6 Descriptive Zoology. 



buoyancy during flight. The work of respiration depends 

 on the abdomen, as the thorax is rigid. The abdomen is 

 made smaller by the action of its muscles, and expands 

 again when they relax. Expiration seems to be accom- 

 plished by active effort, and inspiration by elastic reaction, 

 just the reverse of the breathing* process in man. 



Circulation in the Grasshopper. The circulatory system 

 of the grasshopper is not highly developed. The only dis- 

 tinct organ is the heart (see Figs. 3 and 4), extending along 



Dorsal air-tube 



Air sac 



FIG. 4. AIR TUBES AND AIR SACS OF GRASSHOPPER. 



From Hyatt's Insecta. 



the dorsal part of the abdomen. It is in the form of a 

 tube, closed behind and open at the anterior end. It has 

 several compartments, with valves, which allow the blood 

 to pass forward only. There are also openings with valves 

 at the sides, so that blood enters when the tube widens, and 

 when it contracts the blood is pumped forward. The 

 blood is colorless, or slightly yellowish or greenish, and 

 fills all the otherwise unoccupied spaces of the body, thus 

 bathing all the tissues. The low development of the cir- 

 culatory system is compensated for by the high develop- 



