Feelers(Palpi). In the ordersDipteraandHymenopteraare many larvæ in which these organs have not been certainly discovered; yet Reaumur in that of a common fly (M. meridianaL.) found four retractile nipples[280]which seem analogous to them; and Latreille has observed, that below the mandibles of those of ants are four minute points, two on each side[281]: but in all other larvæ their existence is more clearly ascertained. Themaxillarypalpi vary innumber, many havingtwoon each maxilla and others onlyone. In the perfect insect the former is one of the distinguishing characters of the predaceous beetles (EntomophagiLatr.), but in the larvæ it is more widely extended; since even in the caterpillars ofLepidopterathe inner lobe of the maxilla which represents this feeler is jointed, which is precisely the case with the beetles just named. Cuvier has observed this circumstance in the larva of the stag-beetle[282]; and it belongs to many otherColeopterathat have only a pair of maxillary palpi in the perfect state. Thelabialpalpi are always two, emerging usually one on each side from the apex of the under-lip. With regard to theformof the palpi, those of theLepidopteraare mostly conical; in other orders they are sometimes setaceous and sometimes filiform. Their termination is generally simple, but sometimes the last joint is divided. They are for the most part very short, and the labial shorter than themaxillary. The latter never exceedfourjoints[283], which seems the most natural number; and the former are limited tothree. Both vary between these numbers, andonejoint. The joints, though commonly simple, are sometimes branched. This is the case with one I met with in considerable numbers upon the Turnip, in October 1808, the second joint of the palpi of which sends forth near the apex an internal branch. In the larva of theCossus, as Lyonnet informs us[284], the joints of the palpi are retractile, so that the whole of the organ may be nearly withdrawn.
After thus describing theheadof larvæ, and its principal organs, we must next say something upon the remainder of the body, or what constitutes the
2.Trunk and Abdomen: which I shall consider under one article. These are composed of severalsegmentsor rings, to which the feet and other appendages of the body are fixed. The form of these segments, or that of their vertical section, varies considerably: in manyLepidoptera, the wire-worm, &c., it would be nearly circular; in others a greater or less segment of a circle would represent it; and in some, perhaps, it would consist of two such segments applied together. Their lower surface is generally nearly plane. Their most naturalnumber, without the head and including the anal segment, is twelve: this they seldom exceed, and perhaps never fourteen. The three first segments are those which representthetrunkof the perfect insect, and to which the six anterior legs when present are affixed. In general, they differ from the remaining segments only in being shorter, and in many cases less distinctly characterized; but in Neuropterous larvæ, those ofDytisci, and some otherColeoptera, they are longer than the succeeding ones, and pretty nearly resemble the trunk of the animal in its last state. The surface of the trunk and abdomen will be considered under a subsequent head; I shall not, therefore, describe it here. Theconformationof the different segments varies but little, except of the terminal one, or tail, which in different larvæ takes various figures. In most, this part is obtuse and rounded; in others acute or acuminate; in others truncate; and in others emarginate, or with a wider sinus, and with intermediate modifications of shape which it would be endless to particularize. In some, also, it is simple and unarmed; in others beset with horns, spines, radii, and tubercles of different forms, some of which will come under future consideration. The parts connected with the trunk and abdomen which will require separate consideration, are thelegs, thespiracles, and variousappendages.
Legs.It may be stated generally that the larvæ of the ordersColeoptera,Lepidoptera, andNeuroptera, have legs; and that those of the ordersHymenopteraandDipterahave none. This must be understood, however, with some exceptions. Thus the larvæ of someColeoptera, as the weevil tribes (CurculioL.) have no legs, unless we may call by that name certain fleshy tubercles besmeared with gluten, which assist them in their motions[285];while those ofTenthredoandSirexin the orderHymenopteraare furnished with these organs. At present I know no Dipterous larva that may be said to havereallegs, unless we are to regard as such certain tentacula formed upon a different model from the legs of other larvæ[286]. Rösel has, I think, figured a Lepidopterous apode. No Neuropterous one has yet been discovered.
The legs of larvæ are of two kinds; either horny and composed of joints, or fleshy and without joints[287]. The first of these, as I observed in a former letter[288], are the principal instruments of locomotion, and the last are to be regarded chiefly as props and stays by which the animal keeps its long body from trailing, or by which it takes hold of surfaces; while the other legs, or where there are none, the annuli of its body, regulate its motions. The former have been commonly calledtrue legs(pedes veri), because they are persistent, being found in the perfect insect as well as in the larva; and the latterspurious legs(pedes spurii), because they are caducous, being found in the larva only. Instead of these not very appropriate names, I shall employ for the former the simple termlegs, and for the latterprolegs(propedes)[289].
The legs, when present, are always in numbersix, and attached by pairs to the underside of the three first segments of the trunk. They are of a horny substance, and consist usually of the same parts as those of the perfectinsect; namely,coxa,trochanter,femur,tibia, andtarsus, suspended to each other by membranous ligaments: these parts are less distinctly marked in some than in others. Thus in the legs of a caterpillar, or the grub of a capricorn-beetle, at first you would think there were only three or four joints besides the claw; but upon a nearer inspection, you would discover at the base of the leg the rudiments of two others[290], in the latter represented indeed by the fleshy protuberance from which the legs emerge. In the larvæ of the predaceousColeoptera, the hip and trochanter are as conspicuous nearly as in the perfect insect; and the tarsus, which still consists of only a single joint, is armed with two claws[291]. In those of theNeuropteraorder, in which all the joints are very conspicuous, the tarsi are jointed, as well as two-clawed[292]. The legs of larvæ are usually shorter than those of the perfect insect, and scarcely differ from each other in shape, for they all gradually decrease in diameter from the base to the apex. This is the most usual conformation of them in Lepidopterous, Hymenopterous, and some Coleopterous larvæ, (those of the capricorn-beetles are very short and minute, so as to be scarcely visible,) in which they are so small as to be concealed by the body of the insect[293]. In Neuropterous larvæ, however, andseveralColeoptera, as those ofDytiscus,Staphylinus,Coccinella, &c., they more resemble the legs of the perfect insect, the joints being more elongated, and the femoral one projecting beyond the body[294].
