36 THE BUTTERFLIES OF THE BRITISH ISLES. 



the upright position when fastened in the angle formed by two 

 pales. A position that affords some measure of protection from 

 weather is generally selected. 



Although this butterfly is almost annually to be seen, in 

 greater or lesser numbers, throughout the country, it is occa- 

 sionally scarce, either generally or in some parts of the British 

 Islands. For example, during the past year (1905) it was 

 abnormally plentiful in Ireland, but at the same time com- 

 paratively rare in England. It is a migratory species, and no 

 doubt its abundance in any year in these islands is dependent 

 on the arrival of a large number of immigrants. Possibly in 

 some years none of the migrant butterflies reach our shores, 

 and that it is largely to this failure the rarity of the species 

 in such years is to be attributed. Caterpillars resulting from 

 alien butterflies may absolutely swarm in the autumn of one 

 year, but the eccentricities of an English winter may be too 

 much for the vitality of such of them as escape their enemies, 

 Apanteles glo7neratns, and other so-called " ichneumons," and 

 reach the chrysalis state. So, with immigration on the one 

 hand and destructive agencies on the other, it may be under- 

 stood how it comes about that the Large White is sometimes 

 abundant and sometimes scarce. 



This species seems to range over the whole of the British 

 Islands, with the exception, perhaps, of the Shetlands. Abroad, 

 it has been found in all parts of the Palaearctic Region, except 

 the extreme north, and Eastern Asia. 



The Small White (Pieris rapce). 



The Small White butterfly (Plate 1 1) is, perhaps, more often in 

 evidence then its larger kinsman just referred to. It also is a 

 migrant, and although it never seems to be absent from these 

 islands, in its proper season, its great increase in numbers in 

 some years is almost certainly due to the arrival of immigrants. 



