LETTER XLII.

_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED._

REPRODUCTION.

The reproductive organs of insects in their _general_ denominations and functions correspond with those of the higher classes of animals; but as to _number_, _proportions_, and other _particular_ details of their structure, they differ from them very considerably. I shall not now, however, enter at large upon this subject, but confine myself principally to the consideration of those organs in the female which are appropriated to the formation, fecundation, maturation, exclusion and deposition of their eggs, and other circumstances relating to that subject. The organs connected with this function are the _Sperm-reservoir_; the _Oviduct_; the _Ovaries_; and the _Ovipositor_.

I. The Sperm-reservoir (_Spermatheca_) is an organ connecting the vagina with the oviduct, which, according to Herold, receives the male sperm as into a reservoir[703], and fecundates the eggs in their transit through that passage. This vessel, which consists of a double tunic, in the cabbage-butterfly terminates the vagina, and is connected with the oviduct by a lateral undulating tube: in shape it is a rather irregular oblong, and is surmounted by a small orbicular vesicle, connected by a short tubular footstalk with the main reservoir[704]. A similar organ was discovered by Malpighi in the imago of the silkworm, who denominates it the _uterus_; to which indeed it seems analogous, and which he also regards as a reservoir for the sperm for the gradual fecundation of the eggs[705]. But in that fly the organ is of a rather different shape, and the interior vessel terminates in several spherical vesicles[706]. John Hunter by the most decisive experiments, such as covering the eggs of the unimpregnated moth, after exclusion, with the liquor taken from the spermatheca in those which had been impregnated, and rendering them fertile, he demonstrated that this organ was a reservoir for the spermatic fluid, to impregnate the eggs as they were ready for exclusion, and that coition and impregnation were not simultaneous[707]. It is not improbable that in all insects whose eggs are gradually laid, this provision for their gradual fecundation, if carefully sought for, might be detected[708]. Rifferschweils is of opinion, that in these cases the eggs are fertilized in their transit through the oviduct by sperm adhering to the folds of the _cloacæ_[709]: but this opinion seems less analogous to what takes place in other cases, with regard to the due preparation of the eggs for a safe and effectual transit[710].

II. The Oviduct (_Oviductus_) is the canal, always separate from the vagina, which receives the eggs from the ovary, transmitting them, often by a peculiar and complex instrument in which it terminates, to their proper station. This canal sometimes opens into the anal passage or _cloaca_, and at others, as in the cabbage-butterfly[711], is distinct, and lies between the sexual organ and the anus. In the _Arachnida_ there are _two_ oviducts[712].

III. The Ovaries (_Ovaria_) in insects are the viscera in which the eggs are generated and grow till they arrive at maturity, when they pass through the oviduct, and are extruded or deposited in their appropriate station. They vary considerably in their structure. In all however, except the _Iulidæ_, in which there is only a _single_ ovary[713], the oviduct at its upper or inner extremity terminates in _two_ branches, usually further subdivided into a number of smaller conical ones, which several ramifications constitute the _ovaries_, or egg-tubes as they are sometimes called: these tubes generally consist of a single membrane, and are joined to the oviduct by membranous rugose _cloacæ_[714]: in the _Phalangia_, however, there are _two_ tunics; the outer one of a cellular substance, and the inner one consisting of spiral fibres like _tracheæ_--a kind of structure which renders them capable of great extension[715]. Rifferschweils considers the ovaries as formed upon _two_ primary types.--_First_, _flagelliform_ ovaries, consisting of conical tubes equal in length, and inserted at the same place at the end of the primary branches as in the _Lepidoptera_, the Bee, &c. _Secondly_, _racemose_ ovaries, consisting of short conical tubes, so proceeding from the primary branches as to render the ovary racemose or pinnated, as in certain _Neuroptera_, _Coleoptera_, and _Diptera_[716]: but perhaps their structure will be better understood if they are divided into _agglomerate_ ovaries and _branching_ ovaries: in the _first_ the egg-tubes form _two_ bundles, in which the branches are not discernible, as in the _Ephemera_, the chamæleon-fly, and spiders[717]: and in the _second_ the branches are distinct, as in the _Lepidoptera_ and the majority of insects.

