Legs. It may be stated generally that the larvæ of the orders Coleoptera, Lepidoptera, and Neuroptera, have legs; and that those of the orders Hymenoptera and Diptera have none. This must be understood, however, with some exceptions. Thus the larvæ of some Coleoptera, as the weevil tribes (Curculio L.) 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 of Tenthredo and Sirex in the order Hymenoptera are furnished with these organs. At present I know no Dipterous larva that may be said to have real legs, 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 called true legs (pedes veri), because they are persistent, being found in the perfect insect as well as in the larva; and the latter spurious 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 term legs, and for the latter prolegs (propedes)[289].
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.