Mi ZOOLOGY. 



Class VI.— Insbcta. 



i&eneral Character of Insects.— The triregional division 

 of the body is better marked in the genuine winged insects 

 than in the Myriopods and spiders. They usually have com- 

 pound as well as simple eyes; usually two pairs of wings; 

 three pairs of thoracic legs; often a pair of jointed abdomi- 

 nal appendages, besides an ovipositor or sting which mor- 

 phologically represents three pairs of abdominal legs. 



Order 1. Thysanura.—The, spring-tails (Fodura) and 

 bristle-tails {Lepisina) represent this group. They are wing- 

 less, with some affinities to the Myriopods; and the typical 

 form Gampodea (Fig. 319) is regarded as the ancestral form 

 of the six-footed insects, as it is a generalized 

 type, and forms like it may have been the 

 earliest insects to appear. 



In Podura, the spring-tail, and also in 

 Sniynthurus {Smynthurus quadrisignatus 

 Pack., Fig. 317), the characteristic organ is 

 a forked abdominal appendage or "spring," 

 held in place by a hook; when released the 

 spring: darts backward, sending the insect 

 thufus. a spring- high lu the air. 

 tail. Enlarged. q^^^, commonest Podurau is Tomocerus 



johimheus Linn. (Fig. 318), found all over the northern 

 liomisphere, in North America and Europe. Tiie snow-flea, 

 Acliorutes nivicola Fitch, is blue-black, and is often seen 

 leaping about on the snow in forests. 



The Podurans belong to the suborder Collerribola ; the 

 higher forms, which bear a greater resemblance to the larvas 

 ■of Neuropterous insects and to the young cockroach, are 

 the Cinura. 8colopeiidrella, with its weU-developed ab- 

 dominal legs, represents the suborder Symphyla. 



In the group Oinura there is no spring, but the tail ends 

 ]n two or three bristles; and in Machilis, the highest form, 

 there are compound eyes. In all there are jointed abdominal 

 appendages, which structures are unique among Hexapodous 

 insects. Gampodea staphylimis (Fig. 319) is a small white 



