This and the two following preparations are intended to illustrate the various points of external anatomy in which the larva, pupa, and imago of a Lepidopteron, an insect with perfect metamorphosis, differ from one another. The larva (or caterpillar) belongs to the eruciform type of Packard. It possesses a distinct head, but it has a somewhat vermiform appearance owing to the great homonomy or similarity of the remaining somites, and the fact that the segmentation of the antennae and feet, and the form of the mouth-parts are not obvious to the unassisted eye.
The greater part of the covering of the head is made up by the two large 'parietal scales' of Lyonet, corresponding to the epicranium of the imago. A triangular plate, the 'frontal scale' of the same author, the representative of the clypeus, lies anteriorly between them. The parietal scales are each marked laterally by a dark stripe, at the inferior end of which may be found the six ocelli usually present in a caterpillar, as six whitish spots. The antennae and mouth-parts may be seen with the help of a lens. The antennae are three-jointed, and are situated below and internally to the ocelli. The joints can be retracted one within the other.
There is a pair of large toothed mandibles, but the structures corresponding to the maxillae and labium of the Cockroach are here fused into a plate which closes the mouth behind. The plate consists of a median and two lateral lobes. Each lateral lobe represents a maxilla and carries a jointed appendage, probably the maxillary palp. The median lobe, representing the labium, carries a pair of jointed appendages, the homologues apparently of labial palpi, and a central tubular projection, the spinneret, upon which opens the common duct of the two silk glands.
The three first somites carry each a pair of five-jointed limbs. They represent the thoracic region of the imago, while the remaining ten somites represent its abdomen. Of these ten, the two first somites have no appendages but like the first body-somite (pro-thorax), and unlike the second and third (meso- and meta-thorax), are pierced on each side by a respiratory foramen, the spiracle or stigma. The succeeding four somites also possess spiracles and carry unjointed, sucker-shaped limbs, armed terminally and on the inner side with spines, and known as 'pedes spurii,' or 'prolegs.' The two following somites have spiracles but no prolegs, and the last of the two (the eighth abdominal) carries a dorsal horn characteristic of the family Sphingidae, with the exception of a few species. The body is terminated by two somites, of which the first, representing the large ninth abdominal somite of the imago, is a narrow ring difficult to make out; while the last, the thirteenth larval somite reckoning from the head, is perforated by the anus and carries a pair of anal prolegs.
The anus itself is covered by a triangular anal valve.
The action of the spirit has destroyed the natural colour of the caterpillar. In life it is of a bright green colour, with yellow spiracles, the dorsal horn black above and at the tip, yellow below, and the abdominal region ornamented by seven oblique stripes, lilac above and white below. It feeds upon the privet, ash, etc, and exists in the larval condition from the end of July to the middle or end of September. During this period, it changes its skin six times, increasing much in size after each moult, notably the last. When full grown it has a brief period of quiescence, and then becomes very restless. Its colour, especially on the dorsal surface, tends to a brownish pink, and the pulsations of the heart become very visible and rapid. At last it buries itself in the earth, penetrating to a depth of six to eight inches. Here it hollows out a smooth-walled and waterproof chamber by moistening the soil with the secretion of its silk-glands, and pressing it with its body. It lines the chamber with silk and in about three days' time throws off the caterpillar skin and appears as a pupa or chrysalis.
The labrum is represented by two tubercles connected by a soft skin lying in front of the mandibles. In Pieris it is a corneous plate hinged upon the frontal scale. The mandibles of Sphinx are only slightly toothed. In the embryo the maxillae and labial limbs are separate as in other Insecta. A few Lepidopterous larvae are apodal, e. g. Micropteryx among Tineinae. The coxa and trochanter of the thoracic limbs are but slightly indicated. The tarsus is neither jointed nor clawed. In these points the limb contrasts strongly with the limb of an insect with an active pupa stage, e.g. Cockroach, which closely resembles that of the imago. In a few instances, e.g. in Stauropus Fagi, the second and third pairs of limbs are large with conspicuous joints. The prolegs vary much in number and character. In the embryo Sphinx Populi, as figured by Kowalewsky (Mem. de l'Acad. Imp. St. Petersburg (7), xvi. 1871, Pl. xii. fig. 10), there is a pair to each of the ten abdominal somites. In the Caterpillar they may be reduced to the last and the anal pairs (Geometridae), or to the anal pair alone (some Tineinae). They are as a rule armed with hooks, alternately long and short, arranged either in a complete circle round the terminal disc or only on its inner side.
The anal pair is produced in the Puss Moth (Cerura vinula) into a pair of long whip-like processes, each containing a protrusible filament, used to drive away Ichneumons. Abdominal limbs are also found in the Thysanura, in the larvae of Panorpatae, of Tenthredinidae among Hymenoptera, and of some Coleoptera (?).
Many larval Tenthredinidae closely resemble caterpillars. They differ from them in the following points: the ocellus is single; the antennae are 3-jointed and conical, but 7-jointed in the genus Lyda; the maxillae are membranous, bilobed and furnished with 3-5-jointed palpi; the labium is small, fleshy, and provided with 3-jointed palpi and a spinneret. The majority have prolegs which vary from 6-8 pairs in number. These legs have no hooks, and there is a pair on the second abdominal somite, which never bears one in the Lepidoptera. The larvae of the autumnal brood rest in the cocoon without pupating through the winter. Such a phenomenon is rare among Lepidoptera.