Having given you so full an account of the system ofdigestionin insects, I am now to say something concerning theirsecretions, 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 likeglands, and from their being constantly bathed in thebloodor nutritive fluid, conceives that they separate the peculiar substances they contain, by imbibitionor 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 theorgansof secretion, and next theirproducts.
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,JellyorGluten-secretor,Poison-secretor, andScent-secretors.
i.Silk-Secretors(Sericteria). These organs are most remarkable in the caterpillars of thenocturnal Lepidopteraor moths, especially in that tribe calledBombyces, to which the silkworm belongs: but this faculty is not confined to these insects, but is shared by many otherlarvæin different Orders; and in one instance at least, by theimago. In general, the outlet of the silk-secretors is at themouth; sometimes, however, as in the larva ofMyrmeleonand the imago ofHydrophilus, its exit is at theanus. 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 alwaystwoof 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 afootlong[592], while those of the larva of the goat-moth are little more thanthree inches[593].
Other insects spin silk with theposteriorextremity 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 ofMyrmeleon, 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 therectum; 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].
Thewebof spiders is also a kind ofsilkremarkable 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 analfeeler. Their structure, as far as known, has been before described[597]. The web is secreted in vessels varying in form. In some (Clubionaatrox) 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 bysuction, as theHemiptera,Lepidoptera, andDiptera, 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 besalivalvessels; though he, as well as Ramdohr, thinks they are the same with thesilkvessels 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 bysuction, but are mostly wanting in those thatmasticateit. They have been found, however, inCryptorhynchus Lapathi,Chrysopa Perla, andIulus terrestris. The most usual number of the saliva-secretors istwo[603]; but sometimes, as in the first of the last-named insects, there is onlyone[604]; in others (Pentatoma Baccarum) there arethree, the exterior one consisting of a pair of reservoirs connecting with the gullet by a single capillary tube[605]; inPentatoma prasinathere appear to befour[606]; inNepa cinerea, evensix—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 inSyrphus arcuatusthey are covered withfourrows 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, inNepa, the inner pair terminates in a single vessel of this description[610], and inTabanusandHemerobiusapparently in many[611]. It admits of a doubt however, as was lately observed, whether in theHemiptera, which have usually more than apairof these organs, some are not ratherfood-reservoirsas in theDiptera.
The saliva-secretors open either into theinstrumentsofsuctionthemselves (Tabanus,Musca); or into the entrance of thegullet(Pentatoma, &c.); or, lastly, into that of thestomach(Syrphus,Bombylius). Those which lie at the entrance of thestomachconsist only of a blind uniformtube[612]; but there is commonly to be distinguished in those that open into themouth, areservoir, varying in shape in different species, and terminating in a capillary tube, or tubes, at one or both extremities[613]. In Bugs,twopair 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 theDipterathey 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 entersthe 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 inPhryganea grandis[618].
iv.Jelly-secretor(Corysterium). This is a remarkable organ, related to the preceding, which secretes the jelly ofTrichoptera, someDiptera, &c.; this organ in the former, at least inPhryganea grandis, is of an irregular shape, with four horns or processes[619].
Poison-secretor(Ioterium). This organ, which is most conspicuous in theHymenopteraOrder, has not received much notice, except in the case of the Hive-bee and theScolia: 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 intotwoterminal blind branches[620], though Reaumur could detect butone[621]; in this vessel the poison is secreted and stored up. InScoliathere are two secretory vessels, which enter the reservoir in the middle on each side[622]. In theScorpion, 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 inspidersis a vessel with spiral folds, which seems properly to belong tothishead—thoughTreviranus calls it asaliva-vessel[624]—since in theMygale aviculariaand 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 powerfulscentswhich many of them emit when alarmed and in danger. Concerning theinternalorgans by which these effluvia aresecretedwe possess but little information, but more notice has been taken of theexternalones by which they areemitted. 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. TheIulidæ, at leastIulusandPorcellio[626], cover themselves when alarmed, with afluidof 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 handledIulus 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 thebombardier(Brachinus crepitans), which can fire numerousvolleys 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. Thefirst, 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. Thesecondvessel or reservoir is a small, spherical, brown or reddish body, constant in its form, internally hollow, placed under the last dorsal segment, precisely above therectum, and opening by a small pore into theanus[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 (Eutrechina) 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 inGyrinusconsists of two minute hairy cylindrical retractile tubes, of a red colour[631]. Numerous insects of other tribes and generaemitscentsfrom 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 powderyAphides,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 inChermes 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]. InAphis Fagithe cottony flocoons are almost an inch long[636].
