

—— Die Erscheinungen und Gesetze des organischen Lebens. 2 Bände, Bremen, 1831–1833. (Atmung der Insekten in Bd. i.)

—— Versuche über das Atemholen der niederen Tiere. (Zeitschrift f. d. Physiologie, von F. Tiedemann, G. R. u. L. C. Treviranus, 1832, iv, pp. 1–39.)

=Hausmann, J. F. L.= De animalium exsanguinum respiratione commentatio. Hannover, 1803, vi, p. 70.

=Spallanzani, L.= Memoirs on respiration. London, 1804.

=Sorg, F. L. A. W.= Disquisitiones physiologicæ circa respirationem insectorum et vermium. Rudolstadt, 1805, Part II, p. 146.

=Nitzsch, C. L.= Commentatio de respiratione animalium. Vitebergæ, 1808, 4º, pp. 56.

—— Ueber das Atmen der Hydrophilen. (Reil’s Archiv f. Physiologie, 1811, x, pp. 440–458.)

=Reimarus, J. A. H.= Ueber das Atmen, besonders über das Atmen der Vögel und Insekten. (Reil u. Autenrieth, Archiv f. Physiologie, 1812, xi, pp. 229–236.)

=Dufour, L.= Anatomie de la Ranatre linéaire et de la Nèpe cendrée. (Annal. génér. Scienc. phys., Bruxelles, 1821, vii, pp. 194–213, 1 Pl.)

—— Mémoire pour servir à l’histoire du genre Ocyptera. (Annal. Scienc. natur., 1827, x, pp. 248–261, 1 Pl.)

—— Recherches sur quelques entozoaires et larves parasites des insectes Orthoptères et Hyménoptères. (Ann. Sc. nat. Zool., Sér. 2, 1836, vi, p. 55; Sér. 2, 1837, vii, pp. 5–20.)

—— Note sur le parasitisme. (Compt. rend. Acad. Sc. Paris, 1851, xxxiii, pp. 135–139; Rev. et Mag. de Zool., 1851, pp. 408–412.)

=Dutrochet, R. J. H.= Du mecanisme de la respiration des Insectes. (Ann. Sc. nat., 1833, xxviii, pp. 31–44; Mém. Acad. Sc. Paris, 1838, xiv, pp. 81–93.)

=Newport, G.= On the respiration of insects. (Phil. Trans. Roy. Soc., London, 1836, cxxvi, pp. 529–566.)

=Coquerel, Ch.= Note pour servir à l’histoire de _l’Æpus robini_. (Ann. Soc. Ent. France, Sér. 2, 1850, viii, pp. 529–532.)

=Davy, J.= On the effects of certain agents on insects. (Trans. Ent. Soc. London, 1851, pp. 195–212.)

=Barlow, W. F.= Observations of the respiratory movements of insects. (Phil. Trans. Roy. Soc. London, cxlv, 1855, pp. 139–148.)

=Rathke, H.= Anatomisch-physiologische Untersuchungen über den Atmungsprozess der Insekten. (Schriften d. k. phys.-ökon. Ges. Königsberg, i Jahrg., 1860, pp. 99–138, 1 Taf.)

=Lubbock, J.= On two aquatic Hymenoptera, one of which uses its wings in swimming. (Trans. Linn. Soc. London, xxiv, 1863, pp. 135–142, 1 Pl.)

=Boyle, R.= New pneumatical experiments about respiration. (Phil. Trans., 1870, v, No. 63, pp. 2051–2056.)

=Lambrecht. A.= Das Atmungsgeschäft der Bienen. (Bienenwirtschaftl. Centralblatt, vii Jahrg., 1871, pp. 20–25.)

—— Luftverbrauch eines Biens und die damit zusammenhängenden Lebensprozesse der Glieder desselben. (Ibid., 1871, pp. 115–120.)

=Monnier.= Sur la rôle des organes respiratoires chez les larves aquatiques. (Compt. rend. Acad. Sc. Paris, lxxiv, 1872, p. 235.)

=Liebe, Otto.= Ueber die Respiration der Tracheaten, besonders über den Mechanismus derselben und über die Menge der ausgeatmenten Kohlensäure. (Inaug. Diss. Chemnitz, 1872, pp. 28.)

=Plateau, F.= Recherches physico-chimiques sur les articulés aquatiques. (Bull. Acad. Roy. Belg., Sér. 2, xxxiv, 1872, pp. 271–321.)

—— Recherches expérimentales sur les mouvements respiratoires des Insectes. (Mem. Acad. Belg., 1884, xlv, pp. 219, 7 Pls., 56 Figs.)

—— Recherches physico-chimiques sur les articulés aquatiques. Part I. Action de sels en dissolution dans l’eau. Influence de l’eau de mer sur les articulés aquatiques d’eau douce. Influence de l’eau douce sur les Crustacés marines. (Mém. cour. et Mém. des savants étrang. de Belgique, xxxvi, 1871, pp. 68.) Part II. Résistance à l’asphyxie par submersion, action du froid, action de la chaleur, temperature maximum. (Bull. Acad. Roy. de Belgique, Sér. 2, xxxiv, 1872, pp. 271–321.)

—— Les Myriopodes marins et la résistance des Arthropodes à respiration aërienne à la submersion. (Journ. de l’anatomie et de la physiologie, 1890, xxvi, pp. 236–269.)

=Bütschli, O.= Ein Beitrag zur Kenntnis des Stoffwechsels, insbesondere die Respiration bei den Insekten. (Reichert und Dubois-Reymond’s Archiv f. Anatomie, 1874, pp. 348–361.)

=Ritsema Cz., C.= _Acentropus niveus_ Oliv., in Zijne levenswijze en verschillende toestanden. (Tijdschr. voor Entom, 1876, xxi, separate, pp. 34, 2 Taf.)

=Pott, Rob.= Chemical experiments on the respiration of insects. (Psyche, ii, 1878.)

=Sharp, D.= Observations on the respiratory action of the carnivorous water-beetles. (Journ. Linn. Soc. London, xiii, Zoology, 1878, pp. 161–183.)

=Krancher, O.= Das Atmen der Biene. (Deutscher Bienenfreund., xvi Jahrg., 1880, pp. 49–51.)

=Gissler, C. F.= Sub-elytral air-passages in Coleoptera. (Proc. Amer. Assoc. Advanc. Sc. 29 Meet. (1881), 1881, pp. 667–669.)

=Langendorff, O.= Studien über die Innervation der Atembewegungen. 6. Das Atmungszentrum der Insekten. (Archiv f. Anatomie u. Physiol., Physiol. Abteil., 1883, pp. 80–87.)

=Macloskie, G.= Pneumatic functions of insects. (Psyche, iii, 1883, pp. 375.)

=Chalande, J.= Recherches sur le mecanisme de la respiration chez les Myriopodes. (Compt. rend. Acad. Sc. Paris, civ, 1887, pp. 126, 127.)

=Comstock, J. H.= Note on respiration of aquatic bugs. (Amer. Naturalist, 1887, xxi, pp. 577, 578.)

=Fricken, W. v.= Ueber Entwicklung, Atmung und Lebensweise der Gattung Hydrophilus. (Tagebl., 60, Versamml. deutscher Naturf. u. Aerzte, 1887, pp. 114, 115.)

=Schmidt, E.= Ueber Atmung der Larven und Puppen von _Donacia crassipes_. (Berlin. Ent. Zeitschr., 1887, xxxi Jahrg., pp. 325–334, 1 Taf.)

=Dewitz, H.= Entnehmen die Larven der Donacien vermittelst Stigmen oder Atemrohren den Luftraumen der Pflanzen die sauerstoffhaltige Luft? (Ibid., 1888, xxxii Jahrg., pp. 5, 6, Fig.)

=Müller, G. W.= Ueber _Agriotypus armatus_. (Spengel’s Zoolog. Jahrbücher. Abt. f. Systematik, etc., iv, 1889, pp. 1132–1134.)

—— Noch einmal _Agriotypus armatus_. (Ibid., v, 1890, pp. 689–691.)

=Devaux, H.= Vom Ersticken durch Ertrinken bei den Tieren und Pflanzen. (Naturwiss. Rundschau, vi Jahrg., 1891, p. 231; Compt. rend. Soc. de Biol., 1891, Ser. 9, iii, p. 43.)

See also Dewitz, p. 482; Kolbe, p. 482.

THE ORGANS OF REPRODUCTION

Insects are without exception unisexual, the male and female organs existing in different individuals, no insects being normally hermaphroditic. The reproductive organs are situated in the hind-body or abdomen, especially near the end, the genital glands opening externally either in the space between the 7th and 8th, or 8th and 9th, or 9th and 10th abdominal segments, but as a rule between the 8th and 9th segments (Fig. 299).

The primary or essential male organs are the testes, those of the female being the ovaries. As we shall see, the primitive number of seminal ducts and oviducts was two, this number being still retained in Lepisma and the Ephemeridæ. The reproductive organs of both sexes are at first, in their embryonic condition, of the same shape and structure, becoming differentiated in form and function before sexual maturity. These glands and ducts have a paired mesodermal genital rudiment, the ends of the ducts being often connected with corresponding ectodermal invaginations of the cuticle.

The secondary sexual organs mainly comprise the external genital armature of the male, and the egg-laying organs, or ovipositor of the female. Besides these structures there are other more superficial secondary sexual characters, such as differences in the size and ornamentation as well as coloring of the body, or of parts of it.

The primary sexual organs of insects have been conveniently tabulated by Kolbe, thus:—

I. _Male reproductive organs._

1. Two testes, with testicular follicles. 2. Seminal ducts (_vasa deferentia_). 3. Seminal vesicle. 4. Accessory glands. 5. The common seminal outlet, with the penis. 6. The copulatory apparatus.

