42 THE BOOK OF BRITISH BUTTERFLIES. 



the winter in a stoutly-made web of a similar kind. As 

 spring advances, however, they leave the web and separate. 

 They are full-fed about the end of May, and are then 

 about 2 in. in length (Fig. 20). The divisions between the 

 segments are well marked. The head is black, and so are 

 the six legs, but not the claspers ; the ventral surface is 

 pinkish-grey, and the dorsal reddish-brown with heavy 

 black markings. The dorsal colouring ends abruptly with 

 a broad black band on each side. Along the middle of 

 the back are two fine, somewhat interrupted, black lines, 

 and between these and the broad black lines on each seg- 

 ment are two large black dots, one on each side, and 

 several smaller ones. The colouring of the ventral surface 

 is rather paler near the black division-line, and is elsewhere 

 finely dotted with black, with several larger black marks on 

 the mid-ventral surface. The spiracles are black and the 

 claspers grey. The ventral surface bears a number of fine 

 white hairs; the rest of the caterpillar is slightly downy. 



The chrysalis (Fig. 19), which should be sought for in 

 early June, is somewhat robust and has most of the usual 

 prominences rounded off. It is attached to its late food- 

 plant by the tail and by a band of silk round the thorax. 

 The ground-colour is pale yellowish-white, similar to that 

 of the pupa of Pieris brassicce. It is marked, too, with 

 numerous black dots ; but they are larger and much more 

 pronounced than in brassiccz. The wing-cases are deeply 

 lined with black. 



The imago (Figs. 17 and 18), which is on the wing at 

 mid-summer and a month or so afterwards, and frequents 

 heaths and wooded country, is pale creamy-white in colour, 

 and rather thinly covered with scales. All the wings 

 are rounded and fringeless, the boundary being formed 

 instead by a nervure, black like all the rest, as the 

 common name points out. Where each nervure of the 



