CHAP. XV 



INSECT A : LEPIDOPTERA 



219 



it open the ducts of a pair of silk glands, from which the 

 larva can spin a fine silken thread (Fig. 147, s). 



When full grown, the caterpillar ceases to feed 

 for^Dation ^""^ becomes restless, leaving its food plant and 

 seeking to climb. It climbs even slippery surfaces, 

 such as that of glass, with great ease, giving out from its 

 spinneret a fine white silk thread, which, by means of a side- 

 to-side movement of its head as it climbs, it fixes to the 

 surface as a zigzag silk ladder. It holds on to this with its 

 abdominal feet whilst, with head outstretched, it is adding 

 to it above, rung by rung. (Such a "ladder" is shown in 

 Plate II. below each chrysalis.) Having reached by this 

 means a suitable spot, the caterpillar prepares to pupate. 

 It first spins a little mass of silk against the surface to which 

 it is clinging, and fastens its claspers into this. Then it 

 extends itself along the surface, usually so that it is in a 

 vertical position with head uppermost, and, turning its head 

 right back over its thorax, it spins a little girdle of silk 

 across its body at the level of the second or third segments 

 of the abdomen, fixing it to the glass on each side and 

 repeating the action until the girdle is many strands thick. 

 It then again stretches itself out, and its body shortens and 

 swells, until, after about two days' quiescence, the skin splits 

 and is pushed off at the hind end, whilst 

 the body from within quickly swells up and 

 takes on the pupal or chrysalid form. 



The rudiments of wings, 

 o/chrysaUs. '^®g®' antennae, and compound 

 eyes now appear, and for a 

 moment the wings are free ; quickly, how- 

 ever, a chitinous fluid exudes from the 

 body and hardens all round the pupa, 

 forming a delicate transparent " shell," 

 which is yellowish-green, spangled with 

 yellow and black dots, and beset with 

 little sharp points and angles which prevent 

 it slipping from its silken support. There 

 is one specially large dorsal projection on 

 the thorax, and also one on each side a 

 little further back (Fig. 148, d and X). 

 The silk thread lies between the dorsal and the lateral spines. 



Fig. 148.— Chi-ysalis 

 of Pieris brassicae. 



e, Compound eye ; a, an- 

 tenna ; s, spiracle ; d, 

 dorsal spine ; £, lateral 

 spines. 



