Lyonet counted the muscles contained in the body of the caterpillar of theCossus. In the head he found 228; in the body, 1647; and enveloping the intestines,no less than 2186; which, after deducting 20 that are common to the gullet and the head, gives a total of 4061[893]. In the human subject only 529 have been counted[894]: so that this minute animal has 3532 muscles more than the Lord of the creation!
The muscles of theArachnidaseem less numerous than those of insects. In theScorpionideathey appear to be robust, formed of simple straight fibres, of a whitish gray colour: a muscular web, rather strong, clothes theparietes, but rarely adheres to them, of the abdomen, and envelopes theviscera, with the exception of the lungs, and probably of the heart. The dorsal part of this web gives birth to seven pairs of filiform muscles, which traverse the liver, and are attached to a muscular riband which, passing above the lungs, runs the whole length of the ventralparietes. These muscles when exposed to view resemble extended cords. The abdominal segment preceding the tail is filled with a powerful muscular mass which moves that organ[895]. Treviranus discovered two longitudinal muscles inScorpio europæus, running from the breast to the tail, which above and below each gill were connected by another running transversely across the heart, thus forming a quadrangular area in which the gills are situate[896]. The heart appears to be moved by muscles not very dissimilar to those of theCossus[897], as is likewise that of theAraneidea; inClubiona atroxthe wider part of this organ is muscular, and incloses a considerable cavity[898]. In this tribe the muscles of theabdomen, the skin of which is soft and unfit to act as a lever to them, are attached to a cartilage, and thus their action is better sustained[899].
Having thus laid before you all of importance that I can collect with regard to the apparatus of muscles discoverable in insects, I shall next say something upon a few other points connected with that subject. When I enlarged upon theirmotions, I related a few instances of the extraordinary power of that apparatus[900]inleapingones; but this power is not confined to that circumstance. Theflea, not more remarkable for its compressed form, enabling it to glide between the hairs of animals, and its elastic coat of mail, by which it can resist the ordinary pressure of the fingers, than for its muscular strength, has attracted notice on this account from ancient times. Mouffet relates that an ingenious English mechanic, named Mark, made a golden chain of the length of a finger, with a lock and key, which was dragged by a flea;—he had heard of another that was harnessed to a golden chariot, which it drew with the greatest ease[901]. Another English workman made an ivory coach with six horses, a coachman on the seat with a dog between his legs, a postillion, four persons in the coach, and four lacqueys behind—which also was dragged by a single flea. At such a spectacle one would hardly know which most to admire, the strength and agility of the insect, or the patience of the workman. Latreille mentions a flea of a moderate size dragging a silver cannon on wheels, that was twenty-four times its own weight, which being charged with powder, was firedwithout the flea appearing alarmed[902]. Many caterpillars are accustomed to extend their bodies from a twig, supported merely by the four hind feet, in one fixed attitude, either in an oblique, horizontal, or vertical direction, either upwards or downwards, and that for hours together. We may conceive what prodigious muscular force must be exerted upon this occasion, by reflecting that the most expert rope-dancer, though endued with the power of grasping with his feet like a bird with its claws, could not maintain himself in a horizontal position even for an instant. Bradley asserts that he has seen a stag-beetle carry a wand half a yard long and half an inch thick, and fly with it several yards[903]. Some insects have the faculty of resisting pressure in a wonderful degree. If you take a common dung-chafer (Geotrupes) in your hand and press it with all your strength, you will find with what wonderful force it resists you; and that you can scarcely overcome the counteraction, and retain the insect in your hand: was it not for this quality, the grub of the gad-fly must be crushed probably in passing through the anal sphincter of the horse[904]. But that ofEristalis tenaxaffords a more surprising instance of this power of counteraction:—an inhabitant of muddy pools, it has occasionally been taken up with the water used in paper-making, and strange to say, according to Linné, has resisted without injury the immense pressure given to the surrounding pulp[905]; likeleather-coat Jackmentioned by Mr. Bell[906], who, from a similar force ofmuscle, could suffer carriages to drive over him without receiving any injury. Almost as remarkable is the state of extreme relaxation into which the muscles of some larvæ fall, when their animation is suspended; and the revived tension to which a subsequent resumption of the vital powers restores them. Bonnet having suspended the animation of the caterpillar ofSphinx Ligustriby keeping it submerged, squeezed it between his fingers, until it had wholly lost its cylindrical form and was as flat and supple as the empty finger of a glove; yet in less than an hour the very same caterpillar became as firm, as compact, as cylindrical, and in short, as well, as though it had never been submitted to treatment so rough[907].
It is fortunate that animals of a large size, as has been well remarked, especially noxious ones, have not been endowed with a muscular power proportionable to that of insects. Acockchafer, respect being had to their size, would besixtimes stronger than ahorse; and if theelephant, as Linné has observed, was strong in proportion to thestag-beetle, it would be able to pull up rocks by the root, and to level mountains[908]. Were thelionand thetigeras strong and as swift for their magnitude as theCicindelaand theCarabus, nothing could have escaped them by precaution, or withstood them by strength. Could theviperand therattlesnakemove with a rapidity and force equivalent to that of theIulusandScolopendra, who could have avoided their venemous bite? But theCreatorin these little creatures has manifested his Almightypower, in showing what he could have done had he so willed; and hisgoodnessin not creating thehigher animals endued with powers and velocity upon the same scale with that of insects, which would probably have caused the early desolation of the world that he has made. From this instance we may conjecture, that after the resurrection, our bodies by a change in the structure and composition of their muscular fibre—for we know that their locomotive powers and organs, as far as the muscle is concerned, will then be of a very different nature[909]—may become fitted for motions and a potent agency of which we have now no conception.
