LETTER XLI.

_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED._

SECRETION.

Having given you so full an account of the system of _digestion_ in insects, I am now to say something concerning their _secretions_, and the organs by which they are elaborated. Though no individual amongst them perhaps secretes so many different substances as the warm-blooded animals; yet in general the Class abounds in secretions perhaps as numerous and extraordinary as in the last-mentioned tribes, to some of which a few of them are analogous, while others are altogether peculiar. We know little or nothing of the mode in which the process of secretion in insects is accomplished; in most cases we cannot even discover, except in general, whence the secreted substance originates; and in others, though we are able to trace the vessels that contain it, we are often in the dark as to their structure.--Cuvier, as has been before hinted, from not being able to detect any thing in them like _glands_, and from their being constantly bathed in the _blood_ or nutritive fluid, conceives that they separate the peculiar substances they contain, by imbibition or infiltration, through the pores of the skin[588]; a circumstance which seems to indicate a certain conformation of the pores both as to size and figure, so as to enable them to admit only one peculiar product.

In treating on this subject, I shall first consider the _organs_ of secretion, and next their _products_.

I. _Organs of Secretion._ In general, these are membranous vessels that float in the blood or nutritive fluid, and secrete from it a peculiar substance. They may be denominated according to their products--_Silk-secretors_, _Saliva-secretors_, _Varnish-secretor_, _Jelly_ or _Gluten-secretor_, _Poison-secretor_, and _Scent-secretors_.

i. _Silk-Secretors_ (_Sericteria_). These organs are most remarkable in the caterpillars of the _nocturnal Lepidoptera_ or moths, especially in that tribe called _Bombyces_, to which the silkworm belongs: but this faculty is not confined to these insects, but is shared by many other _larvæ_ in different Orders; and in one instance at least, by the _imago_. In general, the outlet of the silk-secretors is at the _mouth_; sometimes, however, as in the larva of _Myrmeleon_ and the imago of _Hydrophilus_, its exit is at the _anus_. The first is the organ which in the silk-worm provides for us that beautiful substance from which the animal takes its name. There are always _two_ of these vessels, which are long floating tubes, growing slender towards the head of the insect, where they unite to form the spinneret (_fusulus_) before described[589], which renders the silk. Their lower extremity also is commonly more slender than the middle, and is closed at the end. These organs are usually very much convoluted and twisted[590]. According to Ramdohr[591], they consist of two transparent membranes, between which is found a yellow or transparent jelly. The greater the quantity of silk employed by the caterpillar in the construction of its cocoon, &c., the longer are the silk-secretors. Those of the silkworm are a _foot_ long[592], while those of the larva of the goat-moth are little more than _three inches_[593].

Other insects spin silk with the _posterior_ extremity of their body. In the great water-beetle (_Hydrophilus piceus_) the anus is furnished with two spinnerets, with which it spins its egg-pouch[594]; these are in connexion, probably, with the five long and large vessels containing a green fluid, described by Cuvier[595], which surround the base of each branch of the ovaries. The larva of _Myrmeleon_, which also spins a cocoon with its anus, differs remarkably in this respect from other insects, since its reservoir for the matter of silk is the _rectum_; this is connected with a horny tube, which the animal can protrude, and thus agglutinate the silk and grains of sand that compose its cocoon[596].

The _web_ of spiders is also a kind of _silk_ remarkable for its lightness and extreme tenuity. It is spun from four anal spinnerets, which never vary in number; two longer organs peculiar to some species have been mistaken for additional ones, but Treviranus affirms that they are merely a kind of anal _feeler_. Their structure, as far as known, has been before described[597]. The web is secreted in vessels varying in form. In some (_Clubiona atrox_) they consist of two larger and two smaller ones, at the base of which lie many still more minute[598]. The four larger vessels are wide in the middle, branching at top, and below terminating in a narrow canal leading to the spinnerets[599]. Treviranus thinks the fluid contained in the lower minute vessels different from that furnished by the larger ones--but for what purpose it is employed has not been ascertained.

ii. _Saliva-secretors_ (_Sialisteria_). These are organs, rendering a fluid to the mouth or stomach, that are found in many insects, especially those that take their food by _suction_, as the _Hemiptera_, _Lepidoptera_, and _Diptera_, though they are not confined to the perfect insect, being also in some cases visible in the larva. Swammerdam was one of the first that discovered them, and he suspects that they may be _salival_ vessels; though he, as well as Ramdohr, thinks they are the same with the _silk_ vessels of the caterpillar[600]; an opinion which Herold has sufficiently disproved, by showing that at one period of the insect's life they co-exist[601], and Lyonet discovered a very conspicuous pair in the caterpillar of the Cossus, co-existent with the silk-secretors[602]. But the physiologist who has given the fullest account of these organs is Ramdohr:--I shall therefore extract chiefly from him what I have further to communicate with respect to them.