II. _Female reproductive organs._

1. Two ovaries, with the egg-tubes. 2. Two oviducts. 3. Receptaculum seminis; bursa copulatrix. 4. Accessory sac. 5. The common oviduct, vagina, uterus. 6. The ovipositor.

The ducts of the sexual glands in Peripatus being transformed nephridia or segmental organs, it has been inferred that this is also the case with those of insects, though, as Lang states, there is a considerable difference in the two cases, as the greater part of the ducts in Peripatus arises out of the ectoderm, while in the Myriopoda and insects they come from the mesoderm; but he adds that in the Annelids the greater part of the nephridial duct is of mesodermal origin.

While in insects there is but a single pair of genital outlets, the serial arrangement of the testicular (Fig. 458) and egg-tubes (Fig. 459) in some Thysanura (Campodea, Japyx, and Lepisma), where the tubes (5 to 7 on each side) open singly one behind the other in segmental succession, indicates that in their ancestors these egg-tubes opened out on different segments situated one behind the other. Each egg-tube independently opens into one of the two oviducts, which extend through the abdomen as straight canals. The two oviducts open externally by a short unpaired terminal portion, which in Machilis is said to be wanting, only the outer aperture of the two oviducts being in this case common to both. In Campodea and in the Collembola the ovaries and testes on each side are simply tubes. It is to be observed that in the young Lepisma Nassonow found that the external openings of the two ejaculatory ducts are paired (Fig. 458 _B_, _ed._).

In the Stylopidæ, also, though this may be the result of adaptation to the singular parasitic habits of the females whose bodies are mostly situated in the abdomen of their host, the ends of the oviducts are formed by the invagination of the integument of the 2d, 3d, and 4th abdominal segments. In the 2d to 5th segments are situated tubes which open in the cavity of the body with funnel-like ends, so that the ducts have a close resemblance to the segmental organs of worms. (Nassonow.)

Among the winged insects the reproductive organs of the cricket (Fig. 466) are perhaps as simple as any. The testes are separate, and the vasa deferentia very long. The seminal vesicles bear numerous large and short utricles (_utriculi majores_ and _breviores_), the penis being simple and dilated at the end; while in _Phyllodromia germanica_ the testes are functional throughout life, and consist of four lobes each. In the common cockroach (_P. orientalis_) (Fig. 461) the testes are functional only in the young male; they afterwards shrivel and are functionally replaced by the vesiculæ seminales and their appendages, when the later transformations of the sperm-cells are effected. The accessory glands are numerous and differ both in function and insertion. Two sets of these glands (utriculi majores and breviores) are attached to the vesiculæ seminales and the fore end of the ejaculatory duct, while another appendage, called by Miall and Denny the _conglobate gland_, opens separately on the exterior upon a double hook, which forms a part of the external genital armature. The so-called penis is long, slender, and dilated at the end, but is not perforated.

In the locusts (Acrydiidæ) the testes are, unlike those of most other Orthoptera, closely united to each other so as to form a single mass of tubular glands into which penetrate both simple and dilated tracheæ; the entire mass is situated in the 3d, 4th, and 5th abdominal segments, and above the intestine. The anterior end of the testicular mass is rounded and held in place by a broad, thin band, one on each side; two similar bands are situated a little behind the middle of the mass. From the under side, and a little in advance of the middle of the mass, two straight small ducts, as long as the testicular mass, pass obliquely to the sides of the body, at the posterior end of the 7th segment of the abdomen; these are the vasa deferentia. Each vas deferens, with its mate, forms a convoluted mass of tubes, comprising twenty folded bundles (_epididymis_ of Dufour), and two single, long, convoluted tubes, the _vesiculæ seminales_, which are lobed in the 6th and 7th segments of the abdomen. The two vesiculæ unite over the 5th abdominal ganglion, forming a thick, very short canal (_ductus ejaculatorius_), which passes into a large spherical muscular mass (præputium), behind which is the large intromittent organ (_penis_), which forms a short chitinous cylinder, quite complicated in structure, being armed with hooks and projections and affording excellent specific characters. It can be seen in place without dissection by drawing back the orbicular convex piece called the _velum penis_.

In the Hymenoptera the reproductive system is quite simple, as seen in Fig. 462.

The general shape and relations of the female reproductive organs are seen in Fig. 298, of the locust (Acrydiidæ). The ovaries consist of two large bundles of tubes, each bundle tied to the other by slight bands, with air-sacs and tracheæ ramifying among them. These tubes extend along the intestine, passing into the prothorax. The ovarian tubes opening into the oviducts unite to form the vagina, which lies on the floor of the abdomen. (In the cockroach the vagina has a muscular wall and chitinous lining.) Above the opening of the duct, and directly communicating with it, is the copulatory pouch (_bursa copulatrix_), a capacious pocket lined within with several narrow, longitudinal, chitinous bands. Behind the bursa copulatrix lies, partly resting under the fifth abdominal ganglion, the sebific, cement, or colleterial gland (_colleterium_; compare Fig. 299, _sb_), which is flattened, pear-shaped, a little over half as long as a ripe egg of the same insect. From the under side, a little in advance of the middle, arises the sebific duct, which, after making three tight coils next to the ganglion, passes back and empties into the upper side of the bursa copulatrix, dilating slightly before its junction with the latter.

The most primitive type of reproductive organs observed in insects is that of the young Lepisma and the Ephemeridæ, in which the outlets of the oviducts and of the vasa deferentia respectively are double or paired, showing that insects have probably inherited these structures from the segmental organs of their vermian ancestors.

Réaumur had already observed the process of oviposition and seen that the female Ephemera had two openings near the end of the “6th” abdominal segment, from which he saw two masses of eggs pass out at a time (Fig. 463). Eaton afterwards (1871) referred to the oviducts as terminating between the 7th and 8th segments of the abdomen, and after him Joly; but for a detailed monograph on the subject we are indebted to Palmén. He found that the outlets of the sexual glands are paired, not only in the larvæ of all stages, but also in the imagines, and in both sexes. In the males the vasa deferentia pass on the ventral side of the 9th segment through two external appendages, both reproductive organs, at whose tips or sides the openings are situated. In the larvæ the female openings are not formed until after the last moult. In the females the two oviducts open on the ventral side of the hind-body between the 7th and 8th segments.

Palmén suggests that the Ephemerids represent, in respect to the reproductive system among insects, a very primitive type of organization, and he concludes that the inner sexual organs of insects are built up of two different morphological elements; _i.e._ (_a_) internal primitive paired structures (testes with vasa deferentia, ovaria with oviducts), and (_b_) integumental structures, such as the ductus ejaculatorius and vagina.

In the younger larvæ the vasa deferentia form slender cords along which are situated the seminal glands; these cords are inserted in the integument on the hinder edge of the 9th sternite, where afterwards, during the last moult, the copulatory organs grow out. In the older larvæ the sperm collects in the cavities of these cords. Their walls become expanded, and this section then functions as vesiculæ seminales. The ends of the cords remain contracted and act as ductus ejaculatorii. Common unpaired glandular structures are not present. At the last moult the copulatory organs reach their complete development, and the ducts become open externally.

The oviducts in the larva are at first slender, string-like, and bear the egg-follicles. As soon as the eggs pass out of the follicles and collect in the oviducts, the walls of the latter become stretched, and this portion forms two uterus-like structures. The terminal division of the two passages forms their vaginal portions. But since there is no common vagina, there are no unpaired glands and no receptaculum seminis. The two ducts become open after the last shedding of the skin.

Palmén adds that this paired or double nature of the sexual glands and their external ducts in this group of insects occurs in some Myriopoda (Fig. 3, _E_, _F_) and a few Arachnida (Fig. 3, _C_, _D_, the outlets being in this class unpaired), numerous Crustacea, and most worms; and as already stated it is very marked in Limulus, where the paired outlets are in both sexes very simple and wide apart (Fig. 3, _A_). In the worms the paired genital ducts are modified segmental organs. As we have seen, in the young male Lepisma there are two male genital openings. Hence this double nature of the genital passages in the may-flies seems to be very primitive.

In the Dermaptera, also, the genus Labidura was found by Meinert to have two independent ductus ejaculatorii, opening externally in double external slit-like processes (_penes_). The two ducts arise from a single seminal vesicle, which is either paired (_L. advena_), or forms a common passage (_L. gigantea_). In Forficula (Fig. 464, _B_) only one ejaculatory duct persists, the other is obliterated, and one of the penes is atrophied, the other assuming a position in the middle line of the body. Thus the single ejaculatory duct and seminal vesicle arise from the primitive vasa deferentia, and not from the integument of the body, as is the case in the following examples.

According to the researches of Dufour, Loew, etc., most species of Orthoptera (Œdipoda), Libellula, Perla, Panorpa, Rhaphidia, Myrmeleon, Sialis, and Trichoptera (Hydropsyche) have double vasa deferentia and seminal vesicles, and two ejaculatory ducts. The male genital passages of Rhaphidia have a double opening, Loew describing “the two seminal vesicles as lying near each other and at last uniting in a common passage, with an external opening, which, however, must be very short, since I could only once clearly observe it.” This opening is a deep invagination of the external integument, at the bottom of which the two ducts open independently of each other. In such insects, Palmén states that the single ejaculatory duct morphologically arises by an invagination of the integument.

In another group, forming, as regards the genital apparatus, a step next above the Ephemeridæ, viz. the Perlidæ, the oviducts open near each other at the bottom of a median single “vagina,” situated between the 7th and 8th abdominal segment; it is covered beneath by a valve-like, enlarged sternite of the preceding segment, and Palmén homologizes it with the ovi-valvula of some Ephemeridæ. He regards this bell-shaped vagina as a cup-like, deep, intersegmental fold, which projects into the body-cavity and there receives the two ducts.