This wonderful strength of insects is doubtless the result of something peculiar in the structure and arrangement of their muscles, and principally their extraordinary power of contraction, excited by the extent of their respiration: for animals that respire but little, as the fœtus in the womb and the pullet in the egg, have very little contractile muscular power[910]. To get some idea from facts of this extraordinary contractile power in insects,—extract the sting of a bee or a wasp, with its muscles, which appear to be attached to powerful cartilaginous plates[911], and you will find it continue for a long time to dart forth its spicula, almost as powerfully as when moved by the will of the animal. A still more extraordinary instance of irritability is exhibited by theantlia, or instrument of suction of the butterfly. If this organ, which the insect can roll up spirally like a watchspring or extend in a straight direction, be cut off as soon as the animal is disclosed from the chrysalis, it will continue to roll up and unroll itself as if still attachedto its head: and if after having apparently ceased to move for three or four hours it be merely touched, it will again begin to move and resume the same action. This surprising irritability and contractility of muscle doubtless depends upon the peculiar structure of the antlia, which is composed of an infinite number of horny rings, acted upon by muscles, more numerous probably than those which move the trunk of the elephant. The motion only ceases when the muscles become dry and rigid.
I have already, under another head[912], considered theannualsleep, or winter state of torpidity of insects, during which an intermission for the most part of muscular motion and action takes place. I shall now make a few observations with respect to theirdiurnalsleep, which may very properly have its place in the present letter. That insects, usually so incessantly busy and moving in every direction, require their intervals of repose, seems to call for no proof. We see some that appear only in theday, and others only in thenight, others again only at certain hours; which leads to the conclusion, that when they withdraw from action and observation, it is to devote themselves to rest and sleep. The cockchafer flies only in the evening; but if you chance to meet with it roosting in a tree in the earlier part of the day, you will find it perfectly still and motionless, with its antennæ folded and applied to the breast:—we cannot indeed say that its eyes are shut; for as insects have no eyelids, that sign of sleep can never be found in them. Again, if a Lepidopterist goes into the wood to capture moths inthe day-time, he finds them often perched on the lichens that cover the north side of the trunk of a tree, with their wings and antennæ folded, and themselves without motion, and insensible of his approach and their own danger. Thus it was that I captured that rare insect the lobster-moth (Stauropus Fagi) in the New Forest. Some, however, have asserted that the caterpillar of the silkworm, except when they moult, never intermits feeding day or night, and consequently does not sleep: but the accuracy of this statement, both from analogy and observation, admits of great doubt. Malpighi informs us that these caterpillars for an hour and more, twice a day, remain immoveable with their heads bent down as though asleep, and even if disturbed, resume again the same inactive posture[913]; and other larvæ in great numbers certainly seem to have regular intermissions from eating of considerable duration: those called Geometers, for hours together remain motionless projected from a twig, to which they adhere by their posterior prolegs alone; and the processionary caterpillars make onlynightlysorties from their nests, passing the day in inaction and repose[914]. Bees have been often seen by Huber, when apparently wearied with exertion, even in the middle of the day, to insert the half of their bodies into an empty cell, and remain there, as if taking a nap, without motion for half an hour or longer[915]; and at night they regularly muster in a state of sleep-like silence. Mr. Brightwell once observed an individual living specimen ofHaltica concinna, which appeared to remain motionless on the same spot of a wall for three successive days.
Before concluding these remarks on the Internal Anatomy and Physiology of Insects, I shall explain to you, as you will probably feel inclined occasionally to pursue the subject, the best mode ofdissectingthem.—By far the most useful dissecting instruments for this purpose are very fine-pointed and sharpscissors, as these will enable you to divide the integument and separate other parts with much less risk of injuring their delicate structure than any knife. These scissors are what Swammerdam chiefly used; and he had some so extremely small and fine, that he was necessitated to employ a lens when he sharpened them. If to these be added a sharp and fine-pointedknifeor two, someneedlesfixed in handles, also fine-pointed—(you will find them more convenient than any other instrument for detaching minute parts and fibres,) a pair of fine and accurately adjustedpliers, and an assortment of camel's-hairbrushes,—you will be nearly set up as an Entomological dissector. You will still, however, require a small dissecting table, with a projecting and moveable arm for lenses of various descriptions, so as to admit both the hands to be employed upon the subject under examination; and for this purpose probably no contrivance can be better adapted than that of Lyonet, of which the figure in AdamsOn the Microscopewill convey a better idea than any description[916].
Previously to dissecting any insect, it must be killed by plunging it into boiling water, which is recommended by Lyonet, or spirits of wine or of turpentine; and it is often useful to letlarværemain a few days in the latter,by which means the vessels become firmer and stronger. The parts ofpupæbecome much more distinct if they are boiled for a few minutes: and the same mode may be adopted in the examination of spiders.
The most convenient mode of proceeding, which was that also of Lyonet, is to dissect the insect in water, or, to avoid putridity, in diluted spirits,—if small, upon a concave glass, to which it should be fastened by means of a little melted wax; if larger, in the bottom of a common chip box, surrounded with a border of wax to retain the fluid. The integuments of the insect, being carefully divided longitudinally with scissors, should if flexible be turned back, and fixed by small pins stuck in by a fine pair of pliers, while the skin at the same time is stretched by another. After making such observations as present themselves without further dissection, the viscera must be cautiously extracted, washing away the fat which surrounds them with spirits of turpentine, in which it is soluble, applied by camel's-hair pencils. After separation they may conveniently be examined by putting them into water, and gently shaking them so as to cause the parts to unfold. If endowed with the patience of Swammerdam, you may even arrive at injecting these minute parts with wax or coloured fluids, conveyed by delicate glass tubes having one end as fine as a hair, which he also employed to fill the viscera with air; and afterwards drying them in the shade, and anointing them with oil of spike in which a little resin had been dissolved, he succeeded in preserving them. If it is not convenient to finish the dissection of an insect at once, it should be covered with spirits of wine. Swammerdam found a mixture of spirits and distilled vinegarvery useful for keeping caterpillars previously to dissecting them, as it consolidated the parts[917].