They are variously constructed blind vessels, that are present in almost all insects that take their food by _suction_, but are mostly wanting in those that _masticate_ it. They have been found, however, in _Cryptorhynchus Lapathi_, _Chrysopa Perla_, and _Iulus terrestris_. The most usual number of the saliva-secretors is _two_[603]; but sometimes, as in the first of the last-named insects, there is only _one_[604]; in others (_Pentatoma Baccarum_) there are _three_, the exterior one consisting of a pair of reservoirs connecting with the gullet by a single capillary tube[605]; in _Pentatoma prasina_ there appear to be _four_[606]; in _Nepa cinerea_, even _six_--the exterior double pair in this insect, under a powerful lens, is found to consist of spherical vesicles, resembling somewhat a bunch of currants[607]; and in _Syrphus arcuatus_ they are covered with _four_ rows of similar ones[608]. In the flea they consist of two pair of spherical reservoirs, each of which is connected with a short tube, which uniting with that of the other forms a common capillary one connecting with the mouth or gullet[609]; these organs sometimes terminate below in slender vessels;--thus, in _Nepa_, the inner pair terminates in a single vessel of this description[610], and in _Tabanus_ and _Hemerobius_ apparently in many[611]. It admits of a doubt however, as was lately observed, whether in the _Hemiptera_, which have usually more than a _pair_ of these organs, some are not rather _food-reservoirs_ as in the _Diptera_.

The saliva-secretors open either into the _instruments_ of _suction_ themselves (_Tabanus_, _Musca_); or into the entrance of the _gullet_ (_Pentatoma_, &c.); or, lastly, into that of the _stomach_ (_Syrphus_, _Bombylius_). Those which lie at the entrance of the _stomach_ consist only of a blind uniform _tube_[612]; but there is commonly to be distinguished in those that open into the _mouth_, a _reservoir_, varying in shape in different species, and terminating in a capillary tube, or tubes, at one or both extremities[613]. In Bugs, _two_ pair of these vessels are often present, one of which opens into the stomach (_Reduvius_), or gullet (_Pentatoma_), but the other into the instruments of suction[614]. In the _Diptera_ they open into the stomach when the insect feeds only upon the nectar of flowers (_Syrphus_), and into the proboscis when it feeds upon both animal and vegetable juices (_Tabanus_, _Musca_). The function of the fluid secreted by these organs is to moisten or dilute the food before it is received by the instruments of suction and passed to the stomach[615]. When a common house-fly applies its proboscis to a piece of sugar, it is easy to see that it moistens and dissolves it by some fluid.

iii. _Varnish-secretor_ (_Colleterium_). In butterflies, moths, and several other insects, one or more vessels called blind vessels open into the oviduct, concerning the use of which, physiologists are not agreed. In the cabbage butterfly there is a pair of ovate ones, or rather a bilobed one, each lobe of which externally terminates in long perplexed convolutions, not easily traced, filled with a yellow fluid, which Reaumur and Herold think is used for varnishing or gumming the eggs, so that they may adhere to the leaves on which they are deposited: it may probably serve likewise for other uses[616]. Another vessel is also to be found in the above butterfly, which enters the oviduct above this, filled with a thick white fluid, the function of which is, probably, to lubricate the passage[617]. A similar organ is found in _Phryganea grandis_[618].

iv. _Jelly-secretor_ (_Corysterium_). This is a remarkable organ, related to the preceding, which secretes the jelly of _Trichoptera_, some _Diptera_, &c.; this organ in the former, at least in _Phryganea grandis_, is of an irregular shape, with four horns or processes[619].

_Poison-secretor_ (_Ioterium_). This organ, which is most conspicuous in the _Hymenoptera_ Order, has not received much notice, except in the case of the Hive-bee and the _Scolia_: in the former, it is an elliptical membranous vesicle or reservoir, furnished at its lower extremity with a tube which renders to the sting, and at the other by a blind, long, filiform, secretory, vessel, which according to Swammerdam divides into _two_ terminal blind branches[620], though Reaumur could detect but _one_[621]; in this vessel the poison is secreted and stored up. In _Scolia_ there are two secretory vessels, which enter the reservoir in the middle on each side[622]. In the _Scorpion_, we learn from Marcel de Serres that the poison-secretor is clothed externally with a horny thickish membrane, containing two yellowish glands, composed of an infinity of spherical glandules, terminating in a canal, enlarged towards its base so as to form a reservoir, and leading to the extremity of the sting[623]. Connected by a slender tube with each mandible in _spiders_ is a vessel with spiral folds, which seems properly to belong to _this_ head--though Treviranus calls it a _saliva_-vessel[624]--since in the _Mygale avicularia_ and other spiders, the effect of the bite is said to be so venomous as to occasion considerable inflammation, and sometimes death[625].

