ACRIDIUM. 187 



Internal Structure. — Remove the wings, and before opening 

 the body notice the rather large, somewhat transparent tympa- 

 num on each side of the first abdominal segment, very near the 

 base of the leaping leg. The structure of the auditory organ 

 may be easily studied by staining, clearing, and mounting in 

 balsam. (See Packard's ''Text-Book of Entomology" or 

 Brooks's "Hand-book of Invertebrate Zoology.") Remove the 

 dorsal portion of the wall of the abdomen and thorax, and notice: 



1. The heart, which will be found attached to the portion of 

 the wall of the abdomen that has been removed, by means of 

 numerous radiating muscle fibers. You probably will not be 

 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? What is the dis- 

 tribution 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 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 ahmentary canal.^ 



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

 ferent insects. This is correlated with the great differences in feeding 

 habits. 



