ACRIDIUM. 183 



able to determine the structure of the heart in the dissection. 

 Read this up, and determine what the radiating muscle fibers 

 are for. 



2. The space between the muscles and the viscera is filled 

 more or less completely by the fat-body and the trachece. With 

 a lens notice how the tracheae connect with the spiracles and 

 how they branch. Remove a portion of the tissue in which 

 you can see tracheae, mount it in water under a cover, and 

 examine it microscopically. Each tracheal tube is marked by 

 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 seon 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. 



(b) 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 caeca, 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 



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

 different insects. The great differences in feeding habits render this 

 necessary. 



