370 THE BIOLOGY OF INSECTS 



of Lepidoptera, we find the large caterpillars of the hawk- 

 moths (Sphingdae) of a predominantly green (sometimes 

 brown) shade, the area broken by series of obHque, dark, 

 lateral lines — the whole aspect definitely protective among 

 leaves or on twigs. Our two common species of Smerinthus, 

 S. populi and S. ocellatuSy have green caterpillars closely 

 alike in size, colour, markings, but while the horn on the 

 eighth abdominal segment of the populi larva is green like 

 the body generally, that of the ocellatiis larva is blue. It is 

 impossible to regard such differences of any importance in 

 the struggle for existence, and if specific distinctions be 

 due to the action of natural selection they must be not 

 merely important but so important as to decide the alterna- 

 tive of survival or destruction. The only suggestion that 

 has been made to connect characters of this kind with 

 natural selection is the possibility that they may be corre- 

 lated with characters that possess '* survival value " so that 

 the one cannot be inherited without the other. This 

 possibility cannot be dogmatically denied, but can such 

 correlation be reasonably regarded as likely 1 Among 

 insects of very different type from the Lepidoptera — the 

 wingless shore-haunting Collembola or springtails — similar 

 facts can be observed. The same mass of wrack at high 

 drift-mark often harbours hundreds of two species of 

 Achorutes, A. viaticus (Fig. 66) and A, longispinus ^ dis- 

 tinguished by constant structural features in the feet and 

 spring and by the much longer anal spines of the latter 

 (Fig. 8i),yet living amidst identical surroundings to which 

 either kind seems as well suited as the other. 



Yet there are doubtless many cases in which careful 

 study of an insect's habits tend to prove that characters not 

 obviously useful may be really of value in the struggle for 

 existence. Taking for example the African nymphalid 

 butterflies of the genus Precis, a number of forms once 

 regarded as distinct species were shown by G. A. K. 

 Marshall (1896, 1898, 1902) to be alternating seasonal 

 phases of the same markedly variable insect. Precis octavia^ 

 a butterfly with wings predominantly salmon-pink, black 



