376 SOME REPRESENTATIVE INSECTS 



Collembola) wings are absent even in the larval stages. In other 

 insects that are wingless as adults, there are indications, as shown 

 by the presence of wing rudiments in larvee or adults, that two 

 pairs of wings existed in the ancestors but have degenerated in the 

 course of evolution. Thus there are typically two pairs of wings 

 in the majority of insects. The specialization of the anterior pair 

 of wings into wing-covers, as in the Coleoptera and to a lesser 

 extent in the Orthoptera, and the reduction of the posterior pair of 

 wings in Diptera are to be regarded as evolutionary modifications 

 of the more typical four-winged state. Likewise, the simpler 

 insects exhibit direct development. Hence, the extreme meta- 

 morphosis that appears in the Lepidoptera and some of the other 

 more specialized orders has probably been evolved from the more 

 direct, and therefore simpler, type of development. It should be 

 said, however, that the worm-like body which appears in cater- 

 pillars and in the larvse of other insects that exhibit considerable 

 metamorphosis may represent the ancestral type suggested by 

 Peripatus (c/. p. 339) and hence may be the survival of a primitive 

 feature in these more specialized modes of development. 



Body Plan and Characteristics of the Insecta. — The body plan 

 of insects agrees in general with that of other arthropods (Fig. 164, 

 p. 337) as illustrated by the crayfish, and Peripatus (Fig. 165, 

 p. 339). The locust, like the crayfish, is a sufficiently generalized 

 type to be representative not only of its own class but of the 

 phylum as a whole. Like the Annulata, the Arthropoda are bilat- 

 erally symmetrical and metameric animals. The mouth and the 

 anus open ventrally near the two ends of the body, which is 

 usually elongated. The nervous system consists of the dorsal 

 brain, circum-esophageal connectives, and a ventral nerve 

 cord of paired gangHa with their commissures and connectives. 

 The appendages of the arthropod are, however, more com- 

 plex than the simple parapodia of annulates like Nereis, with 

 which they are sometimes compared, since they may be highly 

 developed and are so typically jointed as to have suggested the 

 name " arthropod " for these joint-footed animals. The charac- 

 teristic exoskeleton of chitin, sometimes thickened by additions of 

 carbonate of lime, is formed as a secretion from the underlying epi- 

 dermis and is molted in a characteristic manner. The cuticle 

 secreted by the epidermal cells of most annulates (Fig. 145, p. 302) 

 resembles the skeleton of the arthropod in its mode of origin. In 