Thesecretionsof insects may be considered under the following heads—Silk;Saliva;VarnishorGum;Jelly;Oils;Milk;Honey;Wax;PoisonsandAcids;Odorous fluidsandVapours; andLuminous matter.
i.Silk.This valuable product of insects, while in the silk-secretor, assumes in theLepidopterathe 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 waterbut in the most active solvents, and evenheathas 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 throughtwoorifices is necessarilydoublethrough 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 thesericteriumthe 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 athread, but in abroadriband[640].
ii.Saliva.Many insects have the power of discharging from their mouth a fluid which seems in some degree analogous to thesalivaof larger animals. Thus many, asLepidoptera,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 occasionI 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 minutecœcaor 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 itsprimaryoffice would be thedigestionof the food. We are not however warranted in consideringeveryfluid 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 inSyrphus, as before observed, the saliva-secretors are very peculiar in their structure, as if appropriated to the secretion of a peculiar fluid[649]. Somethingsimilar has been observed by Reaumur with regard to the larva ofCrioceris merdigera, which forms its cocoon with a kind of froth produced from the mouth[650].
iii.VarnishorGum. 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 itscolour, which has been before noticed. SomeLepidopteraalso as we learn from Reaumur and Bonnet[652], use a varnish in the construction of their cocoons.
iv.JellyorGluten. This secretion is particularly conspicuous in theTrichopteraand someDiptera, serving as a bed ornidusfor 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 inCoccinella 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 ofChrysomela Populi. Willughby observed a similar effusion from pores in the upper surface of the body ofAcilius sulcatus; and other insects emit it from other parts of their body[657].
vii.Honey.It is certain that honey is not ananimalsecretion; yet the saccharine matter collected from the nectaries of flowers, from which it is derived, seems to undergo somealterationin 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 withsugar, they will from this substance fill their cells withhoneywhich differs in no respect from the common sort, except that its flavour is a little heightened[659]:—a similarargument may be deduced from the circumstance of the bees imbibing the juices offruitsof 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.Waxgenerally 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 producedpartially[662], but in many other insects it is ageneraltransudation of the body. This is particularly the case with a large number of theHomopterous Hemiptera; and those flocoons that look like cotton, and cover the body of severalChermesandAphides, if closely examined will be found of the nature ofwax: this I have particularly noticed with respect toChermes 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 thefine 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 ofPeltis 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 ofLac, which though regarded as aresin, sinceCoccisometimes certainly producewax[666], probably has some analogy with the latter substance. When the females of thisCoccus(C. Lacca) have fixed themselves to a part of the branch of the trees on which they feed (Ficus religiosaandindica,Butea frondosa, andRhamnus 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 ananimalsecretion, 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 containingspidersin alcohol is said to be a truewax, and may be obtained from these animals by gently heating them[668].
ix.PoisonsandAcids. Thebiteas well as thestingof many insects is followed by inflamed tumours, so that thesialisteriaof somebugs,Diptera,Apteraandspiders, 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 theant—whose celebratedacidmay be considered under the present head,—thebee, thewasp, and thescorpion.
Contrary to the once received doctrine that noacidwas 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 itssialisteria, is employed by a moth to soften its cocoon[669]; and Lister mentions a species ofIuluswhich 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 anacidwith which ants abound, and which may be obtained from them by distillation orinfusion 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 theFormic 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 theAceticandMalic[673]. These acids are in such considerable quantities, and so concentrated in these animals, that, when a number ofFormica rufaare 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 ofFormica rufathat has been deranged, it will be suffocated in five minutes[675]. That which theyejaculatefrom theiranuswhen attacked, as formerly stated[676], must be secreted in anioterium; but their verybloodseems 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 bygreasymoths, that crystals of a salt are occasionally found adhering to them, that they change blue litmus paperred,—it has been inferred that their supposedoilinessis in fact anacidor acid salt, having the property of attracting moisture from the air, the infected moths being in fact not greasy, butwet; 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 calledBombic Acid[679].
Thepoisonofbeesandwasps, 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 thespurge[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 slightredtinge 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 theviper, which however differs in being tasteless, and not affecting vegetable blues. From hence Fontana concludes that this fluid is united with anacid, but in a very small proportion, and not with analkali[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 isremarkable, 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 ofscorpions, 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 fluidsandVapours[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 smellingthe 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 withsweetodours. 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 afetidsmell, yet there are some exceptions to this amongst them. One species (Philonthus suaveolensK. MS.) related toP. 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 thatDytiscus marginaliswhen recently taken smells not unlike liquorice: Bonnet mentions a caterpillar that had the scent of new hay. A little gall-fly (Cynips Quercus Ramuli) hasthe 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 ofNepeta catariaorTeucrium Marum[691]: somePhalangiasmell like walnut leaves[692]; and the various species of the genusProsopis(Melitta* b. K.) have a very agreeable scent ofDracocephalum moldavicum[692].
