ARTHROPOD A 2Q7 



dibles suited to chewing vegetable matter. They are wholly harmless. They 

 have two pairs of legs to each of the numerous segments except the first four. 

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

 are nocturnal in habit (Fig. 137). 



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

 tinct body regions, head, thorax, and abdomen. The head has four segments 

 with appendages, a pair of antennae and three pairs of mouth parts. The thorax 

 has three segments (pro-, meso- and meta-thorax), each of which 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 an exposition of the 

 numerous orders of this enormous group of Hexapoda. Only the more important 

 are suggested below. 



Fig. 137. 



\ 



FlG. 137. Centipede (Scolopendra heros). Photo by Folsom. Four-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 any metamorphosis. The body is covered 

 with scales or hairs. The spring-tails and snow-fleas 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 Fig. 138. 



Order Orthoptera (straight wings}. In this order the metamorphosis is incom- 

 plete or lacking. There are usually two pairs of wings, the anterior often some- 

 what thickened, serving as a cover for the posterior. Mouth parts are adapted 

 to biting and chewing. Here belong the cockroaches, grasshoppers, crickets, 

 locusts, katydids, walking-stick insect. 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 fields of vegetation. Since the settlement of the regions where 