You will find no other than true legs in most Coleopterous, Neuropterous, and Hymenopterous larvæ. But those of the saw-flies (TenthredoL.), and all caterpillars, have besides a number ofprolegs: a few Dipterous larvæ also, are provided with some organs nearly analogous to them. Theseprolegsare fleshy, commonly conical or cylindrical, and sometimes retractile protuberances, usually attached by pairs to the underside of that part of the body that represents the abdomen of the future fly[295]. They vary in conformation and in number; some having but one, others as many as eighteen.
With regard to theirconformation, they may be divided into two principal sections: first, those furnished with terminal claws; and secondly, those deprived of them. Each of which may be divided into smaller sections, founded on the general figure of the prolegs, and arrangement of the claws or hooks.
i. The prolegs of almost all Lepidopterous larvæ are furnished with a set of minute slender horny hooks, crotchets, orclaws, of different lengths, somewhat resembling fish-hooks; which either partially or wholly surround the apex like a pallisade. By means of these claws, of which there are from forty to sixty in each proleg, a short and a long one arranged alternately, the insect is enabled to cling to smooth surfaces, to grasp the smallest twigs to which the legs could not possibly adhere: a circumstance which the flexible nature of the prolegs greatly facilitates[296]. Claws nearly similar are found on the prolegs of someDipterouslarvæ[297], but not in any of those of the other orders. These last, however, are seldom either so numerous, or arranged in the same manner, as in caterpillars. When the sole of the foot is open, the claws with which it is more or less surrounded are turned outwards, and are in a situation to lay hold of any surface; but when the animal wishes to let go its hold, it begins to draw in the skin of the sole,and in proportion as this is retracted, the claws turn their points inwards, so as not to impede its motion[298].
The prolegs with claws may be further divided into four different kinds.
1. In the larvæ of the great majority of butterflies and moths they assume the form of a truncated cone, the lower and smaller end of which is expanded into a semicircular or subtriangular plate, having the inner half of its circumference beset with the claws above mentioned; and, from its great power of dilating and contracting, admirably adapted for performing the offices of a foot. Jungius calls these legspedes elephantini[299]; and the term is not altogether inapplicable, since they exhibit considerable resemblance to the clumsy but accommodating leg and foot of the gigantic animal he alludes to.
2. The larvæ of many minute moths, particularly of the Fabrician generaTortrixandTinea—those which live in convoluted leaves, the interior of fruits, &c., as well as theCossus, and some other large moths,—have their prolegs of a form not very unlike those of the preceding class, but shorter, and without any terminal expansion; the apex, moreover, is wholly, instead of half, surrounded with claws[300]; the additional provision of which, together with a centrical kind of nipple capable of being protruded or retracted, in some measure, though imperfectly, supplies the place of the more flexible plate-like expansion present in the first class.
3. The third class is composed of a very fewLepidopterouslarvæ which have their prolegs very thick and conical at the base, but afterwards remarkably slender,long, and cylindrical, so as exactly to assume the shape of a wooden leg[301]. These, as in the first class, are expanded at the end into a flat plate: but this is wholly circular, is surrounded with claws, and has also in the middle a retractile nipple, as in the preceding class. InCossus, at least in an American species (Cossus Robiniæ), described by Professor Peck[302], the anal prolegs have the claws only on theirexteriorhalf.
4. The remaining description of unguiferous prolegs, if they may not rather be deemed a kind of tentacula, are those of certainDiptera, provided with no true legs; which differ from the three preceding classes, either in their shape, or the arrangement of their claws. In one kind of those remarkable larvæ, which from their long respiratory anal tubes Reaumur denominates "rat-tailed," that ofElophilus pendulus, there are fourteen of these prolegs, affixed by pairs to the ventral segments, the twelve posterior ones of which are subconical, and truncate at the apex, which is surrounded with two circles of very minute claws, those of the inner being much more numerous and shorter than those of the exterior circle; while the anterior pair terminate in a flat expansion, and in shape almost exactly resemble those of a mole[303]. The prolegs of the larvæ of a kind of gnat called by De GeerTipula amphibia, and ofSyrphus mystaceusF., (Musca plumataDe Geer,) are nearly of a similar construction, but in the last are armed with three claws only[304]. Long moveable claws also distinguish the singularprolegs before described[305]of another gnat (Tanypus maculatusMeig.,TipulaDe Geer). The case-worms (TrichopteraK.) and some others, have two prolegs at the anus, each furnished with a single claw[306].
ii. The prolegsdeprivedofclawsare found in the larva of the Hymenopterous tribe of saw-flies (TenthredoL.), in those of someLepidoptera(HepialusF. &c.), and in some few Coleopterous and Dipterous genera. Those of the former are of the shape of a truncated cone, and resemble the second class of unguiculate prolegs, except in the defect of claws. In the latter they are a mere retractile nipple-like protuberance, in some species so small as scarcely to be perceptible. In all they aid in progressive motion; but it is by laying hold of surfaces, and so enabling the body more readily to push itself forward by annular contraction and dilatation, and not by taking steps, of which all prolegs are incapable: to assist in this purpose the protuberance sometimes secretes a gluten[307], which supplies the place of claws. Some larvæ have the power of voluntarily dilating certain portions of the underside of their body, so as to assume nearly the shape and to perform the functions of prolegs. In a Coleopterous (?) subcortical one from Brazil, before alluded to, there are four round and nearly flat areas in each ventral segment of the abdomen, but the last very little raised above the surface, and rough, somewhat like a file; and besides these, the base of the anal segment has ten of these little rough spaces, but of a different shape, being nearly linear, placed in a double series, five on each side. Doubtless these may be regarded as a kind ofprolegs,which enable the animal to push itself along between the bark and the wood[308].
In considering, in the next place, thenumberandsituationof the prolegs, it will contribute to distinctness to advert to these circumstances as they occur in the different orders furnished with these organs.
