ARTHROPOD A 



193 



surface ; hence the name, Hexapoda, for this class. These 

 appendages consist typically of five segments of different 

 lengths, and the fifth or distal segment consists of from three 

 to five very short segments. Here, as in the mouth parts, the 

 structure varies with the function of the appendage, whether it 

 be for walking, running, burrowing, jumping, climbing, or other 

 purposes (Fig. 191). Attached to the dorsal side of the tho- 

 rax there are typically two pairs of wings, the anterior pair 

 on the mesothorax, the posterior pair on the metathorax ; and 

 the shape, size, and 

 texture ot these wings 

 varv much. 



The abdomen is 

 likewise segmented, 

 consisting typically of 

 nine or ten metameres. 

 The anus is at the 

 posterior end of the 

 abdomen, and just 

 anterior to it is the 

 usually single genital 

 opening. The stig- 

 mata are generally 

 numerous : they are 

 absent on the head ; 

 there are at most two 

 pairs on the thorax ; 

 on the abdomen they 

 lie at the lateral 

 boundaries of most of 

 the segments. Of the 

 internal structure of 

 the insects (Fig. 192) 

 we may simply note the high degree of development of the 

 nervous system in many cases ; the brain is a large and compli- 

 cated structure, and the ventral nerve cord, while in some groups 

 possessing ganglionic enlargements in each segment, in the more 

 highly organized insects has these ganglia more or less approxi- 

 mated into a large mass in the thorax. 



mx, 



FlG. 190. Mouth parts of the cockroach. Ibr, labrum ; 

 md, mandible; ?nx, first or anterior pair of maxillae; 

 in, mentum, formed from the fused second maxillae; me 

 and mi, outer and inner divisions of the maxillae ; //, labial 

 palp; pin, maxillary palp; st, stipes; sin, submentum. 

 (From Parker and Haswell's Manual.) 