The number of branches varies in different genera and species. In _Echinomyia grossa_, a large fly, there are only the _two_ primary branches[718]; in the common dung-beetle (_Geotrupes stercorarius_) these appear divided at their apex into fingers[719]: in _Scolia_, a Hymenopterous genus, and the butterfly of the nettle, there are _three_ secondary branches on each side[720]: in many other _Lepidoptera_ and the humble-bee there are _four_[721]; in the common louse there are _five_[722]; in the rhinoceros-beetle and the cockchafer, _six_[723]; in the wasp, _seven_[724]; _eight_ in the cockroach[725]; _twelve_ in the _Carabi_ and the mealworm-beetle[726]; _thirty_ in the large green grasshopper (_Acrida viridissima_[727]); _thirty-two_ in the cheese-maggot-fly[728]; and in the hive-bee more than _a hundred and fifty_[729].

The number of _eggs_ also contained in the ovaries varies. In _Echinomyia grossa_ there is only _one_ egg in each, and only _two_ at once in the matrix[730]: in another fly produced by the cheese-maggot there are _four_[731]; in the louse there are _five_; in the cockchafer _six_[732]; in the hive-bee _sixteen_ or _seventeen_ are visible at the same time[733]; and in the silkworm-moth _sixty_ or _seventy_[734]. Besides the eggs, the tubes contain a pellucid mucus, and at their upper extremity the eggs are lost in a granular mucous mass, in which, however, they may still be discovered with a microscope[735]. With regard to the termination of the ovaries or egg-tubes internally,--in those that have agglomerated ones it is not to be traced, the whole appearing like an oblong obtuse or acute body[736]: but in the branching ones it is more easily traced; at first they converge in most cases to a point; this is seen to advantage in the caterpillar of some butterflies, when near assuming the pupa, in which they are readily discovered, and represent with great truth and elegance the bud of some blossom[737]; but in time they diverge, and sometimes become convoluted[738]; they generally terminate in a slender simple filament, but in the louse in a fork[739]; they are sometimes extremely long, as in the wasp and _Lepidoptera_[740]; in the hive-bee they appear to be shorter[741].

IV. We are next to consider the _Ovipositor_, or instrument by which numerous insects are enabled to introduce their eggs into their appropriate situations, and where the new-born larva may immediately meet with its destined food. As this instrument is one of the most striking peculiarities with which the wisdom of the CREATOR has gifted these little animals, and in many cases is extremely curious and wonderful, both in its structure and modes of operation--though on a former occasion I gave you a brief account of several kinds of them[742], I shall now enter more at large into the subject, and describe these often complex machines, as they are exhibited in most of the different Orders of insects.

With regard to the _Coleoptera_ Order, there are doubtless numerous variations in the structure of this organ; but very few have been noticed, and those chiefly belong to insects whose grubs feed on timber. In these it is usually retractile one part within another, like the pieces of a telescope: in _Buprestis_ it consists of three long and sharp _laminæ_, the two lateral ones forming a sheath to the intermediate one, which probably conveys the eggs[743]: in _Elater_ it is a cylindrical organ, terminating in a pair of conical joints, which seem to form a forceps, and including a tube probably conveying the egg to the forceps, which perhaps introduces it[744]. The Ovipositor of _Prionus coriarius_ differs from that of _Callidium violaceum_, and many Capricorns before described[745]: it consists merely of a long bivalve piece ending in a kind of forceps, and hollowed above into a channel for the passage of the eggs[746].

In the _Orthoptera_ the instrument of oviposition is more simple; in _Locusta_ consisting merely of four robust three-sided pieces, two above and two below, the former pair at the end curving upwards and the latter downwards[747], these pieces seem calculated when they have entered the earth to enlarge the burrow, and the animal appears able to separate them very widely from each other[748]. The ovipositor of _Acrida viridissima_, which like that of many Hymenopterous insects forms a kind of appendage or tail to the body, has been described both by De Geer and Latreille as consisting of _two_ valves only[749]; but in reality it consists of _six_, two upper and four lower, as you may ascertain by means of a pin or the point of a penknife, which will readily separate them. This is confirmed by a figure of Stoll's of a species which seems to connect _Conocephalus_ with _Gryllus_. In this the ovipositor is considerably longer than the body of the animal, and is composed of _six_ distinct pieces; viz. _two_ external ones stouter than the rest, and within these _four_ others finer than a hair and convolute at the apex[750]. There is a considerable variety in the shape of the ovipositors of the _Acridæ_ and the cognate genera:--thus in _A. viridissima_ this organ is straight, in _A. verrucivora_ bent like a sabre, and in _Pterophylla citrifolia_ and some others, the whole machine is short and boat-shaped; in _Scaphura Vigorsii_ it is also rough with sharp little tubercles[751]. I had an opportunity of observing, with respect to the first of these insects, that in boring, as is the case with the _Cicadæ_ and saw-flies, the motion of the valves was alternately backwards and forwards. It appeared also to me that the two outer pieces of each of the apparent valves were fixed in a groove in the margin of the intermediate one. I saw this clearly with respect to the _upper_ pieces, and it is most probable that the lower are similarly circumstanced. In the cricket tribe (_Gryllus_) the ovipositor is as long as the abdomen, very slender, terminating in a knob[752]. It is _apparently_ bivalve like that of _Acrida_, but I believe is resolvable into the same number of pieces.

