INSECTA 



431 



bear the cerci anales (as for instance in the cockroach). In specialized 

 insects the apparent number of abdominal segments may be greatly 

 reduced . 



In insect embryos rudiments of appendages are borne on each of 

 the abdominal segments, but these rudiments disappear in the adult 

 except in the Apterygota. Only those which become the cerci anales 

 in the nth segment are frequently retained. In the 8th and 9th 



t.irM- 



V 



.IV. a 



l.rti. 



P 



.w.a 



Fig. 303. To illustrate the mechanism of wing movement in an Aphid. 

 Wing depression: A, left side view of mesothorax; B, transverse section. 

 Wing elevation: C, left side view of mesothorax; D, transverse section. 

 dv.tn. dorsoventral muscles; l.m. longitudinal muscles; p.zv.a. pleural wing 

 attachment; t.w.a. tergal wing attachment. Effective muscles shown by- 

 dotted lines in A and C. After Weber. 



segments in the female and the 9th segment in the male there are 

 paired structures known as gonapophyses which perform various 

 reproductive functions (oviposition in the female, copulation in the 

 male). It is highly probable that these are modified appendages. 



The alimentary canal (Fig. 304) varies greatly in length; in many 

 larvae it is no longer than the animal itself, but in certain types of 

 insects like the Homoptera, which feed on plant juices, it is much 

 coiled and may be several times the length of its possessor. It consists 



