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.

  1. Larvæ without legs.
    1. With a corneous head of determinate shape (coleopterous and hymenopterous apods—Culicidæ, some Tipulidæ, &c. amongst the Diptera).
    2. With a membranaceous head of indeterminate shape (Muscidæ, Syrphidæ, and other Diptera).
  2. Larvæ with legs.
    1. 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.).
    2. Prolegs only (many Tipulidæ, and some subcutaneous Lepidopterous larvæ, &c.).
    3. 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].