And now having brought to a close my long wanderings in this ample and intricate field, and having threaded, as well as my slender powers and limited knowledge enabled me, the infinite turnings and convolutions of this Dædalean labyrinth—theAnatomyandPhysiologyof insects,—will you not own that the volume of wonders I have laid before you proves irrefragably that, though these minims of nature apparently rank so low in the scale of being, yet in their structure, instead of being, as might be expected, more simple, they are infinitely more complex and highly wrought than those animals that are placed the nearest to ourselves? theCreatorin the latter doing every thing by a beautifulsimplicity; while in the former, the more to magnify his power and skill, because they afford no apparent space for it, by a wonderfully curious and intricatemultiplicity: and whether we study the one or the other, we shall in both trace the footsteps of that adorableLovewhich has shown attention to the comfort and well-being of the lowest insect, as well as of the highest of his creatures.
I am, &c.
Having laid before you what observations I thought might sufficiently explain all the principal features of the Anatomy of insects both external and internal, you will next expect to be informed whether, like the higher animals, they are subject to have the admirable order observable in their frame interrupted byDisease; and you will perhaps imagine, from the multiplicity of their organs and vessels, that they must be peculiarly exposed to derangements of the vital and other functions. That they have their diseases is certain; but, except in the case of their appropriate parasitic assailants, which is a part of their economy, it does not appear that their maladies are more numerous and frequent than those of other animals. The sameAlmighty Powerwhich endowed them with so complex a structure, generally upholds them in health during their destined career, until they have fulfilled the purpose of their creation, whenthey die and return again to their dust[918].
But perhaps I may seem to you as making too great a parade about these little insignificant creatures if I assign a separate letter to the consideration of theirdiseases: but when you recollect that Aristotle has a chapter onthis subject[919], and that the learned Willdenow has devoted a distinct portion of his excellent introductory work on Botany to the diseases of Plants[920],—you will perhaps be of a different mind: indeed, some facts I shall have to communicate are so remarkable and interesting, that I am sure, when you have read this letter, you will not think the subject one that deserves to be slighted.
Insect diseases may, I think, be divided into two great classes; those resulting, namely, from some accidentalexternalinjury orinternalderangement, and those produced byparasiticassailants.
I. Under thefirsthead we may begin withwounds,fractures,mutilations, and otherextraneouscauses of disease. To these—insects are peculiarly subject; and though they are not, like theCrustaceaandArachnida[921]and some other invertebrate animals, endowed with the power ofreproducinga mutilated limb, yet their wounds appear to heal very rapidly, and at the time they are inflicted to produce little pain[922]. But if those important members, theirantennæ, are mutilated, insects seem to suffer a kind of derangement; the great organ of their communication with each other, and in various respects with the external world, being removed, all their instincts at once fail them. I formerly related how the amputation of these affects thequeen-bee[923]. A similar result, as Hubertells us[924], follows, when the same experiment is repeated on theworkersordrones: they immediately become unable to take any further part in the labours of the hive; they can no longer guide themselves except in the light; if they petition one of their fellow-citizens for honey, they are unable to direct their tongue to its mouth to receive it; they remain near the entrance of the hive, and when the light is intercepted they rush out of it to return no more.
Insects occasionally are subject totumoursor a preternatural enlargement of their parts and organs. The antennæ of bees sometimes swell at their extremity so as to resemble the bud of a flower ready to open, becoming at the same time very yellow, as does the fore part of the head[925]. I once saw a specimen of aHydrobius—agreeing withH. fuscipesin every other respect even to the most minute punctum—which had a large tumour on each side of theprothorax, evidently accidental, occasioned probably by the stoppage of the pores by which the superfluous moisture and air escape when it undergoes its last change. The converse of this I have observed to take place sometimes in the same part ofGeotrupes foveatus, the ordinary lateralfoveæbecoming very considerably enlarged;—this was the case with the specimen from which Mr. Marsham made his description of that insect. The species is, however, very distinct in other respects, and may always be known by its small size. It happens now and then also, that these tumours representblisters. I saw one once on one elytrum of a beetle and not on the other. Those ofSerropalpus(asMr. MacLeay, on the authority of M. Clairville, informs me) are particularly subject to this disease. But, of all the organs, the wings are most exposed to derangements of this kind. De Geer, in a specimen ofPieris Cratægijust excluded from the chrysalis, observed that one of these was distended by a considerable quantity of extravasated green fluid—two or three large drops following an incision. This disease appeared to arise from the lower membrane not adhering to the upper; so that the nervures—which are rather longitudinal channels, being open below, than tubes—were not closed to confine the fluid to its proper course. The malady, which might be called a dropsy of the wing, carried off the insect the day after its exclusion[926]. Reaumur observed that the wings of some flies were affected by anair-dropsy, as he calls it, which appeared to arise from the air escaping from its natural channels, and thus separating, the two membranes that form the wing, and filling the cavity produced by their separation[927].
Sometimes alsomonstrositiesare to be met with in these animals, or variations from a symmetrical structure in organs that are pairs. I have a beetle in which the terminal joint of one of the maxillary palpi is short, ovate, and acute; and that of the other, long, semiovate, and rather obtuse. A specimen ofBlaps mortisagain my cabinet, taken by Mr. Denny, besides the terminal mucro of theelytra, has a long diverging lateral one. Goeze had the larva of aSemblisbrought to him in which one of the two fore-legs, though perfect in all its parts, was only half the length of the other[928]; which he regardedas a reproduction, but it seems rather a malformation. Müller mentions a most extraordinary fact of one of theNoctuidæ, which when disclosed from the pupa retained the head of the larva[929]. One of the most remarkable instances of this kind that have fallen under my own observation, may be seen in a specimen ofChrysomela hæmopterain the cabinet of our friend Curtis; in which one of the thighs produces a double tibia, but only one of these is furnished with a tarsus.