v. _Scent-secretors_ (_Osmateria_). Amongst other means with which insects are gifted for the annoyance of their foes and pursuers, are the powerful _scents_ which many of them emit when alarmed and in danger. Concerning the _internal_ organs by which these effluvia are _secreted_ we possess but little information, but more notice has been taken of the _external_ ones by which they are _emitted_. We may conclude in general, that the secretory organs are membranous sacs or vesicles, perhaps terminating in longer or shorter blind filiform vessels, sometimes secreting a fetid fluid, and at others a fetid gaseous effluvium. The _Iulidæ_, at least _Iulus_ and _Porcellio_[626], cover themselves when alarmed, with a _fluid_ of this kind, or emit one, for this faculty is not peculiar to the species noticed by Savi. I observed early in the year, when I handled _Iulus terrestris_, that it was covered with a slimy secretion, of a powerful scent, which stained my fingers of an orange colour. The spiraculiform pores that mark the sides of the animal are the outlets by which this fluid is emitted, and not spiracles as has been supposed: each of these orifices, as we learn from Savi, terminates internally in a black vesicle, which is the reservoir of the fluid[627]. The most remarkable insect for its powers of annoyance in this way, is one on that account called the _bombardier_ (_Brachinus crepitans_), which can fire numerous volleys of stinking vapour at its assailants before its ammunition is exhausted[628]. M. Dufour has given a very particular account of the organ that secretes this vapour;--it consists of a double apparatus, one on each side, in the cavity of the abdomen, both formed of two distinct vessels. The _first_, which is the innermost, presents itself under two different aspects, according as it is contracted or dilated: in the former case it is a whitish, irregularly rounded, soft body, apparently glandular, placed under the last abdominal segments; communicating at one end with the reservoir, and terminating constantly at the other in a very long and slender filament: in the second case, or when it is dilated, it resembles an oblong, membranous, diaphanous sac, filled with air, then occupying the whole length of the abdomen, and appearing free except where it communicates with the reservoir. The _second_ vessel or reservoir is a small, spherical, brown or reddish body, constant in its form, internally hollow, placed under the last dorsal segment, precisely above the _rectum_, and opening by a small pore into the _anus_[629]: so that the tail of this little beetle may be regarded as a battery mounted with two pieces of cannon, which our alert bombardier fires alternately without intermission till all his ammunition is expended. The ground-beetles (_Eutrech_in_a_) in general have a pair of these anal scent-secretors, which discharge an acrid and caustic fluid, and sometimes a volatile one[630]. The external organ of the scent-secretors in _Gyrinus_ consists of two minute hairy cylindrical retractile tubes, of a red colour[631]. Numerous insects of other tribes and genera emit _scents_ from their anus, and from various other parts of the body, of which having before given you a very full account[632], I shall proceed to the consideration of the secretions themselves: but first I must observe, that in many cases, as in some of the cottony and powdery _Aphides_, _Chermes_, &c., the substance secreted appears to be a transpiration through the pores of the body, a kind of excretion from the superabundance of its fluid contents[633]. In many, however, this secretion transpires through appropriate orifices: thus in _Chermes Abietis_, which produces those curious galls resembling the cone of a fir[634], the flocoons of seeming cotton that cover it proceed from little oval concavities on its back, four of which are arranged in a transverse line on each dorsal segment of the abdomen: these concavities have minute tubercles probably terminating in a pore[635]. In _Aphis Fagi_ the cottony flocoons are almost an inch long[636].

* * * * *

The _secretions_ of insects may be considered under the following heads--_Silk_; _Saliva_; _Varnish_ or _Gum_; _Jelly_; _Oils_; _Milk_; _Honey_; _Wax_; _Poisons_ and _Acids_; _Odorous fluids_ and _Vapours_; and _Luminous matter_.

i. _Silk._ This valuable product of insects, while in the silk-secretor, assumes in the _Lepidoptera_ the appearance of a viscid gum, but the moment it is exposed to the air it hardens into a silken thread. It is remarkable for the following qualities:--it dries the instant it comes in contact with the air; it is then insoluble not only in water but in the most active solvents, and even _heat_ has no effect upon it to melt or soften it: indeed, without these qualities it would be of no use to us[637]. As soon as it leaves the spinneret it becomes the thread we call silk, which being drawn through _two_ orifices is necessarily _double_ through its whole length. This thread varies considerably in colour and texture, as has been before stated[638], and sometimes resembles cotton or wool rather than silk. In spiders it is of a much softer and more tender texture than that of other spinning insects; and Mr. Murray seems to have proved that it is imbued, in the case of the gossamer, with negative electricity: in the _sericterium_ the fluid that produces it is sometimes white or grey, and at others yellow[639]. A remarkable gnat (_Ceroplatus tipuloides_), living on an agaric, carpets its station of repose and its paths with something between silk and varnish, which it spins, not in a _thread_, but in a _broad_ riband[640].

ii. _Saliva._ Many insects have the power of discharging from their mouth a fluid which seems in some degree analogous to the _saliva_ of larger animals. Thus many, as _Lepidoptera_, _Hemiptera_, _Diptera_, &c., can dilute their food, and render it fitter for deglutition. I have seen a common fly when not employed in eating, emit a globule of fluid as big as a grain of mustard-seed from its proboscis, and retract it again. On a former occasion I observed to you that many predaceous, carnivorous, and some herbivorous beetles, when alarmed emit a drop of coloured acrid fluid from the mouth[641]. That this is not secreted in any of the ordinary salival vessels is evident from Ramdohr's dissections of those beetles[642], who, had there been such an organ, would doubtless have discovered it: but as the stomach of all of them is distinguished by those minute _cœca_ or blind vessels, which he denominates shags (_zotten_)[643], perhaps these may be the secretors of this fluid, probably analogous to the gastric juice[644]; in which case its _primary_ office would be the _digestion_ of the food. We are not however warranted in considering _every_ fluid effused from the mouth as saliva. The glutinous material with which wasps cement the woody fibres for their paper edifices[645]; that with which some sand-wasps moisten the sand which they scrape away, of which they form the singular tubes that lead to their nests[646]; and that with which the aphidivorous larvæ fix themselves previously to their becoming pupæ[647],--may be a secretion distinct from saliva; possibly intermediate between it and gum or the matter of silk, and secreted by peculiar organs. In the wasp, however, Ramdohr discovered nothing of the kind[648]; and in _Syrphus_, as before observed, the saliva-secretors are very peculiar in their structure, as if appropriated to the secretion of a peculiar fluid[649]. Something similar has been observed by Reaumur with regard to the larva of _Crioceris merdigera_, which forms its cocoon with a kind of froth produced from the mouth[650].

