8 ' THE CLASS OF INSECTS. 



uses to the forward portion of the body. Hence the nervous 

 power is transferred in some degree towards the head ; the 

 cephalothorax containing the nervous centres from which nerves 

 are distributed to the abdomen. Nearly all the organs perform- 

 ing the functions of locomotion and sensation reside in the front 

 region ; while the vegetative functions, or those concerned 

 in the reproduction and nourishment of the animal, are mostly 

 carried on in the hinder region of the body (the abdomen). 



The typical Crustacean cannot be said to have a true head, 

 in distinction from a thorax bearing the organs of locomotion, 

 but rather a group of rings, to which are appended the organs 

 of sensation and locomotion. Hence we find the appendages 

 of this region gradually changing from antennae and jaws to 

 foot-jaws, or limbs capable of eating and also of locomotion ; 

 they shade into each other as seen in Fig. 9. Sometimes the 

 jaws become remarkably like claws ; or the legs resemble jaws 

 at the base, but towards their tips become claw-like ; gill-like 

 bodies are sometimes attached to the foot-jaws, and thus, as 

 stated by Professor J. D. Dana in the introduction to his great 

 work on the Crustacea of the United States Exploring Expedi- 

 tion, the typical Crustaceans do not have a distinct, head, but 

 rat.her a "head-thorax" (cephalothorax). 



When we rise a third and last step into the world of Insects, 

 we see a completion and final development of the articu- 

 late plan which has been but obscurely hinted at in the two 

 lowest classes, the Worms and Crustaceans. Here we first meet 

 with a true head, separate in its structure and functions from 

 the thorax, which, in its turn, is clearly distinguishable from 

 the third region of the body, the abdomen, or hind-body. 

 These three regions, as seen in the Wasp (Fig. 10), are each 

 provided with three distinct sets of organs, 

 each having distinct functions, though all are 

 governed by and minister to the brain force, 

 now in a great measure gathered up from the 

 Fig. 10. posterior rings of the body, and in a more 



concentrated form (the brain being larger than in the lower 

 articulates) lodged in the head. 



Here, then, is a centralization of parts headwards ; they are 



FIG. 10. Phllanthus ventilabris Fabr. A Wood-wasp. From Say. 



