114 



BRANCH ARTHROI'ODA 



The head bears the compound eyes and simple eyes (when they 

 are present), one pair of antennae, and three pairs of mouth 

 parts, which vary according to the character of their food. 

 Hence the mouth parts may be adapted for chewing, lapping, 

 sucking, or piercing — " all referable back to the chewing type. 

 These are, in turn, modified legs."^ 



The thorax has usually three well-marked segments^ — pro- 

 thorax, mesothorax, and metathorax — as in the grasshopper. 

 Each segment bears a pair of jointed ventral legs. The two 

 pairs of wings, when present, are outgrowths of the dorsal por- 



Fig. Sb 



tion of the meso- and metathorax. Sometimes there is but 

 one pair, and in a few cases none. 



The abdominal segments vary in number and usually bear 

 no paired appendages except, sometimes, on the terminal seg- 

 ments. 



Covering. — Over the greater portion of the surface of the body 

 the cuticle or external layer of the skin is made firm and horny 

 by a substance called chitin. This forms an exoskeleton for the 

 protection of the soft parts within, and, by its rough interior 

 surface, provides points of attachment for the numerous small 

 but strong muscles. 



1 Kingsley's Hertwig, " A Manual of Zoology." 