In the _Homopterous Hemiptera_ there seems to be more than one type on which the ovipositor is constructed. In an insect very common with us, the froth froghopper (_Cercopis spumaria_), some approach is made to the ovipositors last described, at least the number of pieces is the same--for it has a pair of external valves forming a sheath, which includes three sharp _laminæ_ resembling the blades of a lancet, the middle one of which can be separated into two; this instrument De Geer had reason to think was scored transversely like a file[753]. In the insects of this Order so noted for their song[754] (_Cicada_), there are only _five_ pieces; namely, two valves forming the sheath, two augers or borers, and an intermediate piece upon which they slide, each being furnished with an internal groove for that purpose, and the middle piece with a ridge to fit; a contrivance of Divine Wisdom, to prevent their dislocation when employed in boring; the augers terminate in a knob which is externally toothed[755]. This structure approaches that of the _Hymenoptera_, especially the saw-flies. With regard to the _Heteropterous_ section of this Order--as they usually do not introduce their eggs _into_ any substance, they have no call for any remarkable ovipositor, and therefore are not so furnished. A remark which will also apply to the _Lepidoptera_ Order.

In the _Libellulina_ amongst the _Neuroptera_, an organ of this kind is sometimes discoverable. In _Agrion_, Reaumur noticed a part which he conjectured to be an _ovipositor_; it consists of four _laminæ_ or lancets, the interior pair slender, the exterior wider, and all externally serrated[756].

The insects of the _Hymenoptera_ Order have long been celebrated for the organs we are describing, whether used as _saws_, _augers_, or _darts_. I formerly gave you a very _general_ account of the _saws_,--I shall now give you a very interesting one in _detail_ copied from an admirable little essay of Professor Peck. "This instrument," says he, "is a very curious object; and in order to describe it it will be proper to compare it with the _tenon-saw_ used by cabinet-makers, which being made of a very thin plate of steel, is fitted with a back to prevent its bending. The back is a piece of iron, in which a narrow and deep groove is cut to receive the plate, which is fixed: the saw of the _Tenthredo_ is also furnished with a back, but the groove is in the plate, and receives a prominent ridge of the back, which is not fixed, but permits the saw to slide forward and backward as it is thrown out or retracted. The saw of artificers is _single_, but that of the _Tenthredo_ is _double_, and consists of two distinct saws with their backs: the insect in using them, first throws out one, and while it is returning pushes forward the other; and this alternate motion is continued till the incision is effected, when the two saws receding from each other, conduct the egg between them into its place. In the artificial saw the teeth are alternately bent toward the sides, or out of the right line, in order that the fissure or kerf may be made sufficiently wide for the blade to move easily. To answer this purpose in some measure, in that of the _Tenthredo_ the teeth are a little twisted, so as to stand obliquely with respect to the right line, and their point of course projects a little beyond the plane of the blade, without being laterally bent; and all those in each blade thus project a little outwards: but the kerf is more effectually made, and a free range procured for the saws, by small teeth placed on the outer side of each; so that while their vertical effect is that of a _saw_, their lateral effect is that of a _rasp_. In the artificial saw the teeth all point outward (_towards the end_) and are simple; but in the saw of the _Tenthredo_ they point inward, or toward the handle, and their outer edge is beset with smaller teeth which point outwards (_towards the end_)[757]." Valisnieri, Reaumur, and De Geer describe the groove as being in the back; but in Mr. Peck's insect, if there is no error in his account, it is, as in the _Cicadæ_, in the saw itself[758]. In the genus _Cimbex_, belonging to the same tribe, the saw differs in shape, being somewhat _sigmoidal_ or resembling the letter S, while in that of other saw-flies it is _cultriform_ with a concave edge: other minor differences distinguish them, which need not be particularized.

