168 ARTHROPODA. 



striations wound around it. Do you know what causes this 

 appearance and what the arrangement is for? Do you under- 

 stand how the tracheal system is arranged ? Why is it extended 

 all over the body 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 lobulated 

 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. 



(6) The crop, which is not sharply separated from the esoph- 

 agus, gradually narrows posteriorly. 



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

 elongated stomach, frequently called the ventriculus. Surround- 

 ing the -anterior end of this portion are a series of rather large 

 diverticula, the gastric cceca, that extend both anteriorly 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 alimentary 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 ali- 

 mentary canal, is the slightly enlarged rectum. The rectum 

 cannot be seen until the ovary is removed, which should be de- 

 ferred until the ducts have been seen. 



Make a drawing showing the position of the parts of the alimen- 

 tary canal in side view. 



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

 different insects. The great differences in feeding habits render this 

 necessary. 



