344: ZOOLOGY. 



Class VI. — Insecta. 



General 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 hare 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 (Podura) and 

 bristle-tails (Lepisma) represent this group. They are wing- 

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

 form' Campodea (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 

 Smynthurus (SmyntJiurus 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 



Fig. 317. —Smyn- i ■ -u • j-i, ■ 



thurus. a spring- high in the air. 



tail. Enlarged. q^^ commonest Poduraii is Tomocerus 



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

 hemisphere, in North America and Europe. The snow-flea, 

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

 leaping about on the snow in forests. 



The Podurans belong to the suborder Collembola ; the 

 higher forms, which bear a greater resemblance to the larvse 

 of Neuropterous insects and to the young cockroach, are 

 the Cinura, to which belong the genera Campodea (Fig. 

 319), Lepisma, and Machilis. 



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

 in 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. Campodea staphylinus (Fig. 319) is a small white 