The diseases of insects which arise from someinternalcause are not very numerous. The first that I shall mention is a kind ofvertigo. "Ants have also their maladies," says M. P. Huber: "I have noticed one extremely singular; the individuals attacked by it lose their power of guiding themselves in a straight line, they can walk only by turning round in a circle of small diameter and always in the same direction. A virgin female shut up in one of my glasses was seized on a sudden with this distemper; she described a circle of an inch in diameter, and made about a thousand turns in an hour, or not quite seventeen in a minute. She continued constantly turning round for seven days, and when I visited her in the night I found her still in motion. I gave her honey—and I think that she ate some of it." He observed that some workers were attacked by a similar disease: one of these, however, had the power of walking from time to time in a straight line; when placed upon its head it continued its gyrations[930]. Similar motions of a little moth, mentioned on a former occasion[931], may perhaps have been produced by the same cause. Bees are alsosubject to vertigo, which has been attributed to their eating poisonous honey[932]—but may not this disease in all these cases arise from some derangement of the nervous system? One of the ants which was so affected had lost one of its antennæ; but as this was not the case with the others, no great stress is to be laid upon the circumstance. Huber does not inform us whether those attacked by this disease recovered or not.
I have observed more than once, that theflesh-flyand some others of the same tribe are subject in particular seasons to a kind ofconvulsions. When thus attacked, they kick and struggle, and seem unable to fly. Sometimes they lie upon their backs without motion, but if a finger be placed near them their convulsive motions are renewed. When thrown into the air, instead of flying, they fall to the ground. Had this distemper occurred earlier or later in the year I should have attributed it to the benumbing effects of cold; but as my observations were made one year (1816) inMay, and in another (1811) in the latter end ofJune, this could scarcely be the case. In the year last mentioned I observed that many flies died under its influence. In wet seasons this tribe is subject to another disease, which proves fatal to many of them, and indeed to otherDiptera. A white crust appears to be formed upon the abdomen both above and below, of a granular appearance, much resembling fine moist sugar. On the back of that part this crust does not cover the margins of the segments, which gives it the appearance of white bands; so that deceived by it, I have often at first flattered myself that I had met withsome new species. The under-side of the abdomen is wholly covered by it, divided in the middle into two longitudinal masses, the anal segment being bare. De Geer has noticed this or a similar disease, which, when flies are attacked by it, causes the abdomen to swell so as even to burst, and the segments become dislocated. Upon opening the abdomen it is found filled with a white unctuous substance, which often accumulates (as above described) on its external surface[933]. Dr. Host says that in this disease when the animal is dead, the wings, which were before incumbent, become extended, and its almost invisible pubescence grows into long hairs[934]. De Geer seems to think that these flies are thus affected in consequence of having eaten some poisonous food[935]; but I rather suspect, as I have observed it become prevalent chiefly in wet seasons, that it arises from a superabundance of the nutritive fluid, or of the fat, so that it seems to be a kind ofplethora. I once observed a fly fixed to a pane of glass, round which was a semicircle of what appeared to be merely vapour, whose radius was nearly three-fourths of an inch. Taking it for an aqueous fluid that had transpired from the dead animal, I paid no further attention to it at that time: but observing from day to day that the moisture did not evaporate, after two or three months had elapsed, I had the curiosity to examine it more closely, and, upon scraping some of it off with a penknife, I found it was a white substance of a fatty nature. In this case, then, the fat must have exploded on all sides with considerable violence from half the body or the abdomen. Probably this wasa more intense degree of plethora. When I examined this appearance the fly had fallen off, and I could not find it.
Mr. Sheppard once brought me a panicle of grass, the glumes of which were rough with hairs, or small bristles, to which several specimens of a fly related toXylota pipiensadhered by their proboscis. At first I thought that having been entrapped by the bristles, and unable to extricate themselves, they had perished from want of food; but since when touched they readily dropped from the glumes, some other cause, perhaps disease, probably occasioned this singular suspension of themselves.
The maladies to whichbeesandsilkwormsare subject are more interesting to us than those offlies, on account of their utility ascultivatedinsects. One of the worst distempers which attacks the first of these animals is a kind ofloosenessordysentery: this happens early in the year, when they are fed with too much honey without any portion of bee-bread[936], and sometimes destroys whole hives. Their excrements, instead of a yellowish red, then become black, and the odour they emit is insupportable; the bees no longer observe their usual neatness, inducing them to leave the hive when they void their excrements, but they defile it, their cells, and each other. Several remedies have been prescribed for this disease. To prevent it, a syrup made by an equal mixture ofgood wine and honey is recommended; and as a cure, to place in the hive combs containing cells filled with bee-bread[937]. But one of the worst maladies to which these useful animals are subject, is that called by SchirachFaux Couvain. It originates with the larvæ; and is caused either by their being fed with unwholesome food, or when the queen, as sometimes happens, lays her eggs so that the head of the grub is not in a proper position for emerging from the cell when the period for its disclosure is arrived:—the consequence is, that in both cases it dies and becomes putrid, which sometimes produces a real pestilence in a hive. The remedy for this evil is to cut away the infected combs, and to make the bees undergo a fast of two days[938]. The hive should be cleaned and fumigated, by burning under it aromatic plants.
