396 SOME REPRESENTATIVE INSECTS 



between the body plan of the Crustacea and that of the Annulata 

 {cj. p. 336) ; and the structure of the Onychophora (Fig. 165, p. 339), 

 which on the one hand resembles that of the Annulata, and on the 

 other the organization that may have existed in the ancestors 

 of the Insecta (Fig. 164, p. 337). This resemblance between 

 Annulata and Arthropoda may not be thus significant, but it is 

 suggestive of a relationship that may have existed in the remote 

 past when the Arthropoda began to be differentiated into the great 

 classes that have since grown and flourished in the ocean, in fresh 

 water, and upon the land surface. As with other phyla, the evi- 

 dence points to the ocean as the primitive habitat. There are 

 many parallels between Insecta and Chordata in this apparent 

 evolution from marine to terrestrial environment. Having 

 become adapted to Ufe upon the land surface, many insects, like 

 the water beetles, have returned to the water for a part or the 

 whole of their life cycles, as have the seals and whales among the 

 Chordata; but their mode of respiration and other features tell 

 the story of their ancestry. Since they have become so well 

 estabHshed upon the land, it is not surprising that a high degree 

 of social organization has been evolved in some types of insects 

 and that there has been a marked increase in intelligence in insects 

 and spiders over anything exhibited by the Crustacea, which are 

 predominantly aquatic. The small bulk of the individual body 

 among Insecta is a factor limiting their evolutionary advancement 

 as compared with the terrestrial vertebrates, but the advantages 

 in rapid reproduction and the innumerable ways in which animals 

 of smaller size can become adapted to the conditions of environ- 

 ment are partial compensations. Alone of all the animal types, 

 the insects among the Arthropoda compete with the terrestrial 

 vertebrates for possession of the land surface, and they may even 

 yet become the dominant forms of terrestrial life when man and 

 his mammalian relatives have run their course. 