iii. _Varnish_ or _Gum_. The eggs of various insects, when they leave the oviduct, are covered with a kind of varnish or gum by which they adhere to the substances that the young larvæ are to feed upon, or are placed in a proper position for their hatching in an appropriate station. Several instances of this have been already mentioned[651]; I shall therefore not enlarge further upon the subject. With regard to the secretion itself, little has been recorded except its _colour_, which has been before noticed. Some _Lepidoptera_ also as we learn from Reaumur and Bonnet[652], use a varnish in the construction of their cocoons.

iv. _Jelly_ or _Gluten_. This secretion is particularly conspicuous in the _Trichoptera_ and some _Diptera_, serving as a bed or _nidus_ for those eggs that are committed to the water,--upon which I have nothing to add to what has been before said[653]. Under this head also may be noticed the fluid, secreted in peculiar vesicles, that lubricates the oviduct and the passages of the sexual organs[654].

v. _Oils._ Oily substances are sometimes produced by insects. The common oil-beetle (_Meloe Proscarabæus_) when touched sends forth a drop of this kind of fluid, of an orange colour, from each joint of its legs[655]: something similar I have observed in _Coccinella bipunctata_: Ray mentions a locust taken in Spain which emits a yellow oleaginous fluid from between the claws of its fore legs[656]; but the precise nature of these substances has not been ascertained, nor whether they are secreted by peculiar organs.

vi. _Milk._ A milky fluid is produced by the larva of _Chrysomela Populi_. Willughby observed a similar effusion from pores in the upper surface of the body of _Acilius sulcatus_; and other insects emit it from other parts of their body[657].

vii. _Honey._ It is certain that honey is not an _animal_ secretion; yet the saccharine matter collected from the nectaries of flowers, from which it is derived, seems to undergo some _alteration_ in the stomach; for the consistence of honey is greater than that of any vegetable nectar, and its taste does not vary greatly, while that of the nectar in different plants is probably not the same. Reaumur also has observed, that each honey-cell in a bee-hive is always covered by a cream-like layer of a thicker consistence than the rest, which apparently serves to prevent the more liquid honey, which from time to time is introduced under it, from running out[658]. Now if honey were the unaltered nectar of plants, it is difficult to conceive how this cream could be collected in proper proportions. The last-mentioned naturalist likewise ascertained, that if bees, in a season in which the fields afford a scarcity of food, be supplied with _sugar_, they will from this substance fill their cells with _honey_ which differs in no respect from the common sort, except that its flavour is a little heightened[659]:--a similar argument may be deduced from the circumstance of the bees imbibing the juices of _fruits_ of various kinds as they are well known to do[660]. It seems therefore evident that the honey collected by bees undergoes some modification in their honey-stomach before it is regurgitated into the cells, and therefore may be regarded in some degree as a peculiar secretion.

Huber says that he has ascertained by a great number of observations that electricity is singularly favourable to the secretion of the substance of which honey is formed by flowers; the bees never collect it in greater abundance, nor is the formation of wax ever more active, than when the wind is in the south, the air humid and warm, and a storm gathering[661].

viii. _Wax_ generally transpires through the pores of the skin of those insects that produce it, either partially or generally, and it is secreted from honey or other saccharine substances taken into the stomach. In the hive-bee, as has been before stated, it is produced _partially_[662], but in many other insects it is a _general_ transudation of the body. This is particularly the case with a large number of the _Homopterous Hemiptera_; and those flocoons that look like cotton, and cover the body of several _Chermes_ and _Aphides_, if closely examined will be found of the nature of _wax_: this I have particularly noticed with respect to _Chermes Fagi_, in which the cotton-like flocoons are often so long as to cause the insect to look like a feather, and a leaf covered by them exhibits a very singular appearance, as if clothed with the fine down of a swan[663]. Probably the white powder or threads that appear to transpire through the skin of many other insects is of a waxy nature. In the larva of a beetle described by Reaumur, the flocoons are so arranged as to give the animal some resemblance to a hedgehog, and when rubbed off they are reproduced in twelve hours[664]. Gyllenhal, speaking of _Peltis limbata_, observes, that when alive it is covered with a white powder resembling mould, which if rubbed off returns again as long as the animal lives[665].

