220 



INTRODUCTION TO ZOOLOGY 



CHAP. 



A butterfly pupa is generally called a Chrysalis because of 

 the golden spots which in so many cases decorate it (Gr. 

 chrysos, gold). The chrysalis can now only move its ab- 

 dominal segments, jerking them laterally if touched. It remains 

 otherwise entirely quiescent, for two or three weeks only, 

 if it is the first brood of the year which pupated in June ; 

 or for the whole winter if it is the second brood which pupated 

 in the autumn. The butterflies of this latter brood emerge in 

 April or May, but so many of the chrysalids get destroyed by 

 birds and other enemies during the winter, that the butterflies 

 in the spring are not nearly so numerous as those that emerge 

 in July and August. 



When the insect is about to emerge, the pupal 



oM^aeo* skin splits along the back of the thorax, and the 



imago gradually pulls itself out. Its wings are at 



first damp and crumpled up, but in the air they gradually 



expand, dry, and stiffen, and then are strong enough to 



support the butterfly. 



In the imago, the division into head, thorax, 



and abdomen is very distinct. The thorax of the 



butterfly is black and hairy, and bears three pairs of jointed 



legs and two pairs of wings. The upper surfaces of the wings 



are yellowish- 

 white with cer 

 tain black spots 

 and smudges on 

 them, which differ 

 in male and 

 female. 



The male has 

 the apex of each 

 front wing either 

 black or brown, 

 and there is a 

 dark stain at the 

 -partly hidden by 

 the front wing — on the front margin of the hind wing ; this 

 is the only dark pigment on the upper side of the wing. 

 The female has, in addition, two dark spots on her front 

 wing with a dark smudge along the inner margin.'- The 



' For terms used in describing the different parts of the wing see Fig. 161. 



Fig. 149. — Pieris brassicae (male). 



base of each 



wmg, 



and a dark smudge- 