A similar structure, with regard to the organ in question, obtains in the rest of the _Hymenoptera_, even those that use it as a weapon of offence; but the backs of the saws in them, composed of a single piece, become a sheath for the darts. The valves, however, vary. In most of those with an exerted sting, as _Pimpla_, they are linear, exerted, and as long as the aculeus itself[759]. In _Proctotrupes_ they appear to be united so as to form a tube for the ovipositor, and are produced by a prolongation of the last abdominal segment. The darts usually run in two grooves of the sheath, and at their apex are retroserrulate[760]. In some cases the sheath itself is serrated[761]. The shanks of the darts are connected with the valves; so that when these open they are pushed out: sometimes on their outer side they have a triangular plate towards the base, which prevents their being pushed out too far[762].

In _Sirex_ and many ichneumons, in which the ovipositor is too long to be withdrawn within the abdomen, it remains always exerted; but in general it is retracted within that part when unemployed. In the gall-fly (_Cynips_) this instrument is really as long as in _Pimpla_, &c.; but as it is infinitely more slender, when in repose it is rolled up spirally and concealed within the abdomen. It is the puncture of this minute organ that produces the curious galls formerly described to you[763]. But the most anomalous ovipositor in this Order appears to be that of _Chrysis_ (_C. ignita_, &c.), which is covered by several demi-tubes or scales enveloping and sliding over each other: when these scales are removed, the true ovipositor appears, which is of a structure similar to that of the rest of the Order, but the valves are long and slender with their summit generally visible without the anus[764].

Though the ovipositor of the majority of _Dipterous_ insects is a tube with retractile joints[765], in the crane-flies this organ is different, and, like that of _Acrida_ above described, consists of what at first sight appear two valves, but each of which is formed of two pieces, the upper ones sharp and longer, and the lower pair blunt. The upper pair forms the auger that bores a hole in the ground, and the lower conducts the eggs into it after it is bored[766].

In the _Aptera_ and _Arachnida_ in general there seems no remarkable instrument of this kind; but Treviranus has described one in spiders for extruding the eggs of a singular construction. It is an oval plate lying between the external genitals and spinning organs, and is composed of a number of small screw-shaped cartilages, connected together in the most wonderful manner. There are few organs, he observes, in the animal kingdom which for their artificial mechanism can be compared with this. Each cartilage inosculates very closely in the adjoining one, and all are besides bound together by a strong skin[767].

The manner in which the eggs of insects are _fecundated_ by the male sperm is one of those mysteries of Nature that are not yet fully elucidated and understood. We can readily conceive that all the eggs may be fertilized by a single intercourse in the case of insects which, like the _Ephemeræ_ and _Trichoptera_, exclude the whole mass at once; or like many moths and butterflies, in a very short time afterwards; but the subject becomes much more difficult to explain when we advert to the female of the hive-bee, the whole number of whose eggs, deposited in _two years_, are, as Huber has demonstrated, in like manner fertilized by a single act[768]:--if you bear in mind, however, what I have lately observed with regard to Malpighi's discovery of a sperm-reservoir in insects, you will more readily comprehend how in this case a _gradual_ fecundation may take place. The principal objection to this solution of the difficulty in the case before us, is derived from the very small size of the organ supposed to be destined for this purpose--it being scarcely bigger than the head of a pin[769]: it seems therefore incredible that it should retain any portion of an extraneous fluid at the end of twelve or eighteen months, and still more unlikely that the fluid should in the interval have sufficed for the slightest moistening of not fewer than 30,000 or 40,000 eggs. The only hypothesis that seems at all to square with this fact, is that of Dr. Haighton,--that impregnation is the result not of any actual contact of the sperm with the eggs, but of some unknown sympathetic influence[770], or rather perhaps of some penetrating effluvia or _aura seminalis_, which, though small in quantity, it may retain the power of emitting for a long period.

Certain female moths, of the species of that family which, from the remarkable cases or sacs the larvæ inhabit, the Germans call _sack--träger_, before noticed[771], have been supposed to have the faculty of producing fertile eggs without any sexual intercourse; and various observers, after taking great pains, appeared to have satisfactorily proved the fact; so that some doubted whether these insects produced any males at all[772]. The enigma was at length explained by the accurate Von Scheven. At first his experiments were attended with the same result as those of his predecessors; but upon making them more carefully, and separating what he conceived to be the female from the male pupæ, he ascertained not only the existence of a _female_ in the species he examined (_Psyche vestita_), but that when thus secluded she laid _barren_ eggs; evidently proving that in the contrary instances above alluded to, an unperceived sexual intercourse must have taken place[773]. Though he thus ascertained that these insects do not in this respect deviate from the general rule, he remarked or confirmed several facts in their economy sufficiently anomalous and striking;--as that the female is not only without wings, but with scarcely any feature of a _moth_, much more closely resembling a _caterpillar_; and that in ordinary circumstances she never attempts to leave the pupa-case in which she has been disclosed, but that being there impregnated by the male, she there also, apparently after the manner of the female _Cocci_, deposits her eggs, which hatching produce young larvæ that make their way out of the case, and thus seem to originate without maternal interference[774].