The cultivators of thesilkwormin France have given names to several diseases to which that animal is subject. One is calledLa Rouge, and is supposed to be occasioned either by too great heat, or by too sudden a transition from cold to heat. It takes place when the caterpillar is first hatched; which lives perhaps, but in a very sickly state, till it should spin its cocoon and assume the pupa, when it expires. Another degree of the same disease is calledLes HarpionsorPassis. A second distemper of this animal isDes Vaches,Le GrasorLa Saune: this is a mortal disease, supposed to be of a putrid nature, and produced by mephitic air; it shows itself after the second moult, but rarely after the subsequent ones. When a caterpillar is first attacked, changing the air may prove a remedy; but when the disease has madeprogress, it is best to burn or bury them, since if the poultry pick them up they might be poisoned by them. A third disease of silkworms is calledLes Morts Blancs, orTripes, which is also occasioned by impure air, when the leaves the animal feeds upon are heaped so as to produce fermentation. The caterpillars attacked by it die suddenly, and preserve after their death the semblance of life and health. Too great heat, whether artificial or natural, occasionsLa Touffe, a fourth, which, when the heat continues long, destroys all those that are arrived at their last stage of existence in their larva state. Black points scattered over different parts of the body, or livid and blackish spots in the vicinity of the spiracles, followed by a yellowish or reddish tint, are symptoms of a fifth malady, calledLa Muscardine. After this the animal soon dies, and becomes mouldy, but does not stink. This disease is not contagious, and is thought to be caused by a moist heat, attended by pernicious exhalations.La Luzette,Luisette, orClairène, is another malady, which shows itself most commonly after thefourthmoult. It seems to arise from some original defect in theegg. The caterpillars attacked by it may be known by their clear red and afterwards dirty white colour; their body becomes transparent, and the matter of silk exudes in drops from their spinnerets; consequently, though as voracious as the rest, they are never able to construct a cocoon, and should be destroyed.Les Dragéesis the name given to cocoons which include a larva that never becomes a pupa. The cause of this disorder has not been ascertained, and whole broods are sometimes subject to it, which, as in the last, seems to imply some defect in theeggs. But as the caterpillar spins its cocoon, and the silk is as good as usual,it is a malady of no great importance. Lastly, sometimes the mulberry leaves have a gummy rather acrid secretion, which purges the silkworms; their excrement is no longer solid; they become weak and languid; and if the secretion is abundant, their transpiration is impeded, and at the time of moulting they are become so feeble as to be unable to cast their skin[939].
In the case of many caterpillars ofLepidopterathat died, Bonnet found by dissection that the disease was remotely occasioned by adiarrhea, which taking place immediately before they became pupæ, prevented the inner membrane of their intestines from being rejected, as it would have been if no extraordinary cause had prevented it, attached to the hard excrement. He found this membrane converted into a jelly occupying great part of the stomach, which he conjectured was the proximate cause of their death[940].
To conclude this head—spidersare reputed to be subject to thestone: I do not sayCalculus in Vesica; but we are informed by Lesser that Dr. John Franck having shut up fourteen spiders in a glass with some valerian root, one of them voided an ash-coloured calculus with small black dots[941].
II. I now come to that class of diseases which appears to prevail almost universally amongst insects—I mean those resulting from the attack ofparasiticenemies. Thus millions and millions annually perish before they have arrived at their perfect state. Diseases of this kind proceed either fromvegetableoranimalparasites. I shall begin with the first, which will not occupy us long.
i. As insects pass often no small portion of their lifein a state of torpidity, in which they remain chiefly without motion, it will not seem wonderful, should any partial moisture accidentally accumulate upon them, that it affords a seed plot for certain minute fungi to come up and grow in. Persoon observes with regard to his genusIsaria, that one species grows upon thelarvæof insects (I. truncata), and another uponpupæ(I. crassa[942]):—as he does not say upondeadlarvæ and pupæ, as upon a former occasion[943], perhaps in these cases these plants may constitute an insect disease; but I lay no stress upon it, and only mention the circumstance here as connected with the history of these animals. Mr. Dickson has described aSphæriaunder the name ofentomorhizathat grows upondeadlarvæ; it has a slender long stipes and spherical granulated head: on the pupa of a species ofCicadain my cabinet, another kind ofSphæria, with a twisted thickish stipes and oblong head, springs up in the space between the eyes. I observed something similar but longer, in the grub of some large beetle in M. Du Fresne's museum at Paris; and I have a memorandum of having noticed something of the kind on the rostrum of aCalandra. Bees and humble-bees have been sometimes thought to have some species ofmucoror otherFungillioccasionally growing upon them; but Mr. Brown is of opinion thatstaminawhich they have filched from flowers have been mistaken for theseFungilli, since he has detected those ofOrchideæin some of this tribe, and upon a beetle shown to him by Mr. MacLeay, one which he knew to be the stamen of anAristolochia. I once observed a bunch of what I mistook for a singularmucorthat adorned the vertex of a humble-bee, between the antennæ, which doubtless wereof the same description; and I even saw one upon its wing. Upon a former occasion I mentioned a parallel circumstance with respect to a species ofXylocopa[944].
ii. Theanimalparasites that infest insects are either themselvesinsects; orworms.
1. Theirinsectinfesters, as far as we know at present, are confined to the OrdersStrepsiptera,Hymenoptera,Diptera, andAptera: they attack them sometimes in theireggstate, most frequently when they are larvæ, occasionally when pupæ, and very rarely in their perfect state. Upon many of these I have formerly enlarged[945], and I shall now add such further circumstances as I then omitted. TheStrepsipteraOrder, as at present known, consists only of two genera,StylopsandXenos; the first being appropriated to the imago ofAndrena, a kind ofbee, and the latter to that of thewasps. Their eggs appear to be deposited in the abdomen of these insects in which they feed, till having attained their full growth they perforate the membrane that connects its segments; and at the proper time their pupa-case bursts, they emerge, and take their flight. Sometimes four or five infest a single bee. Whether the latter dies upon their quitting it I have not been able to ascertain, but from their flying, when the little parasite is very near leaving them, with their usual activity, it should seem that this disease is not mortal; but it probably prevents their breeding: I do not recollect observing the exuviæ of one in amalebee[946].