It will not be improper to include under this head what further account I have to give of _Lac_, which though regarded as a _resin_, since _Cocci_ sometimes certainly produce _wax_[666], probably has some analogy with the latter substance. When the females of this _Coccus_ (_C. Lacca_) have fixed themselves to a part of the branch of the trees on which they feed (_Ficus religiosa_ and _indica_, _Butea frondosa_, and _Rhamnus Jujuba_[667]), a pellucid and glutinous substance begins to exude from the margins of the body, and in the end covers the whole insect with a cell of this substance, which when hardened by exposure to the air becomes lac. So numerous are these insects, and so closely crowded together, that they often entirely cover a branch; and the groups take different shapes, as squares, hexagons, &c., according to the space left round the insect which first began to form its cell. Under these cells the females deposit their eggs, which after a certain period are hatched, and the young ones eat their way out. Though indisputably an _animal_ secretion, many of the properties of lac are not very different from those of the juices of the trees on which the animal feeds, and which therefore would seem to undergo but little alteration.

Wax seems also to form a constituent part of some insects which are not found to secrete it. The yellow substance deposited in vessels containing _spiders_ in alcohol is said to be a true _wax_, and may be obtained from these animals by gently heating them[668].

ix. _Poisons_ and _Acids_. The _bite_ as well as the _sting_ of many insects is followed by inflamed tumours, so that the _sialisteria_ of some _bugs_, _Diptera_, _Aptera_ and _spiders_, may be regarded as producing a poisonous fluid; but we know nothing of the real nature of it, nor of that of other venomous insects, except the _ant_--whose celebrated _acid_ may be considered under the present head,--the _bee_, the _wasp_, and the _scorpion_.

Contrary to the once received doctrine that no _acid_ was to be found in any animal, except as the effect of disease in the alimentary canal, many insects secrete peculiar and powerful ones. I have on a former occasion related an instance in which an acid of this description, secreted in its _sialisteria_, is employed by a moth to soften its cocoon[669]; and Lister mentions a species of _Iulus_ which produced one resembling that of ants[670]; but this last is the most powerful of all. The fact that blue flowers when thrown into an ant-hill become tinged with red has been long known; but Mr. Fisher of Sheffield, about 1670, seems to have been the first who ascertained that this effect is caused by an _acid_ with which ants abound, and which may be obtained from them by distillation or infusion in water[671]. Margraff and other chemists confirmed this discovery[672]; and concluding that this acid was of a peculiar kind, they gave it the name of the _Formic acid_. This name, however, is now exploded; the subsequent experiments of Deyeux, Fourcroy and Vauquelin having ascertained that the acid of ants is not of a distinct kind, but a mixture of the _Acetic_ and _Malic_[673]. These acids are in such considerable quantities, and so concentrated in these animals, that, when a number of _Formica rufa_ are bruised in a mortar, the vapour is so sharp that it is scarcely possible to endure it at a short distance. It also transpires from them, for they leave traces of it on the bodies which they traverse: and hence, according to the experiments of Mr. Coleridge, the vulgar notion that ants cannot pass over a line of chalk is correct; the effervescence produced by the contact of the acid and alkaline being so considerable, as in some degree to burn their legs[674]. The circumstance of much of the food of ants being of a saccharine nature may account for this copious secretion of acid, the use of which is probably to defend themselves and their habitations from the attack and intrusion of their enemies: if a frog be put into a nest of _Formica rufa_ that has been deranged, it will be suffocated in five minutes[675]. That which they _ejaculate_ from their _anus_ when attacked, as formerly stated[676], must be secreted in an _ioterium_; but their very _blood_ seems of an acid nature. It is very probable, as Dr. Thomson has observed[677], that acids may be obtained from many other insects, and that they are various modifications of the acetic.

From the circumstance that water is absorbed by _greasy_ moths, that crystals of a salt are occasionally found adhering to them, that they change blue litmus paper _red_,--it has been inferred that their supposed _oiliness_ is in fact an _acid_ or acid salt, having the property of attracting moisture from the air, the infected moths being in fact not greasy, but _wet_; hence the application of chalk and clay, usually recommended in this case, can have only a temporary and superficial effect. The only effectual remedy, is steeping the body in spirits of wine till all the acid is extracted[678]. This acid is probably the same as Chaussier obtained from silkworms, since called _Bombic Acid_[679].

The _poison_ of _bees_ and _wasps_, as to its chemical qualities, is a transparent fluid, at first sweet to the taste, but immediately afterwards hot and acrid like the milky juice of the _spurge_[680]; soluble in water, but not in alcohol; and separable from the former in the state of white powder, when the latter is added giving a slight _red_ tinge to paper stained with vegetable blue, and when dry and chewed appearing tenacious, gummy and elastic. This last property, as well as solubility in water and not in alcohol, is common also to the poison of the _viper_, which however differs in being tasteless, and not affecting vegetable blues. From hence Fontana concludes that this fluid is united with an _acid_, but in a very small proportion, and not with an _alkali_[681]. The venom of bees is extremely active; a grain in weight, it is conjectured, would kill a pigeon in a few seconds[682]. It is remarkable, however, that while in some constitutions the sting of a single bee or wasp is sufficient sometimes to induce alarming symptoms, in others numerous punctures will produce little or no pain or inflammation. That this fluid, and not the puncture of the sting, is the sole cause of the inflammation that usually follows the wound inflicted by one of these animals, is proved by the facts, that if it be introduced into one made by a needle, the same effect ensues, and that when the whole contents of the poison-bag have been exhausted by the insect's stinging three or four times in succession, its weapon then becomes harmless[683].