But the most remarkable fact bearing upon this head, though as relating to a _viviparous_ insect it does not strictly belong to it, is the impregnation of the female _Aphides_, or plant-lice, before alluded to[775]. If you take a young female _Aphis_ at the moment of its birth, and rigorously seclude it from all intercourse with its kind, only providing it with proper food, it will produce a brood of young ones: and not only this; but if one of these be treated in the same way, a similar result will ensue, and so on, at least to the _fifth_ generation!! to which period Bonnet, who first made an accurate series of observations on this almost miraculous fact, successfully carried his experiments, till the approach of winter and the want of proper food forced him to desist[776]; and Lyonet extended it still further[777]. It is now generally admitted as an incontestible fact, that female _Aphides_ have the faculty of giving birth to young ones without having had any intercourse with the other sex. How are we to explain this most extraordinary fact? Are we to suppose with Bonnet that these insects are truly androgynous, as strictly uniting both sexes in one? This supposition, however, is completely overturned by the circumstance, that there are actually _male_ as well as _female Aphides_, and that these, as was first observed by Lyonet, are united towards the close of the summer in the usual manner[778]. The most likely supposition therefore is, that one conjunction of the sexes suffices for the impregnation of all the females that in a succession of generations spring from that union. It is true that at the first view this supposition appears incredible, contradicting the general laws and course of nature in the production of animals. But the case of the hive-bee, stated above, in which a single intercourse with the male fertilizes all the eggs that are laid for the space of _two_ years, and in the case of a common spider mentioned by Audebert[779], for _many_ years, shows that the sperm preserves its vivifying powers unimpaired for a long period, indeed a longer period than is requisite for the impregnation of all the broods that a female Aphis can produce; and if immediate contact with the fluid be not necessary, who can say that this is impossible? It is, however, one of those mysteries of the CREATOR that human intellect cannot fully penetrate. But this anomaly in nature is not wholly confined to the Aphides; since Jurine has ascertained that the same thing takes place with _Daphnia pennata_ Müll (_Monoculus Pulex_ L.), one of Branchiopod _Crustacea_[780]. It is worth observing whether the female _Aphides_ in their natural state, I mean those of the summer or viviparous broods, have intercourse with the male. I think I have noticed males amongst them; but they seem to become most numerous in the autumn, preparatory to the impregnation of the oviparous females. The object of this law of the CREATOR is probably the more ready multiplication of the species[781].

* * * * *

As to the period of _gestation_, most insects begin to lay their eggs soon after fecundation has taken place: but in some _Arachnida_, as the Scorpion, which seems to be both oviparous and ovo-viviparous, nearly a year intervenes, and the eggs increase to _four_ times the size which they had attained at that period, before they are extruded[782]. The time that is required to lay the whole they are to produce, varies also in _insects_. In this respect they may be divided into two great classes:--those namely which deposit the whole at _once_, as _Ephemerina_, _Trichoptera_, &c., and those which deposit them in _succession_, occupying in this operation a longer or shorter period. Many in the _first_ class, as the _Trichoptera_ or caseworm-flies, envelope their eggs in a gelatinous substance[783], which renders their extrusion in a mass more easy. Of the _second_ class, which includes by far the greater proportion of insects, some exclude the whole number in a very short period, others require two or three days or a week, as the cockroach[784]; and others, as the queen-bee, not less than two years. The eggs in the ovaries of the last vary infinitely in size; those that have entered the oviduct have arrived at maturity, while the rest grow gradually smaller as they approach the capillary extremity of the tubes, where they become at length invisible to the highest magnifier[785]. In many insects the eggs seem nearly to have reached their full growth previously to the exclusion of the female from the pupa; and this exclusion and the impregnation and laying of the eggs rapidly succeed each other. One moth (_Hypogymna dispar_), which is remarkable for the number of eggs she contains, sometimes deposits them, even before they are fecundated, in the pupa-case[786]. But in other cases the sexual union is not so immediate, and some time, longer or shorter, is requisite for the due expansion of the eggs; and the ovaries of the animal swell so much, as often to enlarge the abdomen to an extraordinary bulk: this is seen in a very common beetle (_Chrysomela Polygoni_) that feeds upon the knot-grass; but in no insect is it so striking as in the female of the white ants, whose wonderful increase of size after impregnation I have related to you on a former occasion[787].