The great body of insect parasites, however, belong to theHymenopteraOrder, and chiefly to the Linnean genusIchneumon. The insects of this order have beendenominatedPrincipes, because of the wonderful instincts of ants, wasps, bees, and other gregarious tribes that belong to it; and they merit a name of honour not less for the benefits that they confer upon mankind, by keeping within their proper limits the various insect-destroyers of the produce of the globe. It deserves notice that when these latter increase to a degree to occasion alarm, their parasites are observed to increase in a much greater, so as to prevent the great majority of them from breeding[947]. Though these benefactors of the human race constitute numerous genera, at present not well ascertained, I shall speak of most of them under the common name ofIchneumon.
The appearance of these little four-winged flies puzzled much the earlier naturalists:—that a caterpillar usually turning to amothorbutterflyshould give birth to myriads offlies, was one of those deep mysteries of nature which they knew not how to fathom[948]: even the penetrating genius of our great Ray, though he ultimately ascertained the real fact[949], was at one time here quite at fault; for he seems at first to have thought, when from any defect or weakness nature could not bring a caterpillar to a butterfly, in order that her aim might not be entirely defeated, that she stopped short, and formed them into more imperfect animals[950].
Before I detail more particularly the proceedings of Ichneumons, I shall make a few general remarks upon them. The structure of the instrument by which they are enabled to deposit their eggs in their appropriate station has been before sufficiently described[951]; it is longor short according to the situation and circumstances of the larva which receives them: if this lives in the open air, and the access to it is easy, it is usuallyshortand retracted within the body; but if it lies concealed in deep holes or cavities, or shuns all approach, it is often very long. Thus inPimpla Manifestator, which commits its eggs to the grub of a wild bee inhabiting the bottom of deep holes bored in posts and rails, the ovipositor is nearly an inch and half in length, and in some extra-European species three inches. How the egg is propelled so as to pass in safety from the oviduct, along this extended and very slender instrument to the grub for which it is destined, has not been certainly ascertained: but from an observation of Reaumur's[952]it should seem that it is aided in its passage by some fluid ejected at the same time with it, or is so lubricated as to slide easily without being displaced. The flies we are speaking of, by some authors are calledMuscæ vibrantes, because when searching for the destined nidus of their eggs their antennæ vibrate incessantly, and it is by the use of these wonderful organs that they discover it wherever it lurks. Bergman observed thatFœnus Jaculatorsearches for the latent grub of certain bees and otherHymenopterawith its antennæ[953]: and from Mr. Marsham we learn thatPimpla Manifestator, before it inserts its ovipositor in the nest of the grub ofChelostoma maxillosa, explores it first with one antenna and then with the other, plunging them all the while intensely quivering up to the very root[954]. With respect to theirsize, Ichneumons vary greatly; some being so extremely minute as to be invisible to the naked eye, unless moving upon glass; while others, as totheirlength, emulate the giants amongst insects. The former, unless appropriated to the eggs themselves, usually commit many eggs to a single larva, while the latter are directed by their instinct to introduce into them only one. Some of the former description are endowed with the faculty of leaping[955]. The food of Ichneumons, and indeed of other internal parasites, is chiefly theepiploonor fat of the larva, but they never touch any vital organ; so that it continues to feed, and probably more voraciously, grow, cast its skin, and often it changes to a chrysalis, although at the same time inhabited by an army of these little devourers.
Ichneumons, as far as has been at present ascertained, are parasitic upon other insects chiefly in theirthreefirst states, a solitary instance only having been observed of their inhabiting animago; but from their first exclusion as eggs from the ovary till their assumption of that state they give them no rest. I shall therefore first treat of those that infest theeggs; next those appropriated tolarvæ; and lastly those that devourpupæ.
Vallisnieri appears to have been the first naturalist who discovered that Ichneumons were appropriated to theeggsof other insects. He observed one proceed from those of the emperor-moth (Saturnia Spini): finding two holes in each egg, one larger than the other, he conjectured that one was made when it entered, and the other when it emerged. In this case the egg of the Ichneumon must be fixed on the outside of the egg itwas to feed upon; though some appear to pierce it with their ovipositor, and consequently introduce their egg within: for he says afterwards; "I have seen with my own eyes a certain kind of wild flies deposit their eggsuponother eggs, andboreand pierce others with an aculeus—by which they have introduced the egg[956]." Count Zinanni, a correspondent of Reaumur's, saw an Ichneumon pierce the eggs with her ovipositor repeatedly; which in about fifteen days were filled with the pupa, and in six more produced the imago[957].I. OvulorumL. is the onlyknownspecies of egg-devourers; but most likely there are many, varying in size, according to the size of the egg they inhabit. ProbablyI. AtomusL., andI. PunctumShaw, are of this description[958]. It is wonderful what a number these little flies destroy:—out of a mass of more thansixtyeggs which was brought to De Geer, notonehad escaped the Ichneumon[959]. But the most extraordinary thing is, that even these little creatures we are told are destroyed by another still more minute[960].
Though the animals we are speaking of usually destroy only asingleegg, yet some appear not so to confine themselves. Geoffrey informs us that the larva of one of the Ichneumons whose females are without wings (Cryptus) devours the eggs of the nests of spiders, and from its size—it is nearly a quarter of an inch long—it must require several of them to bring it to maturity[961]. One of those also which destroys the gnat infesting the wheat (I. inserens) appears to devour them intheireggstate, and could not be brought to perfection by the food that a single one would furnish[962].