The venom of _scorpions_, though much more potent, probably resembles that of bees, &c., in many of its chemical qualities: it issues from two pores in the sting before described[684], where, when the animal is irritated, it accumulates under the form of two little drops of a whitish colour: spread upon paper this fluid produces a spot like what would be caused by oil or grease, and this part of the paper becomes by desiccation firmer and transparent[685].

x. _Odorous fluids_ and _Vapours_[686]. The powerful scents which different insects emit are extremely numerous, much more so indeed than the generality of Entomologists have been aware, for there is scarcely a scent odious or agreeable that may not be met with in the insect world. This you will be convinced of, by following a practice which I would recommend to you--that of smelling the insects you take. Some of these scents are peculiar to particular parts or organs, and some are exhaled generally by the whole body; some are emitted by a fluid secretion, and others are gaseous effluvia. On a former occasion I gave you a rather full account of these scents and their organs[687]; I shall relate here only what I there omitted. To begin with _sweet_ odours. Many beetles emit an agreeable scent. The rose-scented Capricorn or musk-beetle (_Cerambyx moschatus_) has long been noted for the delicious scent of roses which it exhales; this is so powerful as to fill a whole apartment, and the insect retains it long after its death. Captain Hancock also informed me that another species of the same genus, _C. sericeus_, has in a high degree a scent resembling that of the cedar[688] on which they feed. Though most of the micropterous tribes (_Brachyptera_) have a _fetid_ smell, yet there are some exceptions to this amongst them. One species (_Philonthus suaveolens_ K. MS.) related to _P. micans_, which I once took, smelt precisely like a fine high-scented ripe pear; another, _Oxytelus morsitans_, like the water-lily; a third, _O. rugosus_, like water-cresses; and lastly, a fourth (_P. fuscipes_) like saffron[689]: _Trichius Eremita_, one of the Petalocerous beetles, is stated to have the scent of Russia leather; _Geotrupes vernalis_, in spite of its stercorarious food, of lavender-water[690]. Mr. Sheppard has observed that _Dytiscus marginalis_ when recently taken smells not unlike liquorice: Bonnet mentions a caterpillar that had the scent of new hay. A little gall-fly (_Cynips Quercus Ramuli_) has the remarkable odour of Fraxinella: the larva of another species of this genus (_C. Rosæ_) has an odour which seemed to Reaumur as attractive to cats as that of _Nepeta cataria_ or _Teucrium Marum_[691]: some _Phalangia_ smell like walnut leaves[692]; and the various species of the genus _Prosopis_ (_Melitta_ * b. K.) have a very agreeable scent of _Dracocephalum moldavicum_[692].

We next come to _fetid_ odours. These in numerous cases are known to be secreted and emitted by appropriate vessels and organs; they are often exhaled from a fluid secretion, of which, in the letter lately referred to, I gave you almost all the known instances. Savi, in his history of _Iulus fœtidissimus_, informs us that it emits a yellow fetid fluid from its supposed spiracles, which if applied in sufficient quantity imparts a red colour to the skin, to be removed neither by friction nor washing, but only disappearing by time; when removed from the black vesicles in which it is stored, it shoots into very transparent octahedral crystals[693].

I have before mentioned the coloured fluid which some insects emit when they are disclosed from the pupa, and that it probably exhales some powerful odour which attracts the males[694].

The great _Hydrophilus_, in its larva state, when first taken into the hand remains without motion; in a minute afterwards it renders itself so flaccid as to appear like a cast skin. Taken by the tail it contracts itself considerably, it then agitates itself briskly, and ejaculates with a slight noise a fetid and blackish fluid[695].

In other cases these odours are produced by _gaseous vapours_. That of the Bombardiers (_Brachinus_) is the most celebrated and remarkable. It is whitish, of a powerful and stimulating odour, very like that exhaled by nitrous acid. It is caustic, producing upon the skin the sensation of burning, and forming instantly upon it red spots which soon turn brown, and which, in spite of frequent lotions, remain several days. It turns blue paper red[696]. That amiable, intelligent, and unfortunate traveller Mr. Ritchie,--whose premature death, when attempting to penetrate to the interior of Africa, all lovers of Natural History so deeply lamented, and whose ardour in the pursuit of that science I had an opportunity of witnessing, when, in company with him, Messrs. Savigny, Du Fresne, and W. S. MacLeay in 1817, I visited the forest of Fontainebleau,--in a letter to the last-mentioned gentleman[697], relates that his companion M. Dupont, near Tripoli took a nest consisting of more than a thousand of a species of this genus. "I am making a few experiments," says he, "on the substance which they emit when they crepitate, but do not know whether I can collect enough to arrive at any conclusion. It made Dupont's fingers entirely black when he took them. It is neither alkaline nor acid, and it is soluble in water and in alcohol." From this we may conjecture that it formed crystals.