This differentiation in the Perlidæ may be regarded as the type for several groups of insects. But in others occur a complication which in some degree modifies the type. Thus the invagination arises out from one segment alone, but several segments during metamorphosis may become so reduced that the ventral portions of all may be invaginated to form the vagina. Thus in the larva of Corethra, according to Leydig, and also Weismann, the two testes are attached by two cords to the integument; the hinder ones are inserted independently, and share in the development of the outlets.

Graber has observed the same relations in the pupa of Chironomus, the efferent genital tubes in both sexes being separate, so that there are two vaginal passages and two penes present. Palmén comments on these relations in the dipterous insects, remarking that during metamorphosis certain parts of the terminal abdominal segments are reduced, while others are hypertrophied; hence the points of insertion of the cords referred to becoming the openings of the vasa are carried within the abdomen; and this part of the integument becomes an unpaired section. In these insects, also, there is an unpaired vesicula seminalis, but its morphological nature (whether formed from the integumental duct or the fused vasa deferentia) can only be settled after special investigation.

In the Lepidoptera, also, it has been shown by Herold, Suckow, Bessels, and recently with full details by Jackson, that the paired larval oviducts are at first solid, but become tubular early in pupal life. A little later, their cavities open into that of the azygos or unpaired oviduct. The paired oviducts open in the female caterpillars on the hind edge of the 7th abdominal segment, afterwards uniting with the unpaired vagina of the 8th segment, which is developed from the hypodermis.

Jackson adds that there are three stages traceable in the evolution of the genital ducts of Lepidoptera: “an ephemeridal stage, which ends towards the close of larval life; an orthopteran stage, indicated during the quiescent period preceding pupation; and a lepidopteran stage, which begins with the commencement of pupal life.”

As a summary of these results it appears that the genital organs of insects consist of two morphologically different elements: 1. the primitive internal paired structures (testes with the vasa deferentia; ovaries with the ovarian tubes), and 2. integumental structures (Fig. 464). In the most primitive winged insects (Ephemeridæ) the latter structures are only represented by the two external sexual openings, the entire reproductive system being paired. The paired parts become in the more highly differentiated forms united into single parts, while, _a_, a common integumental division, grows in, forming the ductus ejaculatorius, or the vagina; or, _b_, the inner passages anastomose together, _i.e._ the openings fuse together; or, _c_, both of these cases occur at once; or, finally, we have _d_, where the superfluous paired parts by reduction become single.

The male ducts open behind the 9th, the female passages of Ephemerids behind the 7th abdominal segment, those of other insects behind the 8th, except in the Stylopidæ (Strepsiptera), in which they open much in front.

Figure 464 graphically shows their relation. In the Odonata (_F_) the chitinous lining or integumental invagination extends inwards where the two oviducts begin, in the Coleoptera (_E_) the vagina, bursa copulatrix, and receptaculum seminis being lined by a thick chitinous layer. While in Perla the two seminal ducts pass directly into the copulatory organ, in the Coleoptera they open into the unpaired ductus ejaculatorius at a distance from the copulatory organ.

The morphological results obtained by Palmén, and for the Lepidoptera by Jackson, were apparently confirmed from an embryological point of view by Nusbaum, from observations on the development of the sexual passages in two genera of Pediculidæ, and are as follows:—

1. The prevalent impression that the larval ducts unite with each other and give origin to the whole system of sexual ducts is incorrect; they form only the vasa deferentia or the oviducts.

2. All other parts of the efferent apparatus (uterus, vagina, receptaculum seminis, ductus ejaculatorius, penis, and appended glands) develop from the hypodermis.

3. The connective tissue and the musculature of the efferent apparatus are derived from mesoblast cells present in the body-cavity.

4. The efferent ducts originate as paired rudiments. All unpaired (azygos) parts (uterus, penis, receptaculum seminis, unpaired glands, etc.) are at first paired. The unpaired efferent apparatus of insects must therefore be regarded as morphologically a secondary and more complicated form.[76]

5. The male and female efferent ducts are strictly homologous.

6. The cavities of the oviducts, uterus, vagina in the female, of the vasa deferentia, appended organs, and ductus ejaculatorius of the male arise independently, and come into connection secondarily.

The presence of two genital openings, viz. a bursa copulatrix or copulatory pouch, and of the primitive oviducal orifice behind the 9th segment, is peculiar to Lepidoptera, and the inquiry naturally arises whether they represent the outlets of two pairs of segmental organs. The question has been fully set at rest, however, by Jackson, who shows that the copulatory pouch is a secondary invagination of the ectoderm, being derived from the hypodermis, while the second aperture is a special adaptation. It is, however, the partial homologue of the vaginal orifice in other orders of insects. It opens behind the sternite of the 8th abdominal segment, the typical position of the vaginal aperture as shown by Lacaze-Duthiers. The lateral position of the bursa and its separation from the azygos oviduct are probably late features in the phylogenetic history of the Lepidoptera, subsequent even to the closure of the furrow.

“The existence of a second or posterior aperture is probably to be attributed to the advantage gained by a terminal position for the aperture through which the ova are laid. The remarkable way in which this aperture shifts backwards seems to point very distinctly to this explanation, especially as the Lepidoptera are entirely devoid of the outgrowths which form the ovipositor in some orders; _e.g._ most Orthoptera.”

The original condition of things appears to have been retained in a moth, _Nematois metallicus_, which, according to Cholodkowsky, possesses but a single external aperture, the bursa opening into the dorsal wall of the unpaired oviduct.

_a._ The male organs of reproduction

Bearing in mind that the testes with their efferent ducts are, like the ovaries and egg-tubes, primitive structures, there are various secondary or adaptive structures which are either due (1) to modifications of the male efferent ducts, or of the ovarian tubes, or (2) to various accessory organs, mostly glandular, resulting from the invagination of the ectoderm.

The male organs are, then, the following:—

1. Two testes (Figs. 465–469, _t_, _H_, _ho_).

2. The two seminal ducts (_vasa deferentia_, _v_, _sl_, _SL_), whose lower or outer (distal) division becomes enlarged and acts as a seminal vesicle (_vesicula seminalis_; Figs. 467–469, _bl_, _SB_).

3. The common ejaculatory duct (_ductus ejaculatorius_), with the penis (Figs. 467–469, _ag_, _uSG_).

4. Accessory glands at the base of the vasa deferentia (_glandulæ mucosæ_, Figs. 465–469, _a. g._, _dr_, _D_), whose secretion mixes with the semen or serves for the formation of the seminal packets (sematophores).

In his paper on the internal male organs of beetles, Escherich states that those of the Carabidæ illustrate the simplest, most primitive condition (Fig. 465). A simple blind tube on each side produces spermatozoa, stores the elements, and secretes mucus. Each of these tubes opens into a somewhat larger duct, and the two unite in a common ejaculatory canal. The terminal portion in these beetles is lined with chitin, and is therefore ectodermal, and not the result of the union of the mesodermic vasa deferentia. The region corresponding to the testes, vasa deferentia, and seminal vesicles are mesodermic. Blaps (Fig. 465, _B_) is intermediate between the Carabidæ and Hydrophilus (Fig. 465, _C_). The accessory glands (_a. g._) are developed, and the seminal vesicles are situated in the middle, and not at the lower end of the vasa deferentia, as in Hydrophilus.

=The testes.=—Each testis is composed of follicles or corresponding parts, which according to the group of insects in which they occur are united in different ways; or each testis consists of a single hank or skein-like blind tube which is enveloped by a membrane, as in the Carabidæ, Dyticidæ, or Lucanidæ.

The number of testicular tubes is small in most Hemiptera, but very great in the Cicadidæ, Orthoptera, Coleoptera, and many Hymenoptera. Although the testes are usually separated from each other, they are closely united in certain Orthoptera (Gryllotalpa, Ephippigera), Coleoptera (Galerucella), in many Lepidoptera, and in a number of Hymenoptera (Scolia, Pompilus, Crabro, and others).

The two testes of most Lepidoptera are so closely grown together or coalesced into a single body that one might regard them as a single testis. But in the different families there occur all grades, from the unpaired testes of most Lepidoptera to Hepialus with separate testes. Cholodkowsky therefore distinguishes four types:—

1. The embryonal or primitive type, with two testes, whose seminal follicles are entirely separate. (Brandt.) These testes are contained, as in all other Lepidoptera, in a well-developed thick chitinous membrane or scrotum, analogous to that of the higher vertebrates, which envelops each separate seminal follicle (_Hepialus humuli_).

2. The larval type, with two testes, whose four follicles are enclosed by a common scrotal membrane (_Bombyx mori_, _Gastropacha quercifolia_, _Ichthyura anachoreta_ and _anastomosis_, _Saturnia pyri_, _Aglia tau_).

3. The pupal type (since it first occurs in the pupa state), with a single testis, which possesses an external median lace-like covering. (Adela, Lycæna.)

4. The imaginal type, with a single testis enveloped by a lace-like scrotum, within which the follicles are wound around the longitudinal axis of the testis. (Most Lepidoptera.)

In Nematois there are twenty seminal follicles, the number of ovarian tubes being the same. (Cholodkowsky.)

In many insects the testes are not composed of tubes (follicles), but of button-like bodies, each of which has its own duct.

The color of the testes is usually white, but they may be orange (Decticus), yellowish green (_Locusta viridissima_), or deep yellow (Chrysopa).

The testes of Asilid flies are enveloped by a common dark-red membrane rich in tracheæ, like that in Lepidoptera which clothes the separate testicular follicles. The two testes of Calliphora are enveloped by an orange-yellow capsule, outside of which is a special membrane formed by the fat-body. (Cholodkowsky.)