To begin with theLepidoptera.—Lepidopterous larvæ have either ten, eight, six, or two prolegs, seldom more[309], and never fewer. Of these, with a very few exceptions, two are attached to the last or anal, and the rest, when present, to one or more of thesixth,seventh,eighth, andninthsegments of the body: none are ever found on the fourth, fifth, tenth, or eleventh segments.
1. Wheretenprolegs are present, as is the case in by far the greatest proportion of Lepidopterous larvæ, there is constantly an anal pair, and a pair on each of the four intermediate segments just mentioned.
2. In caterpillars, which like those of a few species of the generaSphinx,Pyralis, and of theBombycidæ, &c. haveeightlegs, they are placed inthreedifferent ways. In those which have an anal pair, the remaining six are in some fixed to the sixth, seventh, and eighth; in others, to the seventh, eighth, and ninth segments. In those which, likeCerura Vinula, and several other species of the same family, have no anal prolegs; the whole eight emerge from the sixth, seventh, eighth, and ninth segments.
3. The Hemigeometers, asNoctua Gamma, &c. haveonlysixlegs: namely, an anal pair, and two ventral ones, situated on the eighth and ninth segments.
4. The larvæ of the Geometers (GeometræF.) have butfourprolegs; of which two are anal, and two spring from the ninth segment. It should be observed, however, that the larvæ of Hemigeometers, and even of some of those that have ten prolegs, where the four anterior ones are much shorter than the rest, move in the same way as the Geometers. This even prevails in a few where these organs are all of equal length.
5. Many of the larvæ ofTineaL. which live in the interior of fruits, seeds, &c., have butonepair of prolegs, which are attached to the anal segment.
6. The larvæ of Haworth's genusApoda(Hepialus TestudoandAsellusF.), remarkable for their slug-like shape and appearance, move by the aid of two lateral longitudinal pustule-like protuberances, which leave a trace of a gummy slime in their course.
Hymenoptera.—The larvæ of the different tribes ofTenthredoL., almost the only Hymenopterous insects in which prolegs are present, have a variable number of these organs; some sixteen, as the saw-fly of the willow (T. luteaL.), and this is the most numerous tribe of them, including the modern genera,CimbexF.,Pterophorus, &c. Others have fourteen, as that of the cherry (T. cerasiL.); and many others with only nine joints to their antennæ. A third class have only twelve, as that of the rose (T. RosæL.), but this contains but few species. The last class contains those that have no prolegs at all, but only the six horny ones appended to the trunk. Of this tribe, the caterpillars of which have a very different aspect from the preceding, are those of the genusLydaF. (T. crythrocephalaL.)[310]. Two of the prolegs are anal, and the rest intermediate, and none are furnished with claws. This circumstance, in conjunction with the greater number of prolegs, except in the case ofLyda, will always serve as a mark to distinguish thesefausses chenilles, as the French call the larvæ of saw-flies, from truecaterpillars. Thedorsalprolegs of a species ofCynipsdescribed by Reaumur have been before noticed.
Coleoptera.—The larvæ of insects of this order are so little known or attended to, that no very accurate generalization of them in this respect is practicable. Many of them, in addition to their six horny legs, have a proleg at the anus; which in many cases appears to be the last segment of the abdomen, forming an obtuse angle with the remainder of it, so as to support that part of the body, and prevent it from trailing; and in some instances, as inChrysomela Populi, a common beetle, secreting a slimy matter to fix itself[311]. In the larvæ ofStaphylinidæthis proleg is very long and cylindrical; in that ofCicindelait is shorter, and in shape a truncated cone rather compressed; it is very short, also, in those of theSilphæthat I have seen. In the wire-worm (Elater Segetum) it is a minute retractile tubercle, placed in a nearly semicircular space, shut in by the lastdorsalsegment, which becomes also ventral at the anus. This space is in factthe last ventral segment. This seems characteristic of the genus[312]. From the underside of the body of the common meal-worm (Tenebrio Molitor), at the junction of the two last segments, when the animal walks, there issues a fleshy part, furnished below with two rather hard, long, and moveable pediform pieces, which the animal uses in walking[313]. In the larva of another beetle, whose ravages have been before noticed, under the name of the cadelle[314](Trogosita mauritanica), a pair of prolegs are said to be found under the anal segment; and in that of the bloody-nose beetle (Timarcha tenebricosa), that segment is bifid. That of the weevil of the common water-hemlock (Lixus paraplecticusF.) exhibits a singular anomaly: prolegs occupy the usual station of the true legs, being attached to the three segments representing the trunk[315]. This insect, however, does not appear to use them in moving. A pair in each of the twelve segments of the body are found in the grub of another weevil (Hypera RumicisGerm.), the nine last pair being the shortest, which all assist the insect in walking[316]. But the greatest number of prolegs is to be found in the Brazil subcortical larva lately mentioned. Besides the six horny legs of the trunk, this remarkable animal has four prolegs on each of the seven intermediate abdominal segments, and five on each side of the base of the last, making the whole number of prolegs, if so they may be called, amount to forty-four: a far greater number than is to be found in any larva at present known. When I wrote to you upon the motions of insects, I informedyou that some larvæ moved by means of legs upon their back[317], but I was not then aware that any were furnished with them both on the back and the belly at the same time. By the kindness of Mr. Joseph Sparshall of Norwich, a very ardent and indefatigable entomologist, I am in possession of the larva ofRhagium fasciatum, a timber-feeding beetle. This animal on the ten intermediate segments of the underside of the body, which in the centre form a fleshy protuberance, has on it a double series of rasps, as it were, consisting each of two rows of oblique oblong prominences; and on the seven intermediatedorsalsegments there are also in the centre seven rasps of three or four rows each, of similar prominences: so that this animal at the same time can push itself along both by dorsal and ventral prolegs. It is worthy of observation, that a pair of these rasps is between the second and third pair of true legs.