xi. _Phosphorus._ On this remarkable secretion I have so fully enlarged on a former occasion[698], that here I shall merely add a few observations which Mr. Murray obligingly communicated to me. He remarks that in a box in which glow-worms were kept--five luminous specks were found secreted by the animal, which seemed to glow and were of a different tinge of light. One put into olive oil at eleven P. M. continued to yield a steady and uninterrupted light until five o'clock the following morning, and then seemed, like the stars, to be only absorbed by superior effulgence. The luminous spherical matter of the glow-worm is evidently enveloped in a sac or capsule perfectly diaphanous, which when ruptured discloses it in a liquid form, of the consistency of cream. M. Macaire, he observes, in the _Bibliothèque Universelle_, draws the following conclusions from experiments made on the luminous matter of this animal;--that a _certain_ degree of heat is necessary to their voluntary phosphorescence--that it is excited by a degree of heat superior to the first, and inevitably destroyed by a higher--that bodies which coagulate albumen take away the power--that phosphorescence cannot take place but in a gas containing no oxygen--that it is not excited by common electricity, but is so by the Voltaic pile--and lastly, that the matter is chiefly composed of _albumen_.

xii. _Fat._ There is one product found in the body of insects most copiously in their larva state, but more or less also in the imago, which may be called their _fat_. In the former it is a many-lobed mass, occupying the whole of the interior, except the space that is required for the muscles and the internal organs, which it wraps round and protects. It is contained in floating membranes, very numerous, which fill all the interstices, and assume the appearance sometimes of small globules, and sometimes of a thickish mucilage, which easily melts and inflames; in colour it is most commonly white, but sometimes yellow or green. It is imagined to be a kind of _epiploon_ or _caul_, and is accumulated in the _larva_ as a store of nutriment for the growth and development of the organs of the perfect insect while in the _pupa_ state[699]. The blood in which the different organs float that is not required for their nutriment, is supposed to be expended in the formation of this substance. Marcel de Serres is of opinion that it is secreted from the chyle by passing through the pores of the dorsal vessel, formerly called the heart of insects[700].

Under this head I may mention what little is known with regard to the _perspiration_ of these animals[701]. That a considerable quantity of fluid passes off from them when in the pupa state, is sufficiently proved by the loss of weight which they undergo, and by the experiments of Reaumur, who collected the fluid in closed glass tubes; and that in their perfect state they are constantly passing off perspirable matter by the pores of their skin or crust, is not only rendered probable by the succulent nature of their food and the absence of any urinary discharge, but is proved by what takes place in a swarm of bees. These insects, when crowded together in hot weather in a large mass, become heated to such a degree, and perspire so copiously, that those near the bottom are quite drenched with the moisture it produces, which so relaxes their wings that they are unable to fly[702].

I am, &c.

FOOTNOTES:

[588] Cuv. _Anat. Comp._ iv. 163--.

[589] VOL. III. p. 124--.

[590] Malpigh. _De Bombyc._ _t._ v. _f._ 2. Swamm. _t._ xxxiv. _f._ 5. Lyonet, _t._ v. _f._ 1.

[591] _Anat. der Ins._ 59.

[592] _Ibid._ 60. Malpigh. 20.

[593] Lyonet _Anat._ 111.

[594] _N. Dict. d'Hist. Nat._ xv. 483.

[595] _Anat. Comp._ v. 198.

[596] Ramdohr, 60. _t._ xvii. _f._ 1. _f, g, h, r._

[597] VOL. I. p. 403--. Treviran. _Arachnid._ 42.

[598] Treviran. _Arachnid._ 43. _t._ iv. _f._ 42. _o. p._ 9.

[599] _Ibid._ α, y.

[600] Swamm. ii. 21. a. _t._ xxxvi. _f._ 1. _abcd._ Ramdohr 58.

[601] _Schmet._ _t._ iii. _f._ 1.

[602] Lyonet--. 112. _t._ v. _f._ 1. _P, Q, R, S._

[603] Ramdohr _Anat._ _t._ xviii. _f._ 1. _M._ _f._ 5. _F._

[604] _Ibid._ _t._ x. _f._ 1. _m._

[605] _Ibid._ _t._ xxii. _f._ 3. _M L._ Ramdohr regards the double one as a pair; but as they terminate in a single tube, they ought to be reckoned as one.

[606] _Ibid._ _f._ 4.

[607] _Ibid._ _f._ 2. _K, L, M, N._ _t._ xxiii. _f._ 6.

[608] _Ibid._ 177. _t._ xxi. _f._ 3. _F. F._

[609] _Ibid._ _f._ 2. _G, H._

[610] _Ibid._ _t._ xxii. _f._ 2. _L._

[611] _Ibid._ _t._ xxi. _f._ 1. _O._ _t._ xvii. _f._ 6. _n._

[612] Ramdohr _Anat._ _t._ xx. _f._ 6. _D._

[613] _Ibid._ _t._ xxii. _f._ 1. _K, L._ _f._ 2. _I, K, L._

[614] _Ibid._ _f._ 3, 4, 5.

[615] _Ibid._ 57--.

[616] Reaum. ii. 81. Herold _Expl. of Plates_, x. Malpigh. _De Bombyc._ 37. PLATE XXX. FIG. 12. _c._

[617] Herold _Ibid._ x. _t._ iv. _f._ 1. _p, u, y._ Marcel de Serres _Mem. du Mus._ 1819. 141.

[618] Gaede _Anat._ _t._ i. _f._ 3. _d._

[619] _Ibid._ 17. _t._ i. _f._ 4.

[620] _Bibl. Nat._ _t._ xix. _f._ 3. β.

