Part 11

The legs, when present, are always in number _six_, 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 perfect insect; namely, _coxa_, _trochanter_, _femur_, _tibia_, and _tarsus_, 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 predaceous _Coleoptera_, 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 the _Neuroptera_ order, 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, and several _Coleoptera_, as those of _Dytiscus_, _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 (_Tenthredo_ L.), and all caterpillars, have besides a number of _prolegs_: a few Dipterous larvæ also, are provided with some organs nearly analogous to them. These _prolegs_ are 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 their _conformation_, 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, or _claws_, 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 some _Dipterous_ larvæ[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 legs _pedes 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 genera _Tortrix_ and _Tinea_--those which live in convoluted leaves, the interior of fruits, &c., as well as the _Cossus_, 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 few _Lepidopterous_ larvæ 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. In _Cossus_, at least in an American species (_Cossus Robiniæ_), described by Professor Peck[302], the anal prolegs have the claws only on their _exterior_ half.

4. The remaining description of unguiferous prolegs, if they may not rather be deemed a kind of tentacula, are those of certain _Diptera_, 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 of _Elophilus 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 Geer _Tipula amphibia_, and of _Syrphus mystaceus_ F., (_Musca plumata_ De Geer,) are nearly of a similar construction, but in the last are armed with three claws only[304]. Long moveable claws also distinguish the singular prolegs before described[305] of another gnat (_Tanypus maculatus_ Meig., _Tipula_ De Geer). The case-worms (_Trichoptera_ K.) and some others, have two prolegs at the anus, each furnished with a single claw[306].

ii. The prolegs _deprived_ of _claws_ are found in the larva of the Hymenopterous tribe of saw-flies (_Tenthredo_ L.), in those of some _Lepidoptera_ (_Hepialus_ F. &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 of _prolegs_, which enable the animal to push itself along between the bark and the wood[308].

In considering, in the next place, the _number_ and _situation_ of 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 the _Lepidoptera_.--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 the _sixth_, _seventh_, _eighth_, and _ninth_ segments of the body: none are ever found on the fourth, fifth, tenth, or eleventh segments.

1. Where _ten_ prolegs 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 genera _Sphinx_, _Pyralis_, and of the _Bombycidæ_, &c. have _eight_ legs, they are placed in _three_ different 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, like _Cerura 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, as _Noctua Gamma_, &c. have only _six_ legs: namely, an anal pair, and two ventral ones, situated on the eighth and ninth segments.

4. The larvæ of the Geometers (_Geometræ_ F.) have but _four_ prolegs; 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 of _Tinea_ L. which live in the interior of fruits, seeds, &c., have but _one_ pair of prolegs, which are attached to the anal segment.

6. The larvæ of Haworth's genus _Apoda_ (_Hepialus Testudo_ and _Asellus_ F.), 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 of _Tenthredo_ L., 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. lutea_ L.), and this is the most numerous tribe of them, including the modern genera, _Cimbex_ F., _Pterophorus_, &c. Others have fourteen, as that of the cherry (_T. cerasi_ L.); and many others with only nine joints to their antennæ. A third class have only twelve, as that of the rose (_T. Rosæ_ L.), 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 genus _Lyda_ F. (_T. crythrocephala_ L.)[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 of _Lyda_, will always serve as a mark to distinguish these _fausses chenilles_, as the French call the larvæ of saw-flies, from true _caterpillars_. The _dorsal_ prolegs of a species of _Cynips_ described 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 in _Chrysomela Populi_, a common beetle, secreting a slimy matter to fix itself[311]. In the larvæ of _Staphylinidæ_ this proleg is very long and cylindrical; in that of _Cicindela_ it is shorter, and in shape a truncated cone rather compressed; it is very short, also, in those of the _Silphæ_ that 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 last _dorsal_ segment, which becomes also ventral at the anus. This space is in fact the 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 paraplecticus_ F.) 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 Rumicis_ Germ.), 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 informed you 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 of _Rhagium 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 intermediate _dorsal_ segments 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 stercoraria_ De Geer[318] (_Chironomus_ Meig.?), drags itself along by the assistance of a _single_ tubercle, 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 beautiful _Chironomus_ (_C. plumosus_), remarkable for the feathered antennæ of the male[320], has _two_ short prolegs, or pediform but not retractile tentacula in the same situation[321]. Others, as that of _Tanypus maculatus_, &c. have _two pairs_, one attached to the anal and the other to the first segment[322]. _Tipula amphibia_ De Geer in this state has _ten_ prolegs, placed by pairs on the fourth, fifth, eighth, ninth, and tenth _dorsal_ segments[323]; and _Scæva Pyrastri_ F., one of the aphidivorous flies, has not fewer than _forty-two_, arranged in a sextuple series, seven in each row[324].

It may not be useless to close this long description of the _legs_ of larvæ with a tabular view of them, founded chiefly upon these organs; which afford very obvious marks of distinction.

I. Larvæ without legs.

i. With a corneous head of determinate shape (coleopterous and hymenopterous _apods_--_Culicidæ_, some _Tipulidæ_, &c. amongst the _Diptera_).

ii. With a membranaceous head of indeterminate shape (_Muscidæ_, _Syrphidæ_, and other _Diptera_).

II. Larvæ with legs.

i. With legs only, and with or without an anal proleg (_Neuroptera_, and many _Coleoptera_).

1. Joints short and conical (_Elater_, _Cerambycidæ_, &c.).

2. Joints long and subfiliform (_Staphylinus_, _Coccinella_, _Cicindela_, &c.).

ii. Prolegs only (many _Tipulidæ_, and some subcutaneous Lepidopterous larvæ, &c.).

iii. Both legs and prolegs (_Lepidoptera_, _Tenthredinidæ_, and some _Coleoptera_).

1. Without claws (_Tenthredinidæ_, &c.).

2. With claws (_Lepidoptera_, &c.).

I should next say something upon the spiracles, or breathing-pores, or any other external apparatus for the purpose of _respiration_, 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 above _parts_, or under the _clothing_ of 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 genus _Bombyx_ to 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 the _Noctuidæ_, and in most of those of the hawk-moths (_Sphinx_ F.) [_S. Porcellus_, _Vitis_, and a few others excepted; in some of which, as _S. Labruscæ_, &c., this anal horn is replaced by a gibbosity, and in others, as _S. Œnotheræ_, by a callous eye-like plate[326]] in the same situation, but much longer[327], and commonly curving backwards over the tail[328]. Sometimes, however, as in _S. ocellata_ and _S. 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 dermestoides_ F.) 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 in _Bombyx Stigma_ F., 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 yellow stripes, in which these horns are of equal thickness at base and apex, but with the same terminal knob. _Danais Archippus_ has a pair of tentacula at the head, and another pair, but shorter, at the tail; and _D. Gylippus_ has, 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_, and _tridens_ F.) 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 (_Trichoptera_ K.) 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 the remarkable Y-shaped, as it should seem, scent-organs (_Osmateria_) of the beautiful caterpillar of the swallow-tailed butterfly (_Papilio Machaon_ L.), and others of the _Equites_[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].