* * * * *

I shall conclude this subject with a few observations upon _ovo-viviparous insects_; _supposed neuters_, and _hybrids_, which, though they do not fall in regularly under any of the foregoing heads, may very well have a place in this letter.

1. It has already been observed that there are a few _ovo-viviparous_ insects[788], the young of which exist in the ovaries at first as eggs, but are hatched within the body of the mother, and come forth in the living form of a larva and sometimes even of a pupa. Of the first description are certain _Diptera_, the _Aphides_, and the _Scorpion_.

Reaumur has described two modes in which the larvæ of the first are arranged in the matrix of the mother. In some they are heaped together without much appearance of order, being placed merely parallel to each other[789]; but in others they are arranged in a kind of riband--the length of the little animals, which are also parallel, forming its thickness--rolled up like the mainspring of a watch[790]. These larvæ in general are not divided into _two_ masses corresponding with the pair of ovaries in other insects, but form only a single one[791]. You must not suppose that these little fetuses lie naked in the womb of the mother; each has its own envelope formed of the finest membrane, which, however, is not entirely divided from that of those adjoining to it, but appears to be one tube, which becomes extremely slender between each individual, so as when drawn out to look like a chain[792]. Reaumur seems to have thought that in these flies the larvæ were never confined in any other case or egg[793]; but De Geer sometimes found _eggs_ in the body of _Sarcophaga carnaria_, though most generally larvæ, from which he conjectures that it is really _ovo-viviparous_, the eggs being hatched in the body of the mother[794]. As these flies are all carnivorous, and their office is to remove putrescent flesh, you may see at one glance the object of PROVIDENCE in this law of nature--that no time may be lost, and the animal exercise its function as soon as it is disclosed from the matrix.

The _Aphides_, so fruitful in singular anomalies, are ovo-viviparous, as I have before hinted[795], at one period of the year, that is during the summer, but strictly oviparous at its close. From the experiments of De Geer, however, upon _Aphis Rosæ_, it would appear that this faculty is not conferred upon the _same_ individuals, but only upon those of _different_ generations of the same species; all the generations being ovo-viviparous except the _last_, which is oviparous[796]: nor does it appear, as has been sometimes imagined, that it is common to the whole genus. De Geer observed a species in the fir, which makes curious galls resembling a fir cone (_Aphis Abietis_), which appeared never to be ovo-viviparous[797].

With regard to _scorpions_, it does not seem clear that they are _always_ ovo-viviparous: M. Dufour twice found in the midst of the eggs nearly mature, a young scorpion which appeared to him at large in the cavity of the abdomen; it was so large that it was difficult to comprehend how it could possibly be excluded from the animal, without an extraordinary operation[798]. The _pupiparous_ insects (_Hippobosca_, &c.) have been sufficiently noticed before[799].

2. I have already in several of my former letters stated to you what the modern doctrine of physiologists is with respect to certain individuals, usually forming the most numerous part of the community with insects living in society, that were formerly supposed to be _neuters_, or as to their sex neither male nor female--that they are in almost every instance a kind of abortive females, fed with a different and less stimulating food than that appropriated to those whose ovaries are to be developed, and in consequence in most instances incapable of conception[800]. Upon these sterile females, you also heard, devolve in general the principal labours of their respective colonies, showing the beneficent design of PROVIDENCE in exempting them from sexual cares and desires, and meriting for them the more appropriate name, now generally used, of _workers_. The differences in the structure of the female bee and the workers were also then accounted for; and similar reasoning may be had recourse to with regard to those of ants, in which the worker and the female differ still more materially. My reason for introducing this subject here, is to observe to you that I have some grounds for thinking that this system extends further than is usually supposed, and that to each species in some _Coleopterous_ and other genera there are certain individuals intermediate between the male and female; this I seem to have observed more especially in _Copris_ and _Onthophagus_. For in almost every British species in my cabinet of these genera I possess such an individual, distinguished particularly by having a horn on the head longer than that of the female, but much shorter than that of the male. I once observed a pair of _Pentatoma oleracea_, a very pretty bug, _in coitu_, both sexes being ornamented with _white_ spots, and by them stood a third distinguished from them by _red_ ones. I do not, however, build on this circumstance, though singular; but mention it merely that you may keep it in your eye. It would be curious should it turn up, that, to answer some particular end of PROVIDENCE, in some tribes of insects there are two kinds of _males_, as in the gregarious ones two descriptions of _females_.

