TtiE ABDOMEN. 



117 



reach the head, in which are found two air-chambers 

 of considerable size. Reasons for this distribution of 

 the secreting vessels may be found, on the one hand, 

 in the need of the oxygenation of the tissues, 

 especially those connected with the nutrition of the 

 ganglia of highest functions ; and, on the other, in 

 the requirements of buoyancy in the segments 

 relatively the heaviest, and destitute of organs of 

 support in the atmosphere, such as the wings furnish. 



Fig. 33. — Air-sacs of Worker. 



A confirmation of the second of these purposes is 

 derived from the remarkable fact, that in the queen 

 bee, who does not fly more than once or twice in her 

 life, the great air-sacs of the abdomen are almost 

 obliterated, their space being needed for the large 

 ovaries. 



The structure of the trachece is very remarkable. 

 Under a powerful microscope they are seen to con- 

 sist of a double membrane, between the two coats of 