The Ichneumons that are parasitic uponlarvæare the most numerous of all. Some of them are deposited by the parent fly on theoutsideof their prey, and others introduced into itsinterior.Ophion luteumis one of the former tribe; it plants its eggs in the skin of the caterpillar of the puss-moth (Cerura Vinula). Each egg is furnished with a footstalk terminating in a bulb[963], which is so deeply and firmly fixed that it is impossible to extract it without detaching a portion of the animal with it, and even when the caterpillar changes its skin it is not displaced. After it is hatched, the grub, while feeding, keeps its posterior extremity in the egg-shell, to which it adheres so pertinaciously, that it is scarcely possible to disengage it without crushing it. It fixes itself by its mandibles to the skin of the caterpillar, and keeps constantly sucking the contents of its body till it dies: sometimes nine or ten of these larvæ inhabit a single caterpillar[964]. Reaumur has given an account of other external Ichneumons. Upon one caterpillar that he examined, they were so numerous as to render the poor animal quite a spectacle, and they underwent their metamorphosis attached to it[965]. One species of this description avenges the cause of insects upon their most pitiless foes, the all-devouring spider—for in the midst of her toils and lines of circumvallation it makes her its prey. De Geer, meeting one day with a young spider of a common kind, observed with surprise, engaged in sucking it, a small white grub, which was firmly attached to the abdomen near the trunk. Putting it by in aglass, after some days he examined it again; when he observed that it had spun the outline of a vertical web, had stretched threads from the top to the bottom of the glass and from one side to the other, and had also spun the radii that meet in the centre, and this was all;—but what was remarkable, the larva that had fed upon it was suspended in the centre of this web, where it was engaged in spinning its own cocoon, while the spider, exhausted by this last effort, had fallen dead to the bottom of the glass. It cannot be asserted positively that this suspension of the larva of the Ichneumon in the centre of the webalwaystakes place; but if it does, as seems most probable, it shows that this little parasite is endowed with an instinct which causes it so to act upon the spider as may induce it to spin a web so nicely timed as to be sufficiently complete at the period of its death and of the change of the Ichneumon, for the latter to cast it down and assume its station[966].
But the great bulk of the parasitic Hymenopterous devourers of larvæ have their assigned stationwithinthe body. As Entomologists in breeding insects have paid their principal attention toLepidoptera, it necessarily follows that their Ichneumon infesters must be most generally known; but doubtless the larvæ of the other Orders are not wholly liberated from this scourge: they also require to be kept within due limits, and have their appropriate parasites. Some, however, in most of them have been detected: of which I shall now proceed to state to you the most interesting examples, beginning with theColeoptera.
Alysia Manducator[967], remarkable for having mandibulæthat do not close, and toothed at the end, usually attends masses of dung, both of man and cattle, probably for the purpose of depositing its eggs in some of theColeopterouslarvæ that inhabit it. Mr. Stephens, one of the most accurate observers as well as one of the best Entomologists of the present day, informs me that he once captured three specimens ofTimarcha tenebricosa, from each of which forty or fifty minute Ichneumons emerged. An insect also of this Order, that is a great benefactor to mankind, as a destroyer of the plant-lice,—I mean the lady-bird (Coccinella), in its larva state is itself subject to the attack, as we learn from De Geer, of one of these small parasites[968]. He detected them also in that of two species of weevils: and in the pupa of some large grub of a beetle inhabiting the wood of the elm, perhaps that of the stag-beetle, he found the pupa of one of those Ichneumons that have an exserted ovipositor[969]. Doubtless, did we know their history, we should find that numberless species have their internal assailants belonging to this tribe.
Orthopterouslarvæ seem not to have been yet announced as affording a pabulum to these animals: but the late Dr. Arnold, whose tact for observation with regard to the manners and economy of insects has rendered his loss irreparable, discovered that the remarkable parasitic genusEvaniawas appropriated to the all-devouringBlatta. Whether it attacked it in its egg or larva state I have not been informed. This little benefactor is here extremely rare, at least in thecountry; perhaps intowns, where the cock-roach abounds, it may be more common.
The observations of naturalists have chiefly been confined to theHemipterousgenusAphis; but these early attracted their notice. Leeuwenhoek has given a particular and entertaining account of the proceedings ofI. Aphidum. As soon as the little flies approached their prey, they bent their abdomen, which is rather long, between their legs, so that the anus projected beyond the head; then with their ovipositor they pierced the body of the Aphis, at the same time carefully avoiding all contact with it in every other part: whenever they succeeded in their attempt, a tremulous motion of the abdomen succeeded. Only a single egg is committed to one Aphis: when hatched, the latter becomes very smooth and appears swelled; it is, however, full of life, and moves when touched. Those that are thus pricked separate themselves from their sound companions, and take their station on theundersideof a leaf. After some days the inclosed grub pierces the belly of the Aphis, and attaches the margin of the orifice to the leaf by silken threads; upon this it dies, becomes white, and resembles a brilliant bead or pearl[970]. De Geer observed also an Ichneumon on the Coccus of the elm,I. Coccorum[971].
Amongst theNeuropteroustribes likewise, probably theIchneumonidæcommit their usual ravages; but their exploits, as far as I recollect, have met with no historian. I have a small species related toChelonus, which a memorandum made when I took it tells me was obtained fromÆshna viatica; yet I do not remember ever tracing that species to its final change, so that I must have taken this Ichneumon from theperfect insect. It suffices, however, to prove that this tribe is also exposed to the attackof these parasites. Where larvæ and pupæ areaquatic, it seems probable, if any attack is made upon them, that it must take place after they have quitted the water.