I am, &c.

FOOTNOTES:

[703] Herold _Schmetterl._ tab. expl. vii.

[704] Herold _Schmetterl._ _t._ iv. _f._ 1. _x._ &c. PLATE XXX. FIG. 12. _d._

[705] _De Bombyc._ 36.

[706] _Ibid._ _t._ xii. _f._ 1. I. and, _f._ 2. O. M.

[707] _Philos. Trans._ 1792. 186.

[708] Swammerdam, in dissecting the female of _Oryctes nasicornis_, discovered a blind vessel opening into the vagina, and at the other or inner extremity not terminated by any secretory tube, containing a yellowish matter, that seems analogous to the organ mentioned in the text; and in the hive-bee he found a similar organ covered with air-vessels, which he supposes to be connected with the _Colleterium_ (see above, p. 132.), and which he states to contain a slimy matter. _Bibl. Nat._ i. 151. b. _t._ xxx. _f._ 10. _g._ 204. b. _t._ xxix. _f._ 3. _t._ Perhaps likewise the organ discovered by M. L. Dufour in _Scolia_,--which he imagines to belong to the poison-secretor, and which he describes as a sac consisting of a double tunic, the exterior one muscular and the interior membranous, and filled with a blueish-green gelatinous matter (_N. Dict. d'Hist. Nat._ xxx. 388.)--may be a _spermatheca_.

[709] _De Insector. Genital._ 17.

[710] I allude to those organs above described (p. 132.) for the secretion of matter for varnishing the eggs or lubricating the oviduct. It seems most probable, if the fecundation of the eggs takes place gradually, that upon their passing into the oviduct, a special reservoir should be appropriated to the reception of the male sperm, adapted to maintaining in due activity the vivifying principle, or _aura seminalis_.

[711] Herold _Schmett._ _t._ iv. _f._ 2. _m n._

[712] Treviran. _Arachnid._ 36. _t._ iv. _f._ 32. _aa._ Marcel de Serres in _Mém. du Mus._ 1819. 89.

[713] Marcel de Serres, _Mém. du Mus._ 1819. 115.

[714] Rifferschw. _De Genital. Ins._ 11.

[715] Marcel de Serres in _Mém. du Mus._ 1819. 109. PLATE XXX. FIG. 12. _a._

[716] Rifferschw. _ubi supr._ 23--. Swamm. _Bibl. Nat._ _t._ xlii. _f._ 8. _a, f, g, h._

[717] _Ibid._ i. 104. _t._ xv. _f._ 3. ii. 62. _t._ xii. _f._ 8. Treviran. _Arachnid._ _t._ iv. _f._ 32.

[718] Reaum. iv. 391.

[719] Posselt _Anat. der Ins._ _t._ i. _f._ 28, 29.

[720] _N. Dict, d'Hist. Nat._ xxx. 387--. Swamm. _ubi supr._ ii. 23. _t._ xxxv. _f._ 3.

[721] _Ibid._ i. 203.

[722] PLATE XXII. FIG. 2.

[723] Swamm. _ubi supr._ i. 151. Gaede _Anat. der Ins._ _t._ ii. _f._ 3.

[724] Swamm. i. 203.

[725] Gaede _Anat. der Ins._ 20. _t._ i. _f._ 9.

[726] _Ibid._ 25, 28. _t._ ii. _f._ 10.

[727] _Ibid._ 32.

[728] Swamm. ii. 74.

[729] _Ibid._ 203. _t._ xix. _f._ 3.

[730] Reaum. iv. 391--.

[731] Swamm. _t._ xliii. _f._ 19.

[732] Gaede 22.

[733] Swamm. _Bibl. Nat._ i. 203.

[734] _Ibid._

[735] Rifferschw. 11--.

[736] Swamm. _t._ xlii. _f._ 8. Gaede, _t._ i. _f._ 3. _cc._

[737] Herold _Schmett._ _t._ v. _f._ 10. 12.

[738] PLATE XXX. FIG. 12.

[739] PLATE XXII. _f._ 2. _b._

[740] Swamm. _t._ xix. _f._ 4. _b._

[741] _Ibid._ _f._ 3.

[742] VOL. I. p. 355--.

[743] De Geer iv. 127. _t._ iv. _f._ 17.

[744] De Geer iv. 143. _t._ v. _f._ 15.

