142 A BOOK OF INSECTS 



trunks and destroying bark-infesting liehens and algae, 

 has called into being distinct races of dark-winged moths 

 such as are seldom or never seen in other districts. 



Over and above these settled adaptations to a particular 

 environment, we find that certain insects which pass 

 through two or three generations in the year undergo 

 seasonal changes — i.e. the early and late broods of the 

 same species differ in colour and marking. This pheno- 

 menon, which is known as seasonal dimorphism, may be 

 seen in our own white butterflies — individuals of the 

 spring brood being more deeply marked than their de- 

 scendants of the succeeding summer brood ; while in 

 certain exotic species the difference between the wet- and 

 dry-season forms is sometimes so remarkable that the 

 insects were formerly regarded as distinct species. Pro- 

 fessor Poulton tells us that "in northern latitudes the 

 differences between the early and late broods of the same 

 species sometimes correspond to differences in the sur- 

 roundings, and thus promote concealment. In tropical 

 countries the dry-season forms are often better concealed 

 than those of the wet season, when the struggle for exist- 

 ence is less severe." It must be admitted, however, that 

 the significance of these seasonal changes is by no means 

 fully understood. In some cases the application of arti- 

 ficial cold suffices to produce spring forms from summer 

 pupae, while more rarely the summer form is obtained 

 from winter pupae which have been kept in a hot-house. 

 From this it appears that the atmospheric environment of 

 the pupa may directly influence the colours of the perfect 

 insect. 



Before concluding this chapter, reference must be 

 made to the manner in which certain insects conceal 

 themselves by carrying about odds and ends of the sub- 

 stances among which they live. The most familiar 