Diptera.—The larva of a little gnat,Tipula stercorariaDe Geer[318](ChironomusMeig.?), drags itself along by the assistance of asingletubercle, placed on the underside of the first segment of the body, which the animal has the power of lengthening or contracting[319]. That of another beautifulChironomus(C. plumosus), remarkable for the feathered antennæ of the male[320], hastwoshort prolegs, or pediform but not retractile tentacula in the same situation[321]. Others, as that ofTanypus maculatus,&c. havetwo pairs, one attached to the anal and the other to the first segment[322].Tipula amphibiaDe Geer in this state hastenprolegs, placed by pairs on the fourth, fifth, eighth, ninth, and tenthdorsalsegments[323]; andScæva PyrastriF., one of the aphidivorous flies, has not fewer thanforty-two, arranged in a sextuple series, seven in each row[324].
It may not be useless to close this long description of thelegsof larvæ with a tabular view of them, founded chiefly upon these organs; which afford very obvious marks of distinction.
I should next say something upon the spiracles, or breathing-pores, or any other external apparatus for the purpose ofrespiration, in larvæ; but I think it will be best to reserve the consideration of these for a subsequent Letter. We will therefore conclude this detailed description of their parts in their first state, with some account of their other.
iii.Appendages.The generality of larvæ have no other external organs than those already described; but in several of them we observe various kinds of retractile ones and others—protuberances—horn-like processes—rays, &c.; which, though not properly coming either under any of the aboveparts, or under theclothingof these animals, yet require to be noticed. Upon these I shall now enlarge a little.
You must have observed upon the back of the last segment but one of the caterpillar of the silk-worm a horn-like process, rising at first nearly perpendicularly, and then bending forward. A similar horn, though confined in the genusBombyxto the silk-worm and a few others, if we may believe Madame Merian, who, however, often makes great mistakes, is found in the beautiful caterpillar of one of the largest and finest moths that we know (Erebus Strix[325]), the glory of theNoctuidæ, and in most of those of the hawk-moths (SphinxF.) [S. Porcellus,Vitis, and a few others excepted; in some of which, asS. Labruscæ, &c., this anal horn is replaced by a gibbosity, and in others, asS. Œnotheræ, by a callous eye-like plate[326]] in the same situation, but much longer[327], andcommonly curving backwards over the tail[328]. Sometimes, however, as inS. ocellataandS. Stellatarum, it is perfectly straight. These organs towards the apex are horny, and often end in a sharp point; nearer the base they are fleshy. They are without any true joint[329], yet the insect can elevate or depress them at pleasure. Under a lens, they usually appear covered with spinous eminences, arranged like scales. The use of these horns is quite unknown: Goedart fancies that they secrete a potent poison, and are intended as instruments of defence; but both suppositions are altogether unfounded. It has been remarked, that the body of those caterpillars which have these horns, is firmer, and yields less to the touch than that of those which have no such appendages[330]. The larva of a small timber-devouring beetle (Lymexylon dermestoidesF.) has, like the above caterpillars, a long horn, and in the same situation: it has also a singular protuberance on the first segment[331]. Upon some other caterpillars, as inBombyx StigmaF., a singular pair of horn-like appendages arises from the back of the second segment of the body, excluding the head. In a tawny-coloured one from Georgia, with a transverse row of short black spines on each segment, these horns are half an inch long, black, covered with spinous eminences, rather thickest at the base, and terminate in a little knob. They appear to articulate with the body at the lower extremity. I have another species, black, with narrow longitudinal yellowstripes, in which these horns are of equal thickness at base and apex, but with the same terminal knob.Danais Archippushas a pair of tentacula at the head, and another pair, but shorter, at the tail; andD. Gylippushas, besides these, two in the middle of the body[332].
We are equally ignorant of the use of the upright horn found upon the back of the fourth segment in the larva of some moths (Noctua Psi, andtridensF.) which is of a construction quite different from that of those last described. It is cylindrical, slightly thinner at the apex, which is obtuse, fleshy, incapable of motion, of a black colour, and about two lines long. On the same segment, also, in the case-worms (TrichopteraK.) are three fleshy conical eminences, which the animal can inflate or depress, so that they sometimes totally disappear, and then in an instant swell out again. When retracted, they form a tunnel-shaped cavity, varying in depth[333]. Reaumur conjectured that these eminences were connected with respiration, and one circumstance seems in favour of this conjecture, that this segment has not the respiratory threads observable in the subsequent ones. Latreille mentions certain fleshy naked eminences placed upon the ninth and tenth segments of some hairy caterpillars, which, like those just mentioned, the animal can elevate more or less. They are often little cones; but when it would shorten them, the summit is drawn in, and a tunnel appears where before there was a pyramid[334].
In a former Letter I gave you a short account of theremarkable Y-shaped, as it should seem, scent-organs (Osmateria) of the beautiful caterpillar of the swallow-tailed butterfly (Papilio MachaonL.), and others of theEquites[335]; I will now speak of them more fully. That found in the former is situated at the anterior margin at the back of the first segment, close to the head, from which at first view it seems to proceed. At the bottom it is simple, but divides towards the middle, like the letter Y, into two forks, of a fleshy substance[336], which it can lengthen, as a snail does its horns, to five times their ordinary extent, or retract them within the stalk, so as wholly to conceal them. Sometimes it protrudes one fork, keeping the other retracted; and often withdraws the whole apparatus for hours together under the skin, and its place is only marked by two tawny-coloured dots, so that an ordinary spectator would not suspect the existence of such an instrument[337]. Unfortunately this larva is rare in this country, so that I can scarcely flatter you with the hope of seeing this curious organ in a living specimen[338], unless you choose to import a parcel of its eggs from the south of Europe, where it is common. This you will think rather a wild proposition; but why should not Entomologists import the eggs of rare insects, as well as botanists the seeds of rare plants? But if you will be satisfied with the dissection of a dead specimen, I have several,done by the ingenious Mr. Abbott of Georgia, in which this part is well exhibited[339].
