204 ARTHROPODA 



marked by striations wound around it. Do you know what 

 causes this appearance and what the arrangement is for? Do 

 you understand how the tracheal system is arranged? What 

 is the distribution of this system and how is the air made to 

 go in and out? 



3. Near the dorsal surface of the posterior part of the ab- 

 domen, surrounded by the tissues already mentioned, are the 

 gonads. These differ in size and shape according to the sex. 

 In the male the vasa deferentia may be seen leaving the 

 tabulated testes. In the female the oviducts pass around the 

 sides of the intestine. They may be followed later. 



4. Loosen the anterior ends of the gonads and turn them 

 posteriorly to expose the hinder part of the alimentary canal. 1 



(a) The esophagus, which bends backward from the 

 mouth, gradually enlarges as it enters the thorax. 



(b) The crop, which is not sharply separated from the 

 esophagus, gradually narrows posteriorly. 



(c) Following the constriction posterior to the crop is the 

 elongated stomach, frequently called the ventriculus. Sur- 

 rounding the anterior end of this portion is a series of rather 

 large diverticula, the gastric caeca, that extend both ante- 

 riorly and posteriorly from the points where they open into 

 the stomach. 



(d) Some distance behind the posterior ends of the hepatic 

 caeca, quite concealed by the mass of small uriniferous tubes, 

 is a slight constriction and hardening of the alimentary canal 

 that marks the division between the stomach and intestine. 

 It is at this point that the uriniferous tubes join the alimen- 

 tary canal. 



(e) Behind the intestine the alimentary canal becomes 

 much smaller and is known as the hind intestine or colon. 



(/) Behind the colon, forming the hinder portion of the 

 alimentary canal, is the slightly enlarged rectum. The rectum 



1 There is great diversity in the parts of the alimentary canals of 

 different insects. This is correlated with the great differences in feed- 

 ing habits. 