[745] VOL. I. p. 357.

[746] De Geer. v. 62. _t._ iii. _f._ 12.

[747] PLATE XV. FIG. 18.

[748] Stoll _Sauterel._ _t._ xxii. b. _f._ 87, &c.

[749] De Geer iii. 418. _t._ xxi. _f._ 10, 11. Latr. _Gen. Crust. et Ins._ iii. 98.

[750] Stoll _ubi supr._ _t._ xiii. a. _f._ 51.

[751] This insect, which connects _Conocephalus_, _Acrida_, &c. with _Locusta_, is also distinguished by antennæ at first filiform and then setaceous.

[752] De Geer iii. _t._ xxiv. _f._ 1, 12.

[753] _Ibid._ 176. _t._ xi. _f._ 19.

[754] VOL. II. p. 397--.

[755] Reaum. v. 177--.

[756] _Ibid._ vi. 435. _t._ xl. _f._ 6, 7.

[757] _Natural History of the Slug-worm_, 12--. _f._ 12, 13.

[758] Valisn. _Esperienz. &c. Musca dé Rosai._ Reaum. v. 100--. De Geer ii. 916--. The last writer thought he saw in the back of the saw itself a longitudinal cavity (918), which applied to the groove would form an open canal.

[759] PLATE XVI. FIG. 1.

[760] Ibid.

[761] Reaum. v. 347. _t._ xlix. _f._ 10. _d, f._

[762] See above, VOL. III. 390. a.

[763] See above, VOL. I. 448--.

[764] De Geer ii. 835. _t._ xxviii. _f._ 20, 21. PLATE XV. FIG. 22. This figure was drawn by a friend--the organ seems more exerted than in De Geer's. I cannot make out the little appendage at the end.

[765] PLATE XVI. FIG. 2, 3.

[766] Reaum. v. 19--. _t._ iii. _f._ 3-6.

[767] _Arachnid._ 40.

[768] Huber _Nouvel. Observ._ i. 106.

[769] Swamm. _Bibl. Nat._ _t._ xix. _f._ 2.

[770] _Philos. Trans._ 1797. 80.

[771] VOL. I. p. 461.

[772] Compare Reaum. iii. 153. Pallas _Act. Nat. Cur._ 1767. iii. 430. _Wien. Verzeich._ 292.

[773] _Naturfor_ St^k. xx. 59--.

[774] It does not appear to be clearly decided whether the eggs are extruded from the female, or whether dying immediately after fecundation they are hatched within her body. As the young larvæ certainly are hatched in the pupa (not merely within the exterior case of bits of grass, &c., which includes it) which the body of the insect must fill, it does not seem easy to conceive how she can find room for oviposition; and yet Von Scheven expressly says that one female of _Ps. vestita_--which being kept from all access to the male actually left the pupa-case and wandered about the glass which contained them--laid unfruitful eggs.

[775] VOL. I. p. 32, 175.

[776] Bonnet i. 19--.

[777] Reaum. vi. 551.

[778] Reaum. vi. 552.

[779] _N. Dict. d'Hist. Nat._ ii. 284.

[780] _N. Dict. d'Hist. Nat._ ix. 125. Bonnet and Jurine both found that the female Aphides and Branchiopods that were fertile without the usual intercourse of the sexes were less fruitful than their mother, and those of the last generation less so than the first. Latr. _Hist. Nat. des Crust. et Ins._ xi. 292.

[781] See more on the subject of fecundation, VOL. II. p. 154--. 169--.

[782] _N. Dict. d'Hist. Nat._ xxx. 426.

[783] VOL. III. p. 68.

[784] De Geer iii. 533.

[785] Swamm. i. 203. b. _t._ xix. _f._ 3.

[786] Reaum. ii. 66.

[787] VOL. II. p. 36.

[788] VOL. III. p. 64--.

[789] PLATE XXII. FIG. 4.

[790] Ibid. FIG. 3.

[791] Reaum. iv. 414.

[792] _Ibid._ _t._ xxviii. _f._ 14, 15.

[793] _Ibid._ 404.

[794] De Geer vi. 63--.

[795] VOL. I. p. 175.

[796] De Geer iii. 70--.

[797] _Ibid._ 128.

[798] _N. Dict. d'Hist. Nat._ xxx. 426--.

[799] VOL. III. p. 64--.

[800] VOL. II. p. 50, 110--, 118--, 125--, 130--. The neuters of the _Termites_, however, (p. 33.) seem to be a distinct sex, if I may so speak--and to merit that name.