Another small caterpillar, as it should seem, of a geometer, prepared by the same gentleman, exhibits a pair of similar horns on the fifth and sixth segments: in these the common base from which the fork proceeds is very short and wide, and each branch grows gradually more slender from the base to the apex, where it is involute. Whether these are retractile, or whether they correspond with those ofP. Machaonin their nature and use, cannot be ascertained from a dead specimen: as they belong to a larva of a quite different tribe ofLepidoptera, the probability is, that they essentially differ. Two globose retractile vesicles issue from the ninth and tenth segments of those ofArctia chrysorrhea, &c.[340]
A great number of Lepidopterous larvæ, particularly those which are smooth and of a moderate size, have between the under-lip and fore-legs a slender transverse opening, containing a teat-like protuberance of the same construction as the furcate horn of the caterpillar of the beautiful mountain-butterfly,Parnassius Apollo; and, like that, can either be wholly retracted and concealed, or by pressure be extended to the length of one of the legs. In some larvæ this part is of a subhemispherical figure, generally single, but sometimes double. It is commonly, however, more slender and conical; and when of this shape, it is sometimes quadruple[341]. The use of this part is not very clearly known: some have supposed it to be a second spinneret, and to be of use in fabricating the cocoon; butit is more probable that it secretes some other kind of fluid, and is connected with defence.
The singular organ in a similar situation, evidently for that purpose, with which the puss-moth endeavours to annoy its assailants, has been described in a former Letter, to which I refer you[342]. Bonnet, who was the first that discovered this organ, ascertained that it might be cut off without injury to either larva or imago. He also remarked in a caterpillar found in the wild succory (Cichorium Intybus) another short, black, needle-shaped organ between the conical part just described and the under-lip[343]. De Geer mentions a remarkable fleshy horn-like style, which issues from the lower side of the first segment, between the head and the legs of the case-worms (Trichoptera): he does not describe it as retractile, or it might be regarded as analogous to those ofLepidopterasimilarly situated, that I have just noticed[344]. In that of the emperor-moth (Saturnia Pavonia), there are perforated tubercles, which when the animal is molested spirt forth a transparent fluid[345].
The horn-like appendage of the puss-moth (Cerura Vinula) is situated at the tail of the insect, and is composed of two distinct cylindrical diverging branches, each about four lines long, not united at the base. Each of these is hollow, and includes a smaller cylindrical piece, which can be protruded at pleasure, and withdrawn again, as a pencil within its case; or, rather, as the horns of a snail. The two outer horns are tolerably firm, moveable at their base, and beset with black spines; the interior tentacula are fleshy, moveable in every direction, and in full-grownlarvæ of a rose colour. The animal seldom protrudes them, unless in some way disturbed; and frequently it approximates the two outer cases so closely that they resemble a single horn. It appears to use these inner horns, when protruded, as a kind of whip to drive away the flies, especially the Ichneumons, that alight upon its body. When touched in any place, it will unsheath one of them, and sometimes both, and with them strike the place where it is incommoded[346]. A similar organ is found in some otherBombycidæ, asB. TauandFurculaF. Reaumur mentions a caterpillar that to this kind of tail added the resemblance of two ears, or two cylindrical bodies, terminating in a point, which emerged from the first segment behind the head[347]. In another observed by the same author, the legs were replaced by a single horn, but which did not appear to send forth an internal one: from the back of its fourth segment also emerged a single conical or pyramidal fleshy eminence or cleft, terminating in two points[348]. Some of the tropical butterflies also, as may be seen in the figures of Madame Merian, have two diverging anal horns instead of anal prolegs; but it does not appear that they incase tentacula[349]. Wherever these caudal horns are found, the above prolegs are wanting[350].Two conical anal horns also distinguish the caterpillar of one of the moths calledProminents,Notodonta camelina; but these are not terminal, but on the back of the last segment but one[351]. In that of another British moth,N. ziczacF., there are three dorsal prominences, one near the anus, and two more in the middle[352]. Some Geometers (G. fuliginosa, &c.) have two erect horns on the eleventh segment, and others (G. syringaria, &c.) two recurved ones on the eighth[353]. I must not here omit to mention the curious hooks emerging from two tubercles on the back of the eighth segment of the ferocious larva of that beautiful tiger-beetle, theCicindela campestrisL., not uncommon on warm sunny banks. This animal with incessant labour, as we are informed by M. Desmarets, digs a cylindrical burrow, to the enormous depth, the size of the animal considered, of eighteen inches. To effect this, it carries out small masses of earth upon its large concave head; and having often occasion to rest in ascending this height, by means of these hooks[354]it fixes itself to the sides of its burrow, and, having finally arrived at its mouth, casts off its burthen. When these insects lie in wait for their prey, their head, probably in conjunction with the first segment of the body, accurately stops the mouth of the burrow, so as to form an exact level with the surrounding soil; and thus careless insects, walking over it without perceiving the snare, are seized in a moment and devoured[355].
Another kind of appendage, which is found in somelarvæ, is the organ employed by them to carry the excrement; with which, instead of letting it fall to the ground, they form a kind of umbrella to shelter and probably conceal them. All the tortoise-beetles (CassidaL.) have instruments for this purpose, as well as an Indian genus (ImatidiumLatr.) very nearly related to them. This instrument is a kind of fork, half as long as the body, consisting of two branches, growing gradually smaller from the base to the summit, where they terminate in a very fine point, of a substance rather horny, and attached to the body near the anal orifice. They are armed on the outside with short spines, from the base for about a third of their length. When this fork, as it usually is, is laid parallel to the back, with its points towards the head, the anal aperture points the same way. When the animal walks, the fork points the other way, and is in the same line with the body, and the anus assumes a prone position[356].
