INSECTS 129 



stomach into the small intestine. This organ is much smaller in 

 diameter, but the walls have the same general structure as those of 

 the stomach. Here the remainder of the food is digested and 

 then passed into the blood. The small intestine enlarges pos- 

 teriorly to form the rectum which opens to the exterior on the 

 last abdominal segment. 



2. Respiratory and Vascular Systems 



The respiratory organs of the Bee consist of a complex system 

 of thin-walled air tubes, or tracheae, which ramify through the 

 body tissues. These tracheal tubes open to the exterior through 

 ten pairs of breathing apertures, or spiracles, which are situated 

 on the right and left sides of certain of the thoracic and abdominal 

 segments. There are three pairs of spiracles on the thorax and 

 seven pairs on the abdominal segments. The first two pairs on the 

 thorax and the last pair on the abdomen are hidden from view by 

 overlapping portions of the body wall. The spiracles open inside 

 the body into the longitudinal tracheal trunks which extend 

 along either side of the body, and from which the finer ramifying 

 tracheae arise. In the anterior end of the abdomen, the tracheal 

 trunks are very large and constitute definite air sacs, which, when 

 filled with air, are supposed to aid the Bee in flight by lowering 

 the specific gravity of the animal. There is a definite expansion 

 and contraction of these air sacs, possibly corresponding some- 

 what to the respiratory movements of the higher Vertebrate ani- 

 mals. 



This method by which the Insect tissues obtain their supply of 

 oxygen is very efficient. The air containing oxygen comes into 

 direct contact with the body tissues. There is, therefore, no need 

 for a supplementary system to transport the oxygen to the cells, 

 such as is found in the circulatory systems of other animals. As 

 might be expected from this, the vascular system of the Insect does 

 not show so great a development of the closed vascular system as 

 in some of the less complex animals, such, for example, as the 

 Earthworm, but opens freely into large sinuses and connecting 

 channels which occupy considerable portions of the body spaces. 



The only blood vessel in the Bee is the dorsal blood vessel, 

 which extends almost the length of the body in a median line 

 just below the dorsal body wall. The posterior part of this vessel, 

 lying in the abdomen, is differentiated to form the heart. The 



