Many pupae are protected by cocoons. These are masses of silk—very various in form—disposed by the caterpillar around itself during the last stage of its existence. Some of these cocoons are so perfect that the moth has considerable difficulty in escaping when the metamorphosis is complete. Various devices are used for the purpose of emergence; the Puss-moth excretes a corrosive fluid, containing potassium hydroxide, and then protects itself from this by retaining on the head while passing through it a shield formed of a portion of the pupa-skin.[[192]] Lepidopterous pupae usually have the body terminated by a projection of very various and peculiar form called "cremaster." In certain cases these projections are used for the suspension of the pupa, and are then frequently provided with hooks (Fig. 177, C, D). In other cases the cremaster is frequently called the anal armature (Fig. 205, B).
Fig. 168—Wing-rudiments of Pieris brassicae. A, Rudiments of a wing before the first moult of the caterpillar: ce, embryonic cells; ch, external cuticle; h, hypodermis; o, opening of the invagination; tr, trachea. B, posterior wing-rudiment of full-grown caterpillar; b, semicircular pad; c, a bundle of the rolled tracheae; e, envelope; i, pedicel; tr, trachea. (After Gonin.)
The development of the wings of Lepidoptera has recently been much studied. It has been known since the time of Lyonnet, that the rudiments of the wings exist inside the body of the caterpillar when it is nearly adult. Verson considers that he has detected the rudiments in the silk-worm larva even before hatching, and he attributes their origin to a modification of form of those hypodermal cells that occupy the spots where the spiracles of the second and third thoracic segments might be looked for. (It will be recollected that there are no spiracles on these two thoracic segments in Lepidopterous larvae). Gonin has examined the wing-rudiments in the caterpillar, a few days old, of Pieris brassicae,[[193]] and finds that the future wing is then indicated by a thickening and bagging inwards of the hypodermis, and by some embryonic cells and a trachea in close relation with this mass (Fig. 168, A). The structure grows so as to form a sac projecting to the interior of the body, connected with the body-wall by a pedicel, and penetrated by a trachea forming branches consisting of rolled and contorted small tracheae (Fig. 168, B). If the body-wall be dissected off the caterpillar immediately before pupation the wings appear in crumpled form, as shown in Fig. 169. This fact was known to the older entomologists, and gave rise to the idea that the butterfly could be detected in a caterpillar by merely stripping off the integument.
The exact mode by which the wings become external at the time of appearance of the chrysalis is not ascertained; but it would appear from Gonin's observations that it is not by a process of evagination, but by destruction of the hypodermis lying outside the wing. However this may be, it is well known that, when the caterpillar's skin is finally shed and the chrysalis appears, the wings are free, external appendages, and soon become fastened down to the body by an exudation that hardens so as to form the shell of the chrysalis.
Fig. 169—Anterior parts of a caterpillar of P. brassicae, the body-wall having been dissected off, immediately before pupation. a, a′, Anterior and posterior wings; st I, first spiracle; p, p′, second and third legs. (After Gonin.)
Scales and nervures.—Before tracing the further development it will be well to discuss the structure of the scales and nervures that form such important features in the Lepidopterous wing.
If a section be made of the perfect wing of a Lepidopteron, it is found that the two layers or walls of the wing are firmly held together by material irregularly arranged, in a somewhat columnar manner. The thickness of the wing is much greater where the section cuts through a nervure (Fig. 170, A). The nervures apparently differ as to the structures found in them. Spuler observed in a nervure of Triphaena pronuba, a body having in section a considerable diameter, that he considered to be a trachea, and also a "wing-rib" and blood-cells. He remarks that even in nervures, perfectly formed as to their chitinous parts, either wing-rib or trachea or both may be absent.[[194]] Schäffer[[195]] was unable to find any tracheae in the completed wings he examined, and he states that the matrix of the tracheae and even their inner linings disappear. The wing-ribs were, however, found by him to be present (Fig. 170, A and B).