The larvæ of a genus of flies (VolucellaGeoffr.) remarkable for inhabiting the nests of humble bees, are distinguished on their upper side by six long, diverging, pointed, membranous radii; placed in a semicircle round the anus[357]: what the particular use of these organs may be, has not been conjectured. Another in my collection has only four upper radii, but below the anus are two fleshy filiform tentacula. One of a Tipulidan described by Reaumur, has also four upper teeth; but instead of two subanal tentacula, has six[358]. The singular larva of another of this tribe (Chironomus plumosus) has on thetwo last segments four long, fleshy, filiform, flexible tentacula, often interlaced with each other; which, according to the same illustrious author, are used by the animal to fix its caudal extremity, like the geometers, that the other end may be at liberty. Besides these organs round the anus, it has also four other oval ones, of uncertain use: not to mention the two prolegs, which M. Latreille thinks are air-tubes[359]. Jointed anal organs are observable in other larvæ: those in that of a saw-fly described by De Geer (LydaF.) consist of three joints[360]; in that ofHister cadaverinus, a carnivorous beetle, of two[361]. The larva as well as the pupa and imago ofEphemerais furnished with three long diverging multiarticulate tails, which are probably useful as a kind of rudder to assist and direct their motions. That of the smaller dragon flies (AgrionF.) is furnished with three long vertical laminæ, by moving which, as fish do their tails, from side to side, the animal makes its way in the water[362]. That singular one, also, with a hooked head, figured by Reaumur, has a single swimming lamina, or fin, shaped like a fan, and placed in a vertical position under the tail[363].
The whole circumference of the body in some coleopterous larvæ,—for instance, in that of the tortoise-beetle lately mentioned,—is surrounded with appendages like rays. These are sometimes simple, rough with very short spinous points[364]; but I have a dipterous larva, inwhich these radii themselves are beautifully pinnated by a fringe of longish spines on each side. Reaumur has described the grub of a beetle, the genus of which is uncertain, and which feeds upon the larva ofAleyrodes Proletella, whose body is margined on each side by eight triangular fleshy mammular processes, terminating each in a bristle, which give it a remarkable aspect[365]. The curious scent-organs with which the larva ofChrysomela Populiis fringed have been before fully described; and therefore I shall only mention them here[366].
In the larvæ of the lace-winged flies (Hemerobius), and ant-lions (Myrmeleon), the anus is furnished with a small fleshy retractile cylinder, from which proceeds the silken thread that forms the cocoon inclosing the pupa[367]. Providence has many different ways of performing the same operation. From the structure of the oral organs of these animals, the silk could not conveniently be furnished by the mouth; the Allwise Creator has therefore instructed and fitted them to render it by a spinneret at the other extremity of the body.
The respiratory anal appendages of many Dipterous larvæ will be fully described in a subsequent Letter: I shall therefore now only further observe upon this subject, that although there is seldom any alteration in the form of these appendages &c. in the same species, the caterpillars of two moths (Cerura VinulaandAttacus Tau), however, are exceptions. The former, when young, has two hairy projecting ear-like protuberances, which it entirely loses, as I have myself observed, before it assumes the pupa; and the latter, in like manner, after its thirdchange of skin, is deprived of its bent thorn-like points which attend it when young[368]. It is remarkable that these last larvæ, when just excluded from the egg, are also entirely destitute of these appendages; they soon, however, appear, from slight elevations which mark their situation, and rapidly acquire their usual form[369]. Changes of a similar kind, hitherto unobserved, may probably take place in other species.
iii.Figure.I am next to consider the general figure or shape of larvæ. All of them, with but few exceptions[370], agree in having a body more or less constricted at intervals into a series of rings orsegments; usually in number,twelve; often nearly equal in length, but sometimes in this respect very dissimilar[371]. The general outline or shape of the body is extremely various: most frequently it approaches to cylindrical, as in most of the caterpillars ofLepidoptera, and of the Hymenopterous tribe of saw-flies (TenthredoL.). The next most common figure is that more or less oblong or oval one, sometimes approaching to conical, found in many of the larvæ usually called grubs; such as those of the weevil (CurculioL.) andof the capricorn (CerambyxL.), and other coleopterous tribes; of bees, and all Hymenopterous insects but the saw-flies; and also of a large number of flies (Diptera). In some the figure approaches to fusiform, as in most of the moths of the Fabrician genusLithosia. In others, as in those of the water-beetles (Dytiscus, &c.), it approaches to an obovate shape, being widest towards the head, and terminating in a point at the anus. In others, again, it is linear; an example of which is that ofStaphylinus. Some are convex, and others gibbous, above, and flat underneath; as those ofSilpha,Chrysomelaand many other beetles. Others are flat, both above and below, and depressed like a leaf; a remarkable instance of which has been before noticed[372]. Some are very long, as those of mostLepidoptera; others very short, as that of the ant-lion (Myrmeleon). Many other peculiarities of form in individuals might be instanced; but a dry enumeration of these would be of no great use to you. They can only be advantageously learned by the study of good figures, and by watching the actual metamorphosis of the singularly-formed larvæ that you meet with.
Instead, therefore, of any further specification of individual forms, I shall now endeavour to give you, as far as my own knowledge of them and the information I can collect from other sources will enable me, a larger and more general view of the kinds of larvæ; for analytical inquiries lose half their value and importance unless we proceed to apply them synthetically, by forming, if possible, into groups the objects with which we are individually acquainted.