In the honey-bee the testis has two envelopes, the outer of which is formed by the fat-body, the inner coat of connective tissue. The entire testis corresponds to a portion only of that of _Bombyx mori_.

=The seminal ducts.=—The vasa deferentia are fine tubes, which vary much in length; being short in many beetles and locusts, very short in many Diptera (Syrphidæ, etc.), very long in Cicada and many beetles; according to Burmeister, being in Dyticus about five times, in Necrophorus and Blaps eight to ten times, in Cicada 14 times, in _Cetonia aurata_ 30 times, as long as the body. They either resemble a skein of silk, or form a tangled mass.

The distal or lower end of the vasa is in many insects dilated into a sac or seminal vesicle, which serves for the reception and storage of the seminal fluid after it passes through the vasa deferentia. In the honey-bee the vas deferens is given off from the reservoir, forms loops in and outside of the testis, and passes to the seminal vesicle. The canal into which the vesicle narrows does not open into the ductus ejaculatorius, but into the glandulæ mucosæ; its epithelial cells are much vacuolated, and have, therefore, a spongy appearance. (Koschewnikoff.)

=The ejaculatory duct= during coition conducts the sperm into the copulatory pouch of the female. In consequence of the stretching of the integumental membrane the end of the duct can be erected and again withdrawn. For this purpose the end of the duct is thickened and is said to be provided with powerful muscles. The evaginable terminal portion is covered by a strong chitinous membrane forming the penis or intromittent organ (Fig: 462, _h_), which is externally enveloped by a pair of chitinous lobes, which in many beetles are converted into a capsule. The ductus ejaculatorius of the honey-bee is inserted by two chitinous branches into the point of union of the two glandulæ mucosæ; it and the entire copulatory apparatus are devoid of muscles, though it is, however, well developed beneath the mucous glands. (Koschewnikoff.)

=The accessory glands= of the vasa deferentia are tubes whose secretions either directly mix with the semen, or in many cases form seminal packets (_spermatophores_). In Coleoptera, Lepidoptera, and Diptera there is usually one pair. In many insects there are several pairs, as in Hydrophilidæ and Elateridæ; they are branched in Hemiptera, and in Orthoptera bushy. The single glandular tubules are very long, and form a skein-like mass. In Orthoptera, in the larger number of accessory glands, two forms may be distinguished, which differ from each other in their contents (Siebold). In the cockroach (Fig. 461) these glands form the “mushroom” shaped gland of Huxley, which was at first regarded as the testis.

=The spermatozoa.=—These very minute bodies, the sexual homologues of the eggs, abound in the seminal fluid, and are formed in the follicles of the testes from a germinal layer or epithelium, as are the eggs. They are hair- or thread-like, usually consisting of a head, a body or middle-piece, and a long, thread-like tail (flagellum), which vibrates rapidly, causing the spermatozoön to move actively forwards (Fig. 470).

In beetles, according to Ballowitz, there are two main types of spermatozoa, connected, however, by intermediate forms. There is a double-tailed type, already described by Bütschli and v. la Valette St. George, and there are others which are single-tailed. Bütschli showed that in the double spermatozoön one tail-filament is straight and stiff, the other being undulating and contractile. Ballowitz describes this type in Calathus (Fig. 470, _B_), Chrysomela, and Hylobius, etc., and shows that the straight or supporting portion of the tail is elastic, but somewhat stiff, resistant to reagents, and without any fibrillar structure, while the contractile fringe consists of an extremely complicated system of fibrils (Fig. 470). The single-tailed type of spermatozoön, as seen, _e.g._, in Melolontha and Hydrophilus, has no supporting fibres. The tail is twisted in a spiral, corresponds to the contractile fringe of the double type, and exhibits a complicated fibrillar structure. The fringed type works its way ahead like the screw of a steamer.

Each spermatozoön is a modified but complete cell, and the nucleus contains the chromatin, a deeply staining substance of the nuclear network and of the chromosomes and the supposed bearer of heredity.

=Formation of the spermatozoön.=—It arises from a primordial germcell called _spermatogonium_. This cell contains a large, pale nucleus and a dark body, the accessory nucleus of Bütschli. The _spermatogonia_ subdivide, but at a certain period pause in their subdivisions, and undergo considerable growth. “Each spermatogonium is thus converted into a _spermatocyte_, which, by two rapidly succeeding divisions gives rise to four spermatozoa, as follows: The primary spermatocyte first divides to form two daughter-cells, known as spermatocytes of the second order, or sperm mother-cells. Each of these divides again—as a rule without pausing, and without the reconstruction of the daughter-nuclei—to form two _spermatids_ or sperm-cells. Each of the four spermatids is then directly transformed into a single spermatozoön; its nucleus becoming very small and compact, its cytoplasm giving rise to the tail and to certain other structures.... As the spermatid develops into the spermatozoön, it assumes an elongated form, the nucleus lying at one end, while the cytoplasm is drawn out to form the flagellum at the opposite end.” (Wilson’s The Cell, from La Valette St. George.)

Henking finds that the primordial sperm-cells correspond to the primordial ova, both forms of cells in the insect he studied containing the characteristic number of twenty-four chromosomes.

The spermatogenesis of Laphria, according to Cholodkowsky, is very peculiar, and strongly resembles that described by Verson in _Bombyx mori_. In the blind end of the testicular tubes lies a colossal cell visible to the naked eye, the spermatogone, from which the entire contents of the testes originate. In Bombyx this spermatogone appears in the larva state. Such colossal spermatogones also occur in Lepidoptera of different families (Hyponomeuta, Vanessa, and in the pupa of _Chareas graminis_), in Trichoptera, and in Hemiptera (Syromastes); and Cholodkowsky inquires whether they may not be typical of insects. Toyama has observed these colossal cells not only in the testes but also in the ovaries of the silkworm. He regards them as supporting cells.

The spermatozoa are inclined to remain in bundles, and in this state are expelled during copulation. These bundles are either root-like, bushy, string-like, sinuous, or worm-like.

Auerbach has observed the spermatozoa of _Dyticus marginalis_ in their passage through the convoluted seminal vesicles. All those arising from one testicular tube are united in a bundle. Each has a very complex structure, bilateral but unsymmetrical. The right side of the head is concave, the left convex; the whole head is longitudinally curved to right or left; and on the posterior half of the right side there is a projecting ridge bearing a hook-shaped cyanophilous “anchor,” at the free end of which an erythrophilous spherule appears. The most remarkable fact is that the spermatozoa unite in pairs in a perfectly definite way, opposed and crossed in a manner somewhat suggestive of a pair of scissors, with the right sides of the heads in contact. During this conjugation, or “dejugation” as Auerbach calls it, the anchors change their shape, and the little spherules are lost. Hundreds of these double spermatozoa are found together in little balls. The conjugation is a temporary one, but it may permit a molecular exchange of substance, perhaps with the result of mixing the hereditary qualities and limiting variability. (Journ. Roy. Micr. Soc., 1893, p. 622.)

In many insects which lack a true penis, the bundle of spermatozoa are united in the ejaculatory duct, forming packets which are enveloped by the secretion of the accessory glands which stiffens into a hard case. These packets are called _spermatophores_. They are either introduced into the vagina of the female or simply remain outside. Graber has repeatedly observed that the male crickets, in the absence of the female, let their spermatophores fall to the earth; whether it is afterwards made available is not known, because hitherto no case is reported that females seeking impregnation search, as in the case of the Isopod crustacean, Porcellio, for the spermatophores.

In the Gryllidæ and Locustidæ the spermatophore lies in a cup-like cavity under the penis. This is called the “spermatophore cup” (Chadima, 1871), into which the ejaculatory duct of the testis opens.

According to the views of Schneider, the spermatophores, with their capsule, usually consist solely of seminal filaments, which stick closely to each other, and only exceptionally have a capsule formed by a glandular secretion. In Locusta, however, and perhaps also in Gryllus, the sperm is enveloped by the secretion of the accessory glands of the seminal ducts; the spermatophores pass, still fluid, out of the sexual opening of the male into that of the female, but become chilled on the outer surface, so that the sperm, without coming in contact with the air, passes into the receptaculum seminis.

The mode of grouping of the spermatozoa of the Locustidæ as they occur in the spermatheca of the female is remarkable. Their heads lie so close to each other that they form a long shaft, while the numerous threads are arranged so as to look like the two vanes of a feather, the entire mass being like a very long heron’s feather. (Siebold.)

In the honey-bee the spermatophore is likewise enveloped by the secretion of the accessory glands, and thereby becomes a sort of seminal cartridge. This is a peculiar oval body which is carried during the marriage-flight into the air within the upper part of the penis, the so-called penis-bulb. (Leuckart.)

_b._ The female organs of reproduction

The different parts of the female reproductive organs are the following:

1. The two ovaries.

2. The two oviducts.

3. The common egg-passage in nearly all insects (its distal or hindermost part forming the uterus or vagina).

4. The receptaculum seminis, or spermatheca.

5. The bursa copulatrix, or copulatory pouch.

6. The accessory glands (cement, sebific, or colleterial glands, or “oil reservoirs,” glandulæ sebaceæ, coleterium).

=The ovaries and the ovarian tubes.=—As in the testes, so each ovary consists of a variable number of ovarian tubes, by some called _ovarioles_, united by a thread at the distal end, and at the lower or hinder end opening into the oviduct. Each ovarian or egg tube is divided into three sections: (1) the terminal thread; (2) the terminal chamber, and (3) the actual ovarian tube, or chambered main division, this forming the longest part of the egg-tube.

The slender terminal thread serves to attach or suspend each egg-tube near the dorsal vessel (not directly to the heart, as formerly supposed), becoming lost in the fat-body.