[621] Reaum. v. 377. _t._ xxix. _f._ 7. _s._

[622] _N. Dict. d'Hist. Nat._ xxx. 388.

[623] _Ibid._ 427--.

[624] _Arachnid._ 31. _t._ ii. _f._ 21. _p._ 9.

[625] _N. Dict. d'Hist. Nat._ xxii. 114. 117. comp. VOL. I. p. 127.

[626] _Ibid._ xxviii. 6.

[627] _Osservazioni_, &c. 13--.

[628] VOL. II. p. 243--. _N. Dict. d'Hist. Nat._ iv. 308.

[629] _Ibid._ iv. 309.

[630] _Ibid._ v. 252.

[631] De Geer iv. 358. _t._ xiii. _f._ 9. _m._

[632] VOL. II. p. 241--. III. p. 147--.

[633] De Geer iii. 41.

[634] VOL. I. p. 451, where by mistake it is represented as the work of _Aphis Abietis_.

[635] De Geer iii. 111.

[636] Reaum. iii. _t._ xxvi. _f._ 4-6.

[637] _N. Dict. d'Hist. Nat._ vi. 305.

[638] VOL. III. p. 221.

[639] Treviran. _Arachnid._ 44. In Paraguay a spider is found which makes spherical cocoons of yellow silk, which are spun because of the permanence of the colour. This operation occasions a flow of water from the eyes and nose of the spinners. Azara _Voyag._ 212. See also Murray in _Werner. Trans._ 1823. 8--.

[640] Reaum. v. 24.

[641] VOL. II. p. 244--.

[642] Ramdohr _Anat._ _t._ ii.-vi.

[643] _Ibid._ 20. See above, p. 107. As some of the _Sialisteria_ render to the _stomach_ (see above, p. 131), there seems no small affinity between these shags and those organs.

[644] Cuv. _Anat. Comp._ iv. 132, 136.

[645] Reaum. vi. Pref. xxviii. 177--.

[646] _Ibid._ 253--.

[647] _Ibid._ iii. 375.

[648] _Anat._ _t._ xii. _f._ 6.

[649] _Ibid._ xxi. _f._ 3. _I I._

[650] Reaum. iii. 230.

[651] VOL. III. p. 79--.

[652] Reaum. iii. 215. Bonnet ix. 182.

[653] VOL. III. p. 68--.

[654] Marcel de Serres _Mem. du Mus._ 1819. 133, 141.

[655] De Geer, v. 6.

[656] Rai. _Hist. Ins._ 62.

[657] VOL. II. p. 242--. 248. Rai. _Hist. Ins._ 94, 382.

[658] Reaum. v. 448.

[659] _Ibid._ v. 722.

[660] VOL. I. p. 196. II. p. 176.

[661] _Encyclop. Britan._ viii. 205. from _Journ. de Phys._

[662] VOL. II. p. 174.

[663] Reaum. iii. 318--. _t._ xxvi. _f._ 1-6.

[664] _Ibid._ 396--. _t._ xxxi. _f._ 20-29.

[665] _Insect. Suec._ i. 257.

[666] VOL. I. p. 327.

[667] _N. Dict. d'Hist. Nat._ xvii. 189.

[668] Nicholson's _Journ._ i. 298--.

[669] VOL. III. 281.

[670] _Philos. Trans._ 1670.

[671] _Philos. Trans._ _Ibid._ Ray's Lett. 74.

[672] Amoreux _Ins. Venim._ 236--.

[673] _N. Dict. d'Hist. Nat._ xii. 94.

[674] Southey's _Brazil_, i. 645.

[675] _N. Dict. d'Hist. Nat._ ubi supr.

[676] VOL. II. p. 67.

[677] _Syst. of Chemist._ 533.

[678] Germar _Mag. der Ent._ iii. 445--.

[679] _Mem. Dijon_ 1783. ii. 70.

[680] Reaum. v. 354.

[681] _On Poisons_, i. 265--.

[682] _Ibid._ 269.

[683] Reaum. _ubi supr._

[684] VOL. I. p. 124. III. p. 716--.

[685] _N. Dict. d'Hist. Nat._ xxx. 427.

[686] I use the term _odorous_, not in the same sense as _odoriferous_, but to include both sweet and fetid scents.

[687] VOL. II. p. 238--. III. p. 147--.

[688] A Brazilian wood so called, but differing from the common cedar.

[689] Dotharding _Insect. Coleopt. Danic._

[690] Sturm _Deutsch. Fn._ i. 27.

[691] Reaum. iii. 494.

[692] _Mon. Ap. Angl._ i. 136.

[693] _Osservaz. sullo Iulus, &c._ 14.

[694] VOL. III. p. 297--.

[695] _N. Dict. d'Hist. Nat._ xv. 487.

[696] _N. Dict. d'Hist. Nat._ iv. 308.

[697] Dated Tripoli in the West, January 21, 1819.

[698] VOL. II. p. 418--.

[699] Reaum. i. 145. Lyonet _Anat._ 106--. _N. Dict. d'Hist. Nat._ xvi. 224. PLATE XXI. FIG. 5. _a._

[700] See above, p. 90. note^a.

[701] See above, p. 78.

[702] Huber i. 273.