Partial attempts at a synthetical arrangement with regardto the larvæ ofLepidopteraand the saw-flies (TenthredoL.) have been made both by Reaumur and De Geer. M. Latreille also has recently given aTableau méthodique et généralof articulated animals furnished with jointed legs, considered in their first state[373]. The former of these is chiefly founded upon the number of the prolegs, and the latter upon the metamorphosis, prolegs, habits, head, and parts of the mouth, without any other notice of the configuration. Mr. Wm. MacLeay, who, though young in years, is old in science and critical acumen, has started a perfectly new hypothesis upon this subject. In the progress of his inquiries into the natural arrangement of animals, particularly ofinsectsin theLinneansense, he has been the first to observe, that the relation which organized objects bear to each other is of two kinds; one of realaffinity, and the other only ofanalogy, or resemblance. This important distinction, upon which I shall enlarge in a future Letter, when I come to treat of Systems of Entomology, he has applied, in a way quite original, to larvæ in general, but more particularly to those of theColeopteraorder. For the basis of his system he assumes a relation of analogy between thelarvæof Insects that in the progress of their metamorphosis assume wings, and those that do not, which form his classAmetabola, so that the prototypes of the former shall be found amongst the latter[374]. But though Mr. MacLeay appears to consider the analogy between these two asprimary, he extends it in asecondarysense to theCrustacea, at least in several instances[375]. Upon this occasionhe very judiciously remarks, that "in terming larvæChilognathiformesorChilopodiformes, it is not meant that they areScolopendræorIuli, or even near to them in affinity; but only that they are so constructed that certain analogical circumstances attending them strongly remind us of theseAmetabola[376]." This remark you will bear in mind while I am treating of this subject. It should seem from another part of the same paragraph, that the comparison which our learned Physiologist recommends, is between theyoungof theAmetabolaand the larvæ of the corresponding groups ofColeoptera. This must be understood to refer chiefly to the young of theChilopodaandChilognatha, since they approach nearer to them in that state, having then only six legs; but the rest of theAmetabolashould certainly be brought to this comparison in their adult state: and even the two former orders in that state more strongly resemble numerous coleopterous larvæ, than when they are young and much shorter. I before called your attention to the remarkable circumstance that contrasts very many larvæ of Hexapod insects that become winged in their perfect state with adultMyriapoda: namely, that in one the progress to this state is by losing their prolegs and shortening their body; while in the other, the reverse of this takes place, numerous prolegs and additional segments being gained before they arrive at maturity[377]. As the multiplication of organs is a sign of imperfection, it may be affirmed of the former of these tribes, that their progress is towards greater perfection; while that of the other may be called a degradation. As larvæ may be regarded as a stepping-stone by which approach is madefrom the apterous to the winged tribes of Insects, it seems most consistent with general analogy that each should connect with the other in that state in which the resemblance is greatest. Now theMyriapodaresemble larvæ, as we have just seen, most when in theiradultstate; therefore the comparison should be between larvæ and adultMyriapoda.
Mr. MacLeay dividescoleopterouslarvæ into five tribes thus characterized:—
1.A carnivorous hexapod larva, with an elongate linear flattened body, having a large head armed with two sharp falciform mandibles, and furnished with six granular eyes on each side.This kind he denominatesChilopodiform, as having for its type in theAmetabola,ScolopendraL. The examples he gives areCarabusandDytiscus.
2.A herbivorous hexapod larva, with a long and almost cylindrical body, so fashioned that the posterior extremity being curved under the breast, the animal when at rest necessarily lies like anIuluson its side.This tribe he denominatesChilognathiform, fromIulusL. His examples are, the larvæ ofPetalocerousinsects, asScarabæusL.,LucanusL. &c.
3.An apod larva, having scarcely the rudiments of antennæ, but which is furnished instead of feet with fat fleshy tubercles; which, when continued along the back and belly, give the animal a facility of moving in whatever way it may be placed.These he denominatesVermiform, from certain of theVermes intestinaandMolluscaof Linné which he has associated with hisAnnulosa[378]. His examples are,CurculioL. andCerambyxL.
4.A hexapod and distinctly antenniferous larva, with asubovate rather conical body, of which the second segment is longer and of a different form from the others, so as to give the appearance of a thorax.His denomination for these isAnopluriform, fromPediculusL., forming Dr. Leach'sAnoplura. His examples are,CoccinellaandChrysomelaL.
5.A hexapod antenniferous larva of an oblong form, having like the former vestiges of a thorax, besides two or more articulated or inarticulated setaceous or corneous appendages to the last segment of the abdomen.This tribe he callsThysanuriform, fromLepismaandPoduraL., forming M. Latreille's orderThysanura. His example isMeloewith a note of interrogation[379].
The system here stated, of naming and characterizing larvæ from the resemblance and analogy, in many cases very striking, that they bear to the apterous tribes, is a very happy and original one, and does its author great credit; yet I think in some instances, as I shall soon have occasion to point out to you, the application of it is not so happy as the first idea. But this is always the case when a new law of nature is discovered; the proper application of it is gradually developed, and it does not at all detract from the merit of the first discoverer, that all the bearings of such law do not strike him as it were intuitively.
Having, however, got the vantage-ground afforded by this discovery of my friend, let us see if by standing upon it we cannot get a tolerable generalization of the larvæ of all orders of insects that undergo a metamorphosis. But first I must observe, that as in the perfect animals, so in their larvæ, the different groups are connected by certaintransitionspecies, exhibiting characters common to two or more of them; and likewise that in many cases, which you will see as we proceed, the analogy is as strong or stronger between them and theCrustacea(and in a few instancesArachnida, and evenMollusca) than theAmetabola. My denominations, therefore, will be taken from those tribes where the analogy appears to me the most striking, and not from theAmetabolaalone.
I shall begin by drawing up for you a list of the Primary forms that I seem to have observed, and their characters; and then going through the orders, shall give you the examples of each, with such observations upon them as the case may require.
Primary Forms of Larvæ.
Characters.