The terminal chamber contains undifferentiated cell elements, supposed to be the remains of the ovarian rudiments. From these arise (either in the embryo or larva) first, the follicle epithelium of the ovarian tubes; and, second, the material for the formation of the new eggs, and nutritive cells. “In the terminal chamber these cell-elements remain undifferentiated, excepting when required for the removal of the follicle epithelium, eggs, and nutritive cells in the adult insect.” (Lang.) This portion of the ovariole is called the _germarium_. In Blatta it is filled with protoplasm in which numerous small nuclei are imbedded. (Wheeler.) The chambered main division of the egg-tube contains the ripening eggs, one in each compartment, the tube appearing like a string of beads.

The egg-tubes are of two types: (1) those without, and (2) those with nutritive cells, the first kind being the simplest, and occurring in the Synaptera (except Campodea) and in Orthoptera. As an example may be cited that of the cockroach (Fig. 473), where in each tube there is a simple continuous row of eggs from the terminal chamber to the oviduct. The tube being constricted between these consecutive eggs, gives it a beaded appearance.

In the cockroach (_Periplaneta orientalis_) each egg-tube has a beaded appearance. Its wall consists of a transparent elastic membrane, lined by epithelium, with an external peritoneal layer of connective tissue. The terminal filament (_tf_) is filled with a clear protoplasm, with a few nuclei. In the terminal chamber (_tc_) are large nucleated cells, with separate nuclei, both entangled in a network of protoplasm. In the third, or egg-chamber (_ec_), are about twenty ripening eggs, arranged in a single row. “Between and around the eggs the nuclei gradually arrange themselves into one-layered follicles, which are attached, not to the wall of the tubes, but to the eggs, and travel downwards with them. As the eggs descend, the yolk which they contain increases rapidly, and the germinal vesicle and spot (nucleus and nucleolus), which were at first plain, disappear. A vitelline membrane is secreted by the inner surface and a chitinous chorion by the outer surface of the egg-follicle.

“The lowest egg in an ovarian tube is nearly or altogether of the full size; it is of elongate-oval figure, and slightly curved, the convexity being turned towards the uterus. It is filled with a clear albuminous fluid, which mainly consists of yolk. The chorion now forms a transparent yellowish capsule, which, under the microscope, appears to be divided up into very many polygonal areas, defined by rows of fine dots. These areas probably correspond to as many follicular cells.” (Brandt, from Miall and Denny.)

In the second type, _i.e._ those egg-tubes with nutritive cells, there are two kinds. In the first the egg-chambers and yolk- or nutritive chambers alternate, each of the latter containing one or more nutritive cells, which serve for the nourishment of the ripening egg contained in the neighboring chamber. “The egg- and yolk-chambers may be distinctly separated externally by constrictions (Hymenoptera and many Coleoptera), or one nutritive and one egg-chamber may lie in each section of the ovarian tube, which is externally visible as a swelling (Lepidoptera, Diptera).”

In the second kind with nutritive cells, the actual tube consists (Fig. 474, _C_) of ovarian chambers only; the nutritive cells here remain massed together in the large terminal chamber. The single egg in the tube is united with the terminal chamber by connective strands (_d. s._), which convey the nutritive material to the eggs. (Lang.)

Egg-cells, nutritive cells, and the cells of the follicle-epithelium (epithelium of the chambers of the ovarian tubes) are, says Lang, according to their origin, similar elements, like the egg and yolk-cells of the flat worms (Platodes); division of labor leads to their later differentiation. Only a few of the numerous egg-germs develop into eggs, the rest serving as envelopes and as food for these few.

Korschelt considers that all the chief elements of the egg-tubes, viz. egg, nutritive, and epithelial cells, arise by a direct transformation of the elements of the terminal chamber, and that the last may be traced to the indifferent elements of the terminal thread, the elements in question originating from the nuclear elements by a breaking down of the syncytium (or masses of protoplasm with nuclei scattered through it) composing it (Fig. 475).

The latest work is that of Wielowiejski (Zoologische Anzeiger, ix, 1886, p. 132), whose observations are based on a study of the ovarian tubes and the growing eggs of the Hemiptera (Pyrrhocoris), the Coleoptera (Telephorus, Saperda, Cetonia and Melolontha, Carabidæ, and Hydradephaga), etc.

Wielowiejski divides the ovaries of insects into three groups:—

1. Comprising such ovaries in the ends of whose egg-tubes (terminal filament) the embryonal cells in the early stages are accumulated, and are transformed into egg-, yolk-, and epithelial cells respectively. (Ovaries of Orthoptera, geodephagous and hydradephagous Coleoptera, Lepidoptera, Diptera, and Hymenoptera).

2. Comprising ovaries whose ends above the egg-cells and egg-germs (_Eianlagen_) possess throughout life a more or less voluminous solid accumulation of cells (terminal chamber), but which stand in no close relation with the first. (Ovaries of Coleoptera, with the exception of the Geodephaga and Hydradephaga, and Aphidæ in part.)

3. Comprising ovaries whose ends above the egg-germs contain a well-developed mass of cells functioning as a yolk-forming organ, between whose special elements grow root-like offshoots of nearly ripe egg-cells. (Hemiptera.)

When the egg is ripe the food-chamber disappears because its contents have served for the formation of the egg below it. In Lepidoptera especially, the egg-tubes resemble strings of pearls because most of the numerous eggs ripen simultaneously and are likewise deposited at the same period, which is naturally not the case in those insects whose eggs gradually ripen (Fig. 477). In other cases the egg- or food-compartments are transformed into each other, but only one egg- and one food-compartment can be situated in the same dilatation of the ovarian tube. Finally, there are insects in whose egg-tubes the egg-compartments are arranged in a single row, while the capacious terminal chamber contains a large mass of food-cells.

Egg-cells, nutritive cells, as well as the cells of the follicle epithelium (epithelium of the chambers of the ovarian tubes), originate as similar or homologous elements, division of labor leading to their later differentiation. Only a few of the numerous egg-germs develop into eggs, the rest serving as envelopes and also as food for these few. (Lang.)

In many insects the egg-tubes open into an egg-calyx (Fig. 478, _c_), in which the ripe eggs collect before passing into the oviduct (_ov_).

As the result of his investigations on the origin of the cellular elements of the ovaries of insects Korschelt concludes:—

1. The different cell-elements of the egg-tubes, eggs, nutritive cells, and epithelium arise from identical undifferentiated elements situated in the contents of the earliest germ of the egg-tubes.

2. The first formation of the cellular elements present, and the differentiation of the individual compartments of the egg-tube, occur during embryonic and larval life.

3. The undifferentiated elements of the terminal chamber correspond to the embryonic condition, while in post-embryonic time, and even during imaginal life, a new formation of the different kinds of cells takes place.

4. The mode of origin of the different kinds of cells from the undifferentiated elements varies greatly in different insects.

5. From their histological nature, and from the mode of origin of their elements, the most complex egg-tubes and those provided with nutritive compartments are phylogenetically derived from those without such nutritive compartments.

6. The nutritive cells in certain cases originate in the same way and at the same time as the germ-cells, and are therefore to be regarded as germ-cells which have abandoned the function of egg-making, and exchanged it for the production of nutritive material.

7. In the egg-tubes with numerous nutritive compartments the nutritive cells can originate at the same place as the egg-cells, and they afterwards still lie intermingled with these in the beginning or upper part of the egg-tubes.

8. While the capability of egg-making of the germ-cells originally situated in the extremity of the terminal chamber gradually becomes transferred to those at the base of the terminal chamber, and the first transform into nutritive cells, egg-tubes with nutritive compartments at the base may be found.

9. The nutritive cells of certain forms arise independently of the germ-cells and therefore could not have previously originated from them.

10. The epithelium has in all forms nearly the same mode of formation; it everywhere shows a close similarity to the undifferentiated elements of the terminal chamber, out of which it directly develops. As to the fact of formation of epithelium through the germ-vesicles (_Keimblaschen_), nutritive-cell nuclei, or the so-called “oöblasts,” I could not feel certain.

11. Neither the eggs of Hemiptera or of other insects arise through the agency of “oöblasts,” but like the epithelial and nutritive cells arise by a gradual differentiation from the indifferent elements of the ovarian tubes.

12. The different elements of the egg-tubes, also the eggs, have the morphological value of cells.

=Origin of incipient eggs in the germ of the testes.=—Heymons has detected in the germ of the testes of the male larvæ of _Phyllodromia germanica_ 7 mm. in length, young or incipient eggs, similar to those seen in the ovarian tubes of the female larva of the same size. In another male larva of the same size also occurred short cylindrical tubes each with a terminal thread, which had the appearance of rudimentary egg-tubes. Hence he thinks that every part of the genital germs (_Anlagen_) in the male, which are not concerned in the formation of testicular follicles, represents the germ of a female genital gland. As is well known, no insects are hermaphroditic, but this case of the practical origin of eggs and egg-tubes in the lowest division of the male efferent passage, which is homologous with the egg-producing division of the female ovarian tubes, points back to hermaphroditic ancestors. And Heymons suggests that the frequent occurrence of hermaphroditism in insects probably confirms this view.

=The bursa copulatrix.=—The copulatory pouch in most insects is a special cup-shaped appendage of the vagina adapted for the reception of the male organ during sexual union. Its mode of formation in the cockroach is thus described by Haase:—

“By the retreat of the female sexual aperture, situated in the 8th ventral plate, a considerable space, the genital pouch, is produced; this is formed chiefly by the extended connective membrane between the elongated 7th and 8th ventral plates. This serves for the development of the egg-cocoon, which is retained by the internal appendages of the posterior gonapophyses.”

The fertilization of the female takes place once for all a long time previous to oviposition; the semen in the receptaculum seminis passes out as the eggs slip down the egg-passage, and a spermatozoön gains entrance into the interior of the egg through the micropyle. In Œcanthus, according to Ayers, fecundation probably takes place while the egg is passing into the vagina, “since it is hardly possible that the male element could gain access to the follicles before the chorion is secreted.”

