298 ZOOLOGY 



Both centipedes and millipedes inhabit the land, and frequent dark places. Many 

 are nocturnal in habit. 



Class IV. Hexapoda {six feet; Insects). — Tracheate arthropods with three dis- 

 tinct body regions, — ^head, thorax, and abdomen. The head is a fusion of at least 

 six segments of which fotir are represented by appendages, — a pair of antennae 

 and three pairs of mouth parts. The thorax has three segments (pro-, meso- and 

 meta- thorax), each of which, in adult insects, bears a pair of legs; the meso-thorax 

 and the meta-thorax may each bear a pair of wings. The abdomen has a variable 

 (often obscure) number of segments. Its appendages are usually entirely wanting 

 or much reduced. A metamorphosis frequently occurs. The larval condition 

 often suggests the annelids and the myriapods in the similarity of its segments, 

 and in the numerous appendages. 



The student is referred to more comprehensive works for a complete exposition 

 of the numerous orders of this enormous group Hexapoda. Only the more im- 

 portant are described below. 



Fig. 137. 



Fig. 137. Centipede {Scolopendra heros). Photo by Folsom. Pour-fifths natural size. 

 Questions on the figure. — What differentiation of segments is apparent? Are 

 there any fusions into body-regions? What is the law of the occurrence of ap- 

 pendages? What diversity is there among them? 



Order Aptera {without wings). — This order embraces a number of minute, 

 wingless insects which do not undergo metamorphosis. The body is covered 

 with scales or hairs. The spring-tails and snow-fieas are examples. These make 

 their leaps by suddenly straightening out a tail-like structure which is bent under 

 the body when at rest. They are not the only wingless insects and hence the 

 name is somewhat misleading. See Pig. 138. This group is often regarded as 

 representing two distinct orders, Thysanura (the bristle-tails) and CoUembola (the 

 spring-tails). 



Order Orthoptera {straight wings). — In this order the metamorphosis is incom- 

 plete. There are usually two pairs of wings, the anterior often somewhat thick- 

 ened, serving as a cover for the posterior. Mouth parts are adapted for biting 

 and chewing. Here belong the cockroaches, grasshoppers, crickets, locusts, katy- 

 dids, walking-stick insects. The order is of considerable economic importance. 

 Most of its members are vegetable feeders and when they are gregarious are often 

 very destructive. The Rocky Mountain locust, so named because it breeds on 

 the plateau at the eastern base of these mountains, in 1873 and again in 1878, 

 migrated eastward over Nebraska and Kansas in search of food, literally stripping 