1.Anopluriform.Carnivorous; hexapod; antenniferous: with a shortish oblong depressed body, and distinctthoracic shield. Example:Psocus,Coccinella, mostHemiptera[380].2.Thysanuriform.Polyphagous; hexapod; antenniferous: body with segments of trunk distinctly marked; anus often furnished with setæ or mucro. Ex.Meloe[381]?Thrips,Aphis.3.Chilopodiform.Carnivorous; subhexapod; antenniferous: body depressed, elongate, linear, with falcate acute mandibles, a distinct thoracic shield, and an anal proleg. Ex.Cicindela[382]?CarabusL.4.Chilognathiform.Herbivorous; body subcylindrical, elongate, linear; no thoracic shield; often many prolegs, sometimes a retractile one, and sometimes none.—Ex.Elater,Petalocera, mostLepidoptera,TenthredoL.[383]5.Vermiform.Polyphagous; apod or hexapod: with very short legs; antennæ nearly obsolete; body fleshy, plicate, with sides often plicato-papillose;no distinct thoracic shield. Ex.CurculioL.,CerambyxL.,MuscaL., and many otherDiptera[384].6.Araneidiform.Carnivorous; hexapod: body very short; mandibles long, suctorious; animal lying in wait for its prey in a pitfall it has prepared; motion retrogressive. Ex.MyrmeleonL.[385]Cicindela?7.Isopodiform.Saprophagous; hexapod; antenniferous, with longer antennæ: body oblong; thoracic shield distinct; anus styliferous or laminiferous. Ex.BlattaL.SilphaL.?8.Onisciform.Herbivorous; polypod; antenniferous: body short, oblong, depressed, margined. Ex.Erycina,Lycæna, in theLepidoptera, and some species ofTenthredoL.[386]9.Idoteiform.Subcortical; hexapod; antennæ obsolete: body much depressed, with the last segment elongate, terminating in three or more mucros. Ex. Larva from Brazil. Perfect insect at present unknown.
1.Anopluriform.Carnivorous; hexapod; antenniferous: with a shortish oblong depressed body, and distinctthoracic shield. Example:Psocus,Coccinella, mostHemiptera[380].
2.Thysanuriform.Polyphagous; hexapod; antenniferous: body with segments of trunk distinctly marked; anus often furnished with setæ or mucro. Ex.Meloe[381]?Thrips,Aphis.
3.Chilopodiform.Carnivorous; subhexapod; antenniferous: body depressed, elongate, linear, with falcate acute mandibles, a distinct thoracic shield, and an anal proleg. Ex.Cicindela[382]?CarabusL.
4.Chilognathiform.Herbivorous; body subcylindrical, elongate, linear; no thoracic shield; often many prolegs, sometimes a retractile one, and sometimes none.—Ex.Elater,Petalocera, mostLepidoptera,TenthredoL.[383]
5.Vermiform.Polyphagous; apod or hexapod: with very short legs; antennæ nearly obsolete; body fleshy, plicate, with sides often plicato-papillose;no distinct thoracic shield. Ex.CurculioL.,CerambyxL.,MuscaL., and many otherDiptera[384].
6.Araneidiform.Carnivorous; hexapod: body very short; mandibles long, suctorious; animal lying in wait for its prey in a pitfall it has prepared; motion retrogressive. Ex.MyrmeleonL.[385]Cicindela?
7.Isopodiform.Saprophagous; hexapod; antenniferous, with longer antennæ: body oblong; thoracic shield distinct; anus styliferous or laminiferous. Ex.BlattaL.SilphaL.?
8.Onisciform.Herbivorous; polypod; antenniferous: body short, oblong, depressed, margined. Ex.Erycina,Lycæna, in theLepidoptera, and some species ofTenthredoL.[386]
9.Idoteiform.Subcortical; hexapod; antennæ obsolete: body much depressed, with the last segment elongate, terminating in three or more mucros. Ex. Larva from Brazil. Perfect insect at present unknown.
I have placed this larva, which was described above[387], amongst crustaceous forms, because of the remarkable resemblance which the last segment of the body bears to that of theIdoteidæ; but I do it with considerable hesitation, since in other respects its type seems to be in theAmetabola. In its want of antennæ, very short legs, and ventral asperities, it resembles some of theVermiformlarvæ; in its small head, distinct thoracic shield, and oblong shape, it approaches theAnopluriform; and in itsvery depressed body, but not at all in other respects, theChilopodiform. At any rate, it appears of a primary Type.
10.Læmodipodiform.Herbivorous; hexapod; antenniferous, with long antennæ: body elongate, subcylindrical; second segment of the trunk the longest; anterior pair of legs distant from the other two. Ex.Phasma.11.Amphipodiform.Herbivorous; hexapod; antenniferous, with long antennæ: body shortish, compressed, saltatorious. Ex.GryllusL.[388]12.Stomapodiform.Carnivorous or saprophagous; hexapod; antenniferous, with long antennæ: body elongate, subdepressed, with raptorious hands, and abdomen wider than the trunk; in aquatic species furnished with lateral gills. Ex.Mantis,Ephemera? Sialis?13.Decapodiform.Carnivorous; hexapod; antenniferous: body elongate, narrow, convex, compressed, tapering towards the tail; tail with natatorious laminæ. Ex.DytiscusL.,AgrionF.[389]14.Branchiopodiform.Carnivorous?; aquatic; apod; antenniferous: head distinct: body transparent, flexile, furnished with a respiratory tube just above the tail. Ex.Culex[390].15.Limaciform.Herbivorous; apod, or with very short legs: body ovate or obovate, slimy. Ex.ApodaHaw.,Tenthredo CerasiL.
10.Læmodipodiform.Herbivorous; hexapod; antenniferous, with long antennæ: body elongate, subcylindrical; second segment of the trunk the longest; anterior pair of legs distant from the other two. Ex.Phasma.
11.Amphipodiform.Herbivorous; hexapod; antenniferous, with long antennæ: body shortish, compressed, saltatorious. Ex.GryllusL.[388]
12.Stomapodiform.Carnivorous or saprophagous; hexapod; antenniferous, with long antennæ: body elongate, subdepressed, with raptorious hands, and abdomen wider than the trunk; in aquatic species furnished with lateral gills. Ex.Mantis,Ephemera? Sialis?
13.Decapodiform.Carnivorous; hexapod; antenniferous: body elongate, narrow, convex, compressed, tapering towards the tail; tail with natatorious laminæ. Ex.DytiscusL.,AgrionF.[389]
14.Branchiopodiform.Carnivorous?; aquatic; apod; antenniferous: head distinct: body transparent, flexile, furnished with a respiratory tube just above the tail. Ex.Culex[390].
15.Limaciform.Herbivorous; apod, or with very short legs: body ovate or obovate, slimy. Ex.ApodaHaw.,Tenthredo CerasiL.