In the Lepidoptera, as has been stated, the copulatory pouch opens separately from the opening of the oviduct (vagina), but a slender canal connects the pouch with the vagina (Fig. 310, _bc_). The outlet (“vagina” of Burgess) of the copulatory pouch opens between the 7th and 8th segments, that of the oviduct (vagina) on the 9th segment being “situated immediately below the anus and hardly separated from it, between the lappets of the 9th segment.” (Burgess.) The opening of the copulatory pouch is, as we have seen, the genuine or primitive sexual opening.

=The spermatheca.=—This is a sac or pouch for the reception and storage or preservation of the semen. While in most of the higher insects it opens into the dorsal wall of the vagina (Fig. 472, _f_), in the cockroach, locusts, and grasshoppers it opens into the bursa; but in other European Orthoptera, as in most insects, it lies upon the dorsal wall of the vagina. (Berlese.) In the cockroach, it is a short tube dilated at the end and wound into a spiral of about one turn. “From the tube a cœcal process is given off, which may correspond with the accessory gland attached to the duct of the spermatheca in many insects (_e.g._ Coleoptera, Hymenoptera, and some Lepidoptera). The spermatheca is filled during copulation, and is always found to contain spermatozoa in the fertile female. The spermatozoa are no doubt passed into the genital pouch from time to time, and there fertilize the eggs descending from the ovarian tubes.” In Meloë the spermatheca is exceedingly large. (Miall and Denny, pp. 170, 171.)

=The colleterial glands.=—We have already briefly referred to these glands. Those of the cockroaches form a number of long blind tubes opening into the vagina. They furnish the material for the egg-capsule or oötheca, viz. chitin and large crystals of oxalate of lime.

In _Phyllodromia germanica_ “these glands are glistening white till the time of oviposition approaches, when they assume a yellow tint, and the octahedral crystals are seen imbedded in a viscid substance which fills their lumina. This viscid substance is soluble in potassium hydrate, and is consequently not chitin. When excreted to form the oötheca, it slowly hardens, deepens in color, and becomes insoluble in potassium hydrate. Light has nothing to do with this change, which is possibly produced by the oxygen in the air. It is the same change which is undergone by the cuticula of the insect itself immediately after ecdysis.” (Wheeler.)

=The vagina or uterus.=—This is simply the end of the common oviduct, which, when dilated, is called the vagina, and, in the pupiparous forms, the uterus.

In the cockroach the vagina opens by a median vertical slit situated in the 8th sternite, into the genital pouch or bursa, upon the dorsal wall of which the orifice of the spermatheca is situated. In the sheep-tick the oviduct is enlarged to form the so-called uterus, which furnishes a milk-like secretion for the nourishment of the larva during its intra-uterine life.

In insects in general, the external opening of the vagina is simple, the chitinous structures (valves) at the opening being adapted to receive the male intromittent organ.

When the eggs are to be deposited deep below the surface of the earth, or in wood, or in wood-boring larvæ, or in the body of caterpillars, etc., they are inserted by the ovipositor (see p. 167).

_Signs of copulation in insects._—Leydig has collected, partly from his own observations and partly from those of others, a number of cases in which female insects bear traces of having had sexual union, in the form of tags or plates attached to the body, and apparently formed from material secreted by the male. Such probably is the “pouch” on the abdomen of _Parnassius apollo_, and a somewhat similar structure in _Fulgora laternaria_, and such is the plate which is found on the hinder part of the abdomen of _Dyticus latissimus_ and _D. marginalis_. Leydig compares these structures with the white plate in _Astacus fluviatilis_, and with the little white lid on the spider Argenna, and finds analogues among vertebrates. (Arbeit. Zool. Zoot. Inst. Wurzburg, x, 1891, pp. 37–55, 2 Figs.)

LITERATURE ON THE ORGANS OF REPRODUCTION

_a._ General

=Hunter, J.= Observations of bees. (Phil. Trans. Roy. Soc. London, 1792, lxxxii, pp. 128–195.)

=Hegetschweiler, J. J.= Dissertatio inauguralis zootomica de insectorum genitalibus. Turici, 1820, pp. 28, 1 Pl.

=Audouin, V.= Recherches anatomiques sur la femelle du Drile jaunatre et sur le male de cette espèce. (Ann. Sc. nat., ii, 1824, pp. 443–462, 1 Pl.)

=Müller, Johannes.= Ueber die Entwickelung der Eier im Eierstock bei den Gespenstheuschrecken. (Nova Acta Acad. Leop.-Carol., xii, 1825, pp. 555–672, 6 Taf.)

=Dufour, L.= Recherches anatomiques sur les Carabiques et sur plusieurs autres insectes coléoptères. Organes de la génération (Ann. Sc. nat., vi, 1825, pp. 150–206, 6 Pls.; pp. 427–468, 4 Pls.).

—— Recherches anatomiques sur l’Hippobosque des cheveaux. (Ibid., 1825, vi, pp. 299–322, 1 Pl.)

—— Recherches anatomiques sur les Labidoures. Appareil de la génération. (Ibid., 1828, xiii, pp. 354–359, 2 Pls.)

—— Recherches anatomiques et considérations entomologiques sur quelques insectes coléoptères, compris dans les familles des Dermestins, des Byrrhiens, des Acanthopodes et des Leptodactyles. Appareil génital. (Ibid., Sér. 2, Zool. i, 1834, pp. 76–82, 2 Pls.)

—— Résumé des recherches anatomiques et physiologiques sur les Hémiptères. (Ibid., Sér. 2, i, pp. 232–239.)

—— Mémoire sur les métamorphoses et l’anatomie de la _Pyrochroa coccinea_. Appareil génital. (Ibid., Sér. 2, Zool., xiii, pp. 337–339, 1 Pl.)

—— Histoire des métamorphoses et de l’anatomie des Mordelles. (Ibid., Sér. 2, xiv, pp. 235–238, 1 Pl.)

—— Anatomie générale des Diptères. Appareil génital. (Ibid., Sér. 3, Zool., i, 1844, pp. 250–264.)

—— Histoire des métamorphoses et de l’anatomie du _Piophila petasionis_. Appareil génital. (Ibid., Sér. 3, Zool., i, 1844, pp. 378–386.)

—— Études anatomiques et physiologiques sur les insectes diptères de la famille des Pupipares. Appareil génital. (Ann. Sc. nat., Sér. 3, Zool., iii, 1845, pp. 73–93, 2 Pls.)

—— Recherches sur l’anatomie et l’histoire naturelle de _l’Osmylus maculatus_. Appareil génital. (Ibid., Sér. 3, Zool., ix, 1848, pp. 349–356, 1 Pl.)

—— Recherches anatomiques sur les Hyménoptères de la famille des Urocerates. Appareil génital. (Ibid., Sér. 4, Zool., i, 1854, pp. 216–234, 1 Pl.)

—— Fragments d’anatomie entomologique. Sur les ovaires du _Nemoptera lusitanica_. (Ibid., Sér. 4, viii, 1857, pp. 9–10, 1 Pl.)

—— Fragments anatomiques sur quelques Élatérides. (Ibid., Sér. 4, viii, 1857, pp. 365–372, 1 Pl.)

—— Recherches anatomiques et considérations entomologiques sur les Hémiptères du genre Leptopus. Appareil génital. (Ibid., Sér. 10, 1858, pp. 356–362, 1 Pl.)

—— Recherches anatomiques sur _l’Ascalaphus meridionalis_. Appareil génital. (Ibid., Sér. 4, xiii, 1860, pp. 203–206, 1 Pl.)

—— Sur l’appareil génital male du _Coræbus bifasciatus_. (Thomson’s Archiv Ent., 1857, i, pp. 378–381.)

=Suckow, F. W. L.= Geschlechtsorgane der Insekten. (Heusinger’s Zeitschr. f. organ. Physik., 1828, ii, pp. 231–264, 1 Taf.)

=Rathke, M. H.= Miscellanea anatomico-physiologica. Fasc. 1. De Libellarum partibus genitalibus. Regiomonti, 1832, p. 38, 3 Pls.

=Dutrochet, R. J. H.= Observations sur les organes de la génération chez les pucerons. (Ann. Sc. nat., 1833, xxx, pp. 204–209.)

=Doyère, L.= Observations anatomiques sur les organes de la génération chez la Cigale femelle. (Ann. Sc. nat., 1837, vii, pp. 200–206, Fig.)

=Siebold, C. Th. E. von.= Ueber die weiblichen Geschlechtsorgane der Tachinen. (Wiegmann’s Archiv f. Naturgesch., 1838, iv, pp. 191–201.)

—— Ueber die inneren Geschlechtswerkzeuge der viviparen und oviparen Blattläuse. (Froriep’s Notizen, 1839, xii, pp. 305–308.)

—— Ueber das Receptaculum seminis der Hymenopteren-Weibchen. (Germar’s Zeitschr. f. Ent., 1843, iv, pp. 362–388, 1 Taf.)

=Loew, H.= Beitrag zur anatomischen Kenntniss der inneren Geschlechtsteile der zweiflügligen Insekten. (Germar’s Zeitschr. f. Ent., 1841, iii, pp. 386–406, 1 Taf.)

—— Horæ anatomicæ, Abth. I. Entomotomien. Heft i-iii, Posen, 1841.

—— Beiträge zur Kenntniss d. inneren Geschlechtstheile der zweifl. Insecten. (Germar’s Zeitschr. f. Entomologie, iii, 1841, pp. 386–406, 1 Taf.)

=Stein, F.= Vergleichende Anatomie und Physiologie der Insekten. I, Monographie. Ueber die Geschlechtsorgane und den Bau des Hinterleibes bei den weiblichen Käfern, Berlin, 1847, i, pp. 139, 9 Taf.