We next come tofetidodours. 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 ofIulus 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 greatHydrophilus, 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 bygaseous 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 obliginglycommunicated 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 elevenp. 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 theBibliothèque Universelle, draws the following conclusions from experiments made on the luminous matter of this animal;—that acertaindegree 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 ofalbumen.
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 theirfat. 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 ofepiploonorcaul, and is accumulated in thelarvaas a store of nutriment for the growth and development of the organs of the perfect insect while in thepupastate[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 theperspirationof 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.
The reproductive organs of insects in theirgeneraldenominations and functions correspond with those of the higher classes of animals; but as tonumber,proportions, and otherparticulardetails 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 theSperm-reservoir; theOviduct; theOvaries; and theOvipositor.
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 asmall 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 theuterus; 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, thatin these cases the eggs are fertilized in their transit through the oviduct by sperm adhering to the folds of thecloacæ[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 orcloaca, and at others, as in the cabbage-butterfly[711], is distinct, and lies between the sexual organ and the anus. In theArachnidathere aretwooviducts[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 theIulidæ, in which there is only asingleovary[713], the oviduct at its upper or inner extremity terminates intwobranches, usually further subdivided into a number of smaller conical ones, which several ramifications constitute theovaries, or egg-tubes as they are sometimes called: these tubes generally consist of asingle membrane, and are joined to the oviduct by membranous rugosecloacæ[714]: in thePhalangia, however, there aretwotunics; the outer one of a cellular substance, and the inner one consisting of spiral fibres liketracheæ—a kind of structure which renders them capable of great extension[715]. Rifferschweils considers the ovaries as formed upontwoprimary types.—First,flagelliformovaries, consisting of conical tubes equal in length, and inserted at the same place at the end of the primary branches as in theLepidoptera, the Bee, &c.Secondly,racemoseovaries, consisting of short conical tubes, so proceeding from the primary branches as to render the ovary racemose or pinnated, as in certainNeuroptera,Coleoptera, andDiptera[716]: but perhaps their structure will be better understood if they are divided intoagglomerateovaries andbranchingovaries: in thefirstthe egg-tubes formtwobundles, in which the branches are not discernible, as in theEphemera, the chamæleon-fly, and spiders[717]: and in thesecondthe branches are distinct, as in theLepidopteraand the majority of insects.
The number of branches varies in different genera and species. InEchinomyia grossa, a large fly, there are only thetwoprimary branches[718]; in the common dung-beetle (Geotrupes stercorarius) these appear divided at their apex into fingers[719]: inScolia, a Hymenopterous genus, and the butterfly of the nettle, there arethreesecondary branches on each side[720]: in many otherLepidopteraand the humble-bee there arefour[721]; in the common louse there arefive[722]; in the rhinoceros-beetle and the cockchafer,six[723]; in the wasp,seven[724];eightin the cockroach[725];twelvein theCarabiand the mealworm-beetle[726];thirtyin the large green grasshopper (Acrida viridissima[727]);thirty-twoin the cheese-maggot-fly[728]; and in the hive-bee more thana hundred and fifty[729].
The number ofeggsalso contained in the ovaries varies. InEchinomyia grossathere is onlyoneegg in each, and onlytwoat once in the matrix[730]: in another fly produced by the cheese-maggot there arefour[731]; in the louse there arefive; in the cockchafersix[732]; in the hive-beesixteenorseventeenare visible at the same time[733]; and in the silkworm-mothsixtyorseventy[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 thebud 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 andLepidoptera[740]; in the hive-bee they appear to be shorter[741].
IV. We are next to consider theOvipositor, 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 theCreatorhas 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 theColeopteraOrder, 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: inBuprestisit consists of three long and sharplaminæ, the two lateral ones forming a sheath to the intermediate one, which probably conveys the eggs[743]: inElaterit 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 ofPrionus coriariusdiffers from that ofCallidium 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 theOrthopterathe instrument of oviposition is more simple; inLocustaconsisting 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 ofAcrida 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 oftwovalves only[749]; but in reality it consists ofsix, 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 connectConocephaluswithGryllus. In this the ovipositor is considerably longer than the body of the animal, and is composed ofsixdistinct pieces; viz.twoexternal ones stouter than the rest, and within thesefourothers finer than a hair and convolute at the apex[750]. There is a considerable variety in the shape of the ovipositors of theAcridæand the cognate genera:—thus inA. viridissimathis organ is straight, inA. verrucivorabent like a sabre, and inPterophylla citrifoliaand some others, the whole machine is short and boat-shaped; inScaphura Vigorsiiit 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 theCicadæand 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 theupperpieces, 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 isapparentlybivalve like that ofAcrida, but I believe is resolvable into the same number of pieces.