In theHymenopteraOrder itself, almost every genus has been ascertained to have its Ichneumon parasites. Not even the fortified habitations of the gall-flies (Cynips) can escape them, almost every species becoming their prey; a circumstance which puzzled not a little some of the older naturalists, when they at one time saw a fly not remarkable for its colours or brilliancy emerge from the curious moss-likeBedeguarof the wild rose, and at another were struck by the appearance of one of those splendid minims of nature which almost dazzle the sight of the beholder[972]. Immunity, however, from this pest seems to have been granted to thegregarious Hymenoptera; at least none has yet been discovered to attack the ant, the wasp, the humble-bee, or the hive-bee; in which last, had there been one appropriated to it, it could never have escaped the notice of the Reaumurs and the Hubers. Thesolitarybees, however, as we have seen above[973], do not escape; andEpipona spinipes, asolitarywasp which feeds its own young with a number of green caterpillars[974], is itself, when a larva, though concealed in a deep burrow, the prey of the grub of an Ichneumon, which by means of a long ovipositor introduces its egg into its body[975]. Even these parasites, whose universal office it is in their first state to prey upon insects, are themselves subject to the same malady. Ichneumonidan devourers are kept in check by otherIchneumonidan devourers. These in some cases are so numerous as to destroy the tithe of the kinds they attack[976]. Thus an ever-watchfulProvidenceprevents these parasites from becoming so numerous as to annihilate in any place the species necessary for the maintenance of the general economy and proportion of animal and vegetable productions. Amongst the assailants of theHymenoptera, none seem to have a more laborious task assigned them than those that pierce the various galls in which the larvæ of theCynipstribe are inclosed. To look at an oak-apple, we should think it a work of difficulty, requiring much sagacity and address, for one of our little flies to discover the several chambers lurking in its womb, and to direct their ovipositor to each of them. ItsCreator, however, has enabled it instinctively to discover this, and furnished it with an appropriate elongated instrument, which will open a way to the deep and hidden cells in which the grubs reside, penetrate their bodies, and to each commit an egg. When it prepares to perforate the gall, the Ichneumon begins by depressing this organ, that it may extricate it from its sheath; it next elevates its body as high as possible, and bending the instrument till it becomes perpendicular to the body and to the gall, so as to touch the latter with its point, it then gradually plunges it in, till it is quite buried[977]. A very remarkable Hymenopterous parasite (Leucospis), which when unemployed turns its ovipositor over the back of its abdomen, so that its end points to its head, is said to deposit its eggs in the nest of the mason-bee, most probably in the larva: but the curious observations that are stated to have beenmade by M. Amédée Lepelletier upon its history have not yet been given to the public[978].
Dipterousinsects, likewise, do not escape from these pests of their Class: but few observations, however, have been recorded as to the species assailed by them. We learn from De Geer, that a gnat (Cecidomyia Juniperi), which forms galls upon the juniper is devoured by an external Ichneumon[979]; that which injures the wheat in the ear, whose ravages I formerly mentioned to you[980], affords food to three of these parasites,—one I lately mentioned as probably devouring its eggs; another pierces the glumes of the floret, where its destined prey is concealed; and the third enters it. I once placed a number of the larvæ of the gnat upon a sheet of paper, at no great distance from each other, and then set down one of these last Ichneumons in the midst of them. She began immediately to pace about, vibrating her antennæ very briskly: a larva was soon discovered, upon which she fixed herself, the motion of her antennæ increasing intensely; then bending her abdomen obliquely under her breast, she inserted her ovipositor, and while the egg was depositing these organs became perfectly motionless. The larva when pricked gave a violent wriggle. This operation was repeated with all that had not already received an egg, for only one is committed to each larva. I have often seen it mount one that was already pricked, but it soon discovered its mistake, and quitted it untouched[981]. The Hessian Fly also (Cecidomyia Destructor) related to the preceding, whose alarming ravages I formerly described to you[982], has a peculiar parasiteattached to it, which keeps it in check. The only other Dipterous insects that I have seen mentioned as affording pabulum to an Ichneumon, are—one of the aphidivorous flies mentioned by De Geer, who does not note the species, to the larva of which the Ichneumon commits only a single egg, producing a grub that intirely devours its interior[983];—and two described by Scopoli, one, the larva of aflyfrequenting hemp; and the other, which feeds on aBoletus, that of agnat[984].
TheLepidoptera, however, is the Order over the larvæ of which the Ichneumons reign with undisputed sway; attacking all indiscriminately, from the minute one that forms its labyrinth within the thickness of a leaf, to the giant caterpillar of the hawk-moth. The most useful of all, however, the silkworm, appears at least with us, exempted from this scourge. De Geer, out of fifteen larvæ that were mining between the two cuticles of a rose-leaf, belonging to the first tribe here alluded to, found thatfourteenwere destroyed by one of these parasites, only one coming forth to display itself in all its brilliancy and miniature magnificence[985]. One of the most useful to us is that which destroys the clothes-moth, which the same writer also traced[986]. Another, equally serviceable, takes up its abode in the caterpillar that ravages our cabbagesand brocoli (Pontia Brassicæ) which perish by hundreds from its attacks. As this falls frequently under our notice, it will not be uninteresting to give a fuller account of it. Reaumur has traced and related its whole history. One of these little flies that he observed, was so intent upon the business in which she was engaged, that she suffered him to watch her motions under a lens, without being discomposed. She pursued nearly the same plan of proceeding with that of the Ichneumon of the wheat-gnat just described; except that she repeated her operations frequently on the same caterpillar in different parts, alternately plunging in and extracting her ovipositor. She seemed to prefer the spot where the segments of the body are united, particularly where the eighth meets the ninth, and the ninth the tenth. When the fly had completed its work and quitted the caterpillar, Reaumur gave it food, and it did not seem less lively and vigorous than others of its kind; in less than a fortnight it assumed the pupa; and in four days the whole of its interior being devoured, it died: but its parasites, perhaps not finding a sufficient supply of nutriment in it, never came to perfection[987]. Sometimes, however, these little grubs arrive at maturity before the caterpillar has become a chrysalis, when they pierce the skin and begin to emerge. First appears a little white tubercle, which gradually elevates itself in a direction perpendicular to the body; while this is doing, a second appears in another place; and so on, till fifteen or sixteen are seen on each side, giving the caterpillar a very grotesque appearance. By the alternate contraction and relaxation of their bodies the grubs effect their complete liberation, which takesplace with respect to the whole in less than half an hour. When entirely disengaged, they place themselves close to the sides of the caterpillar: even before this they begin spinning, and draw unequal threads in different directions, of which they form a cottony bed, which serves as the base of the separate cocoon of each individual, which they next construct of a beautiful silk thread of a lovely yellow, which, if it could be unwound and in sufficient quantity, would yield a silk unrivalled in lustre and fineness[988].