=Brauer, F.= Beitrag zur Kenntniss des inneren Baues und der Verwandlung der Neuropteren. (Verhandl. d. zool. botan., Vereins in Wien, 1855, pp. 1–26, 5 Taf.)

=Haliday, A. H.= Note on a peculiar form of the ovaries observed in a hymenopterous insect, constituting a new genus and species of the family Diapriadæ. (Nat. Hist. Review, 1857, iv, pp. 166–174, 1 Pl.)

=Laboulbène, A.= Recherches sur les appareil de la digestion et de la reproduction du _Buprestis manca_. (Thomson’s Archiv Ent., 1857, i, pp. 204–236, 2 Pls.)

=Lubbock, John.= On the ova and pseudova of insects. (Phil. Trans. Roy. Soc., London, cxlix, 1860, pp. 341–369.)

=Landois, H.= Ueber die Verbindung der Hoden mit dem Rückengefäss bei den Insekten. (Zeitschr. f. wissens. Zool., xiii, 1863, pp. 316–318, 1 Taf.)

=Leydig, F.= Der Eierstock und die Samentasche der Insekten. (Nova Acta Acad. Leop.-Carol., xxxiii, 1867, pp. 88, 5 Taf.)

—— Beiträge zur Kenntniss des thierischen Eies im unbefruchteten Zustande. (Spengel’s Zool. Jahrbücher, 1889. Abth. f. Anat., iii, pp. 287–432, 7 Taf.)

=Bessels, E.= Studien über die Entwicklung der sexual Drüsen bei den Lepidopteren. (Zeitschr. wissens. Zool., xvii, 1867, pp. 545–563.)

=Rajewsky.= Ueber die Geschlechtsorgane von _Blatta orientalis_, etc. (Nachr. d. k. Gesellschaft d. Moskauer Universität, xvi, 1875. Testes of cockroach. In Russian; for abstract, see Hoffmann u. Schwalbe, Jahresbericht, 1875, p. 425.)

=Brehm, Siegfr.= Comparative structure of the reproductive organs in _Blatta germanica_ and _Periplaneta orientalis_. (Horæ Ent. Soc. Rossicæ, St. Petersburg, viii, 1880.) (In Russian, male organs only.)

=Cholodkowsky, N. A.= Ueber die Hoden der Schmetterlinge. (Zool. Anzeiger, iii Jahrg., 1880, pp. 115–117.)

—— Ueber den Bau der Testikel bei Schmetterlingen. (Zool. Anzeiger, 1880, iii, pp. 214–215.)

—— Ueber die Hoden der Lepidopteren. (Zool. Anzeiger, 1884, pp. 564–568.)

—— Ueber den Geschlechtsapparat von _Nematois metallicus_. (Zeitschr. f. wissens. Zool., xliii, pp. 559–568, 1885.)

=Tichomirow, A.= Ueber den Bau der Sexualdrüsen und die Entwickelung der Sexualprodukte bei _Bombyx mori_. (Zool. Anzeiger, iii, 1880, pp. 235–237.)

=Nusbaum, F.= Zur Entwicklungsgeschichte der Ausführungsgange der Sexualdrüsen bei den Insekten. (Zool. Anzeiger, v, 1882, pp. 637–643.)

——On the developmental history of the efferent passages of the sexual glands in insects. Lemberg, 1884 (in Czech).

=Berlese, Ant.= Ricerde sugli organi genitali degli ortotteri. (Atti della R. Acad. dei Lincei. Ser 3, xi, 1882.) (Genital organs of European Orthoptera.)

=Balbiani, G.= Le Phylloxera du chêne et le Phylloxera de la vigne. Paris, 1884, pp. 45, 11 Pls.

—— Contribution à l’étude de la formation des organes sexuel chez les insectes. (Recueil Zool. Suisse, 1885.)

=Schneider, Anton.= Die Entwicklung der Geschlechtsorgane bei den Insekten. Zool. Beiträge, Breslau, i, 1885.

=Beauregard, H.= Recherches sur les insectes vésicants. Suite. (Journal Anat. Phys., Paris, 1887, xxii Année, pp. 528–548, 1 Pl.; xxiii Année, pp. 124–163, 6 Pls.)

—— Les insectes vésicants. Paris, 1890. Chap. v, Appareil de la génération, pp. 103–159, Pls. 10–12.

=Nassonow, N.= Études morphologiques sur les Lepisma, Campodea et Podura. (Mém. Soc. Imp. Anthropologie et d’Ethn. Moscou, iii, 1887, pp. 85, 2 Pls., 68 Figs.)

—— _Xenos rossii_; seine Anatomie und Entwicklungsgeschichte. (Bull. de l’Université de Varsovie, 1892, pp. 74, 2 Taf.) (In Russian.)

—— Position des Strepsiptères dans le systeme selon les données du développement post-embryonal et de l’anatomie, p. 11, Warsaw, 1892.

=Grassi, B. J.= Progenitori dei Miriapodi e degli Insetti. Memoria vii, Anatomia comparata dei Tisanuri. (Atti d. R. Acad. de’ Lincei, Cl. scienc. e fis., Serie 4, iv, 1888, pp. 435–606, 5 Pls.)

=Heymons, R.= Ueber die hermaphroditsche Anlage der Sexualdrüsen beim Mannchen von _Phyllodromia germanica_. (Zool. Anzeiger, 1890, pp. 451–457.)

=Koschewnikoff, G.= Zur Anatomie der männlichen Geschlechtsorgane der Honigbiene. (Zool. Anzeiger, xiv, 1891, pp. 393–396.)

=Verhoeff, C.= (See p. 186.)

=Ingenitzky, J.= Zur Kenntniss der Begattungsorgane der Libelluliden. (Zool. Anzeiger, 1893, xvi Jahrg., pp. 405–407, 2 Figs.)

—— On the fauna and organization of dragon-flies of Russian Poland, 1893, 1 Pl. (In Russian.)

=Escherich, K.= Anatomische Studien über das männliche Genitalsystem der Coleopteren. (Zeitschr. f. wissens. Zool., lvii, pp. 620–641, 1894.)

=Kluge, Max H. E.= Das männliche Geschlechtsorgan von _Vespa germanica_. Inaug. Diss. Leipzig, 1895, pp. 1–45, 1 Taf.

=Verson, E.= La borsa copulatrice nei Lepidotteri (Atti e Mem. Accad. Sc. Lett. ed Arti, Padova, 1896, xii, pp. 369–372, 4 Pls.).

=Klapálek, Fr.= Über die Geschlechtstheile der Plecopteren, mit besonderer Rücksicht auf die Morphologie der Genitalanhänge. (Sitzungsb. k. Akad. Wissens. Wien. Math.-Naturw. Cl., cv, 1896, pp. 56, 5 Taf.)

=Fenard, A.= Recherches sur les organes complémentaires internes de l’appareil génital des Orthoptères. (Bull. Sc. France Belg., xxix, 1897, pp. 390–527, 528–533, 5 Pls.)

Consult also the Works of Ayres, Balfour, Burgess, Burmeister, Bütschli, Claus, Dzierzon, Gensch, Henking, Honert, Huxley, Kluge, Kramer, Landois, Leuckart (art. Zeugung), Leydig, Ludwig, Metschnikoff, H. Meyer, Minot, Müller, Pfitzner, Schneider, Seeliger, Scholz, Siebold, Suckow, Swammerdam, Tichomiroff, Wagner, Waldeyer, Weismann, Wheeler, v. Wielowiejski, Will, Witlaczil, and Ziegler.

_b._ Formation of the egg (oögenesis)

=Claus, C.= Beobachtungen über die Bildung des Insekteneies. (Zeitschr. wissens. Zool., xiv, 1864, pp. 42–54.)

=Brandt, A.= Ueber die Eiröhren der _Blatta orientalis_. (Mém. Acad. Imp. Scienc. de St. Petersbourg, Sér. 7, xxi, 1874, p. 30.)

—— Vergleichende Untersuchungen über die Eiröhren und die Eier der Insekten. (Nachr. d. Gesellsch. Freunde d. naturwiss. Moskau, xxiii, 1876; also xxiv, 1877, pp. 77–79.)

—— Das Ei und seine Bildungsstatte. Ein vergleichenden-morphologischer Versuch mit Zugrundelegung der Insecteneies. Leipzig, 1878.

=Kadyi, H.= Beiträge zur Vorgänge beim Eierlegen der _Blatta orientalis_. Vorläufige Mittheilung. (Zool. Anzeiger, 1879, pp. 632–636.) (Formation of the egg-capsules of cockroach.)

=Brass, Arn.= Das Ovarium und der Eibildung und der ersten Entwicklungsstadien bei viviparen Aphiden. Halle, 1883. (Zeits. f. Naturwiss. in Halle, Jahrg. 1882.)

—— Zur Kenntniss der männlichen Geschlechtsorgane der Dipteren. (Zool. Anzeiger, 1892, pp. 178–180.)

=Will, Ludvig.= Zur Bildung der Eies und des Blastoderms bei den viviparen Aphiden. (Arbeiten Zool. Inst. Univ. Würzburg, vi, 1882, pp. 217–258.)

=Korschelt, E.= Zur Frage nach dem Ursprung der verschiedenen Zellenelemente der Insectenovarien. (Zool. Anzeiger, 1885, pp. 581–586, 599–605.)

=Wielowiejski, H. V.= Zur Morphologie des Insektenovariums. (Zool. Anzeiger, 1886, ix Jahrgang, pp. 132–139.)

—— Zur Kenntniss der Eibildung bei der Feuerwanze (_Pyrrhocoris apterus_). (Zool. Anzeiger, 1885, pp. 369–375.)

