304 THE BIOLOGY OF INSECTS 



same time as those which started several months ahead of 

 them. 



The caterpillars of other noctuids — the Cabbage Moth 

 (Mamestra hrassicae) for example — are fully grown in 

 autumn, and by October or November are lying as pupae 

 buried in their earthen chambers until the time for emergence 

 next season. The pupal is the characteristic wintering stage 

 of many insects, our common White Butterflies (Pieris),for 

 example, whose angular pupae, supported by the pad of 

 silk to which the hooked tail-region (cremaster) of the pupa 

 is anchored, and by a silken girdle around the waist, may be 

 seen on tree- trunks, palings, and such resting-places from 

 October to November until April or May, when the spring 

 butterflies emerge. While the Turnip Moth and its allies 

 are partially double-brooded, only a minority of the race 

 completing their transformations in less than a year, the 

 White Butterflies are regularly double-brooded, the cater- 

 pillars hatched in spring pupating during the summer so 

 as to develop into a hot-season brood of butterfliies in July 

 and August. Enough examples have now been given to 

 illustrate in how many respects the life-cycles of insects 

 are fitted to the circle of the seasons, not the structure only, 

 but the habits of the creatures in the various stages of their 

 growth being adapted so as to ensure the survival of enough 

 individuals to provide for the continuance of the race, while 

 the endlessly diverse ways in which the adaptation is brought 

 about suggests the power of adjustment to new conditions. 

 To this power there are, however, definite limits. Early 

 in this chapter reference was made to the yearly immigra- 

 tion into our islands of the Painted Lady Butterfly (Pyrameis 

 cardui). C. B. Williams (1923-4) describes vividly the 

 northward and westward flight of these insects from Egypt 

 over the Mediterranean, giving reason for believing that the 

 starting-place of their immigration is much farther south 

 and that they occasionally travel northward as far as Iceland. 

 Abundant almost every summer in our countryside, these 

 beautiful insects leave no progeny because their winged 

 adults succumb to the conditions even of our milder winters. 



