INSECTS 119 



the blood herein contained which receives a considerable portion 

 of the absorbed food materials and later transfers them to the cells 

 as the fluid slowly circulates through the body tissues. Another 

 point of interest in this connection is the large fat body, particu- 

 larly prominent in the larval stages, which lies in the hemocoel 

 surrounding the alimentary canal and which receives excess food 

 for storage. 



2. Respiratory and Vascular System 



Respiration. The Insect respiratory system, which is well- 

 shown in the Grasshopper, is unlike that found in any other 

 phylum of animals. It has as its distinctive functional feature 

 the direct transfer of the respiratory gases to and from the cells. 

 This is accomplished by means of special tubes, the tracheae ; 

 the external openings, or spiracles, of which are seen on the right 

 and left sides of the two posterior thoracic and the eight anterior 

 abdominal segments as previously noted. (W. fs. 54, 116.) 



The Grasshopper breathes through the spiracles, and the in- 

 coming air, rich in oxygen, is conveyed directly to the cells in all 

 parts of the body by the minute tracheal branches. These are 

 present in such abundance throughout the tissues of the body that 

 all the cells are supplied. The waste carbon dioxide from the 

 cells is eliminated through the same system. Certain abdominal 

 tracheae, in close connection with the spiracles, may be enlarged 

 and united to form two large air sacs — one lying on either side 

 of the abdomen. The air sacs are particularly well-developed in 

 the more actively flying Insects, such as the Bee. 



Vascular System. Thus in the Grasshopper and other In- 

 sects, the tracheal system permits a direct interchange of respira- 

 tory gases without the assistance of the vascular system as a trans- 

 portation agent, as previously observed in the Earthworm and 

 Crayfish. As might be expected under such a condition, the 

 vascular system is not so highly developed in the Insects. This is 

 noted particularly in the general absence of the blood vessels 

 (arteries and veins) which are abundantly present throughout the 

 tissues in animals in which the vascular system serves for the 

 transportation of the respiratory gases to and from the cells. 



The tubular vascular system of the Grasshopper consists of a 

 single large contractile dorsal blood vessel, or heart, lying 

 close to the dorsal body wall and extending from a blind ending 



