THE NEW EVOLUTION 



special bodily secretions. There are the various 

 types of silk produced by insect larvas and by spiders, 

 the paper made by wasps, the wax produced by bees, 

 aphids (figs. 7-9, p. 2.1) and other insects, sweet sub- 

 stances secreted by aphids and other types, narcotics 

 used to stupefy the prey, poisons used to kill the prey, 

 antiseptic substances used to protect the eggs of 

 internal parasites, and various kinds of poisons and 

 reagents for special and restricted uses. 



But here we become involved with the chief differ- 

 ence, other than the structural, between the insects 

 and the vertebrates. In their relations to the world 

 about them the insects are mainly guided by the chem- 

 ical senses which in us are represented by taste and 

 smell, whereas in the vertebrates the eyes and ears 

 are commonly the main controlling organs, often com- 

 bined with a delicate sense of touch, and smell and 

 taste are relatively unimportant, even though the 

 former may be, as in the dogs, highly developed. So 

 the extensive use of chemical processes by the insects 

 is quite in line with the largely chemical nature of 

 their external contacts. 



The very diverse, ingenious and effective snares of 

 spiders and of some insect larvae, as the young of some 

 caddis-flies and the New Zealand glow-worm, are 

 really most extraordinary structures. They show, 

 most of them at least, an almost perfect adjustment 

 in the relation of each part to the strains and stresses 

 which that part must meet. The pit-falls dug by 

 the young of ant-lions are equally effective and 

 ingenious. 