=Blochmann, F.= Ueber die Richtungskörper bei Insekteneiern. (Morph. Jahrb., 1887, xii, ix 544.)

Also the writings of Leydig (p. 509).

_c._ On the spermatozoa

=Treviranus, G. R.= Ueber die organischen Körper des tierischen Samens und deren Analogie mit dem Pollen der Pflanzen. (Zeitschr. f. d. Physiologie, von F. Tiedemann, G. R. und L. C. Treviranus, 1835, v, pp. 136–153, 2 Taf.)

=Siebold, C. Th. E. von.= Ueber die Spermatozoen der Crustaceen, Insekten, Gasteropoden und einiger anderer wirbellosen Tiere. (Müller’s Archiv f. Anatomie, 1836, pp. 13–52, 2 Taf.)

—— Fernere Beobachtungen über die Spermatozoen der wirbellosen Tiere. (Ibid., 1836, p. 232; 1837, pp. 381–432, 1 Taf.)

—— Ueber die Spermatozoen der wirbellosen Tiere, iv. (Ibid. 1837, pp. 392–433.)

—— Lange Lebensdauer der Spermatozoen bei _Vespa rufa_. (Wiegmann’s Archiv f. Naturgesch., 1839, v, pp. 107, 108.)

—— Ueber die Spermatozoiden der Locustinen. (Nova Acta Acad. Leop.-Carol., 1845, xxi, pp. 249–274, 1 Pl.)

=Kölliker, A.= Beiträge zur Kenntnis der Geschlechtsverhältnisse und der Samenflüssigkeit wirbelloser Tiere, nebst einem Versuch über das Wesen und die Bedeutung der sogenannten Samentiere, Berlin, 1841, pp. 88, 3 Taf.

—— Die Bildung der Samenfaden in Bläschen als allgemeines Bildungsgesetz. (Neue Denkschr. d. allg. Schweiz. Ges., viii, 1847, pp. 28, 3 Taf.)

—— Physiologische Studien über die Samenflüssigkeit. (Zeitschr. f. wissens. Zool., vii, 1856, pp. 201–272, 1 Taf.)

=Yersin, A.= Observations sur le _Gryllus campestris_. (Bull. Soc. Vaudoise sc. nat., 1853, iii, pp. 128.)

=Lespès, Ch.= Mémoire sur les spermatophores des grillons. (Ann. Sc. nat., Sér. 4, iii, 1855, pp. 366–377, 1 Pl.; iv, pp. 244–249, 1 Pl.)

=Landois, H.= Entwicklung der büschelförmigen Spermatozoiden bei den Lepidopteren. (Schultze’s Archiv f. Anat. u. Physiol., 1866, pp. 50–58, 1 Taf.)

=Bütschli, O.= Vorlaufige Mitteilungen über Bau und Entwicklung der Samenfaden bei Insekten und Crustaceen. (Zeitschr. f. wissens. Zool., xxi, 1871, pp. 402–415.)

=Bütschli, O.= Nähere Mitteilungen über die Entwicklung und den Bau der Samenfaden der Insekten. (Ibid., xxi, 1871, pp. 526–534, 2 Taf.)

=La Vallette St. George, A. V.= Ueber die Genese der Samenkörper, III. Mitteilung. (Archiv f. Mikroscop. Anat., 1874, x, pp. 495–504, 1 Taf.)

—— Spermatologische Beitrage: II. Mitteilung (Ibid., 1886, xxvii, pp. 1–122 Taf.). IV. Mitteilung (Ibid., 1886, xxviii, pp. 1–13, 4 Taf.) V. Mitteilung (Ibid., 1887, xxx, pp. 426–434, 1 Taf.)

=Schneider, A.= Das Ei und seine Befruchtung, pp. 88, 10 Taf.; Arthropoden, pp. 57–68 und 79. Taf. 8–10, 1883.

=Wielowiejski, H. de.= Observations sur la spermatogénèse des Arthropodes. (Archiv Slav. de Biologie, 1886, ii, pp. 28–36.)

=Ballowitz, E.= Zur Lehre von der Struktur der Spermatozoen. (Anat. Anzeiger, i Jahrg., 1886, pp. 363–376.)

—— Untersuchungen über die Struktur der Spermatozoen, zugleiche in Beitrag zur Lehre von feineren Bau der kontraktilen Elemente. Die Spermatozoen der Insekten. I. Coleopteren. (Zeitschr. f. wissensch. Zool., 1, 1890, pp. 317–407, 4 Taf.)

—— Zu der Mittheilung des Herrn Professor L. Auerbach in Breslau über merkwürdiger Vorgänger am Sperma von _Dytiscus marginalis_. (Anat. Anzeiger, 1893, viii Jahrg., pp. 505–506.)

—— Die Doppelspermatozoen der Dyticiden. (Zeitschr. f. wissensch. Zool., lxvi, 1895, pp. 458–499, 5 Taf.)

=Beauregard, H.= Note sur la spermatogénèse chez la cantharide. (Compt.-rend. Soc. Biol., Paris, 1888, iv, pp. 331–333.)

=Gilson, G.= Étude comparée de la spermatogénèse chez les Arthropodes, in La Cellule, Recueil de Cytologie et d’Histologie gén., i, 1888, 8 Pl.

=Verson, E.= Zur Spermatogenesis. (Zool. Anzeiger, xii Jahrg., 1889, pp. 100–103, Fig.)

—— La spermatogenesi nel _Bombyx mori_. (Padova, 1889, 25 pp. und 3 Taf.)

—— Zur spermatogenesis. (Zool. Anzeiger, xii, 1889, pp. 100–103.)

—— Zur spermatogenesis bei der Seidenraupe. (Zeitschr. f. wissens. Zool., lviii, 1894, pp. 303–313, 1 Taf.)

=Henking, H.= Ueber Reductionsteilung der Chromosomen in den Samenzellen von Insekten. (Internat. Monatsschr. f. Anat. und Phys., 1890, vii, pp. 243–248.)

—— I. Untersuchungen über die erste Entwicklungsorgänge in der Eiern der Insekten. II. Ueber Spermatogenese und deren Beziehung zur Eientwickelung bei _Pyrrhocoris apterus_. (Zeits. wissens. Zool., xlix, 1890, pp. 503–564; li, 1891, pp. 685–736.) III. Specielles und Allgemeines. (Ibid., 1892, liv, pp. 1–274, 12 Taf.)

=Sabatier, A.= De la spermatogénèse chez les Locustides. (Comptes rend. Acad. Paris, 1890, cxi, p. 797.)

=Cholodkowsky, N.= Zur Frage über die Anfangsstadien der Spermatogenese bei den Insecten. (Zool. Anzeiger, 1894, pp. 302–304.) See also Zool. Anzeiger, 1892, p. 179.

=Auerbach, Leopold.= Ueber merkwürdige Vorgänge am Sperma von _Dytiscus marginalis_. (Sitz. Ber. Akad., Berlin, 1893, pp. 185–203, 2 Figs.)

—— Zu dem Bemerkungen des Herrn Dr. Ballowitz betreffend das Sperma von _Dytiscus marginalis_. (Anat. Anzeiger, viii Jahrg., 1893, pp. 627–630.)

=Toyama, K.= On the spermatogenesis of the silkworm. (Bull, ii, No. 3, Coll. Agric. Imp. Univ. Tokyo, pp. 125–157, 1894, 2 Pls.)

=Wilcox, E. V.= Spermatogenesis of _Caloptenus femur-rubrum_ and _Cicada tibicen_. (Bull. Mus. Comp. Zool., xxvii, 1895, pp. 32, 5 Pls.)

—— Further studies on the spermatogenesis of _Caloptenus femur-rubrum_. (Ibid., xxix, 1896, pp. 193–202, 3 Pls.)

=Wilson, Edmund B.= The cell in development and inheritance. (New York, 1896.) Also the writings of Platner, Waldeyer.

_d._ On the paired genital efferent passages

=Loew, H.= Abbildungen und Bemerkungen zur Anatomie einiger Neuropterengattungen. (Linnæa Ent., 1848, pp. 345–385, 6 Taf.)

=Meinert, F.= Anatomia Forficularum, i, Kjöbenhavn, 1863, 1 Pl.

—— Om dobbelte Saedgange hos Insecter. (Naturhist. Tidsskrift, 3 Raekke, v, 1868, pp. 278–294.)

=Palmén, J. A.= Zur vergleichenden Anatomie der Ausführungsgänge der Sexualorgane bei den Insekten. (Morph. Jahrb., ix, 1883, pp. 169–176.)

—— Ueber paarige Ausführungsgänge der Geschlechtsorgane bei Insekten. Eine morphologische Untersuchung. Helsingfors, 1884, pp. 108 und 5 Taf.

=Nusbaum, J.= Zur Entwicklungsgeschichte der Ausführungsgänge der Sexualdrüsen bei Insekten. (Kosmos, Lemberg, 1884, ix Jahrg., pp. 256–266, 393–408, 462–474, 2 Taf. In Polish with résumé in German.)

=Spichardt, C.= Beitrag zur Entwickelung der männlichen Genitalien und ihrer Ausführgänge bei Lepidopteren. (Verhandl. d. naturwiss. Vereins zu Bonn, 1886, xliii Jahrg., pp. 1–34, 1 Taf.)

=Jackson, W. H.= Studies in the morphology of the Lepidoptera. I. (Zool. Anzeiger, xii Jahrg., 1889, pp. 622–626.) (See p. 389.)

=Lowne, B. Th.= On the structure and development of the ovaries and their appendages in the blow-fly (_Calliphora erythrocephala_). (Journ. Linn. Soc. London, 1889, xx, pp. 418–442, 1 Pl.)

See also Meinert (1897), Heymons (1897).

END OF PART I