TRACHEATA 327 



union give off a trachea to the generative organs. Each of the 

 dorsal loops of the system thus formed breaks up into six or 

 eight smaller tracheae, which pass dorsally, giving off fine 

 branches to the various viscera, and frequently terminating in 

 vesicles ; the ventral loops also give off smaller branches, and 

 each of the last six gives rise to a long trachea, which passes 

 backward and opens into a large swelling in the fifth abdom- 

 inal segment; similar branches come from both sides of the 

 body, thus the right and left tracheal system is in communi- 

 cation within the body. 



Each trachea consists of a tube of very thin transparent 

 chitin, which is strengthened and kept expanded by a spiral 

 thickening of the chitin ; this gives the trachea its character- 

 istic spiral appearance. The chitin is secreted by a layer of 

 polygonal cells, with conspicuous nuclei, which surround the 

 tracheae. The vesicles are simply oval swellings on the 

 tracheae. The finest branches ramify between the cells which 

 compose the various tissues, and thus in a tracheate animal 

 the cells are directly supplied with air, and are not dependent 

 upon the blood for their supply of oxygen. Owing to the 

 complete intercommunication of the various tracheae the whole 

 system could be filled with air from any one stigma, so that 

 if anything rendered the supply from some of them inefficient, 

 it could be made right by the others. The vesicles when 

 charged with air doubtless serve to render the body of the 

 beetle lighter during flight. 



The heart of the cockchafer lies in the median line immedi- 

 ately beneath the dorsal integument ; it consists of a tube 

 closed behind but open in front, with contractile muscular 

 walls. In the abdominal section of this tube there are eight 

 pairs of ostia, with valvular lips ; through these the blood enters 

 the heart, and by its contractions is propelled forwards. The 

 ostia mark the limits of the eight chambers into which the 

 heart is sometimes said to be divided ; anteriorly it is continued 

 into a vessel, the so-called aorta, which passes as far forward 

 as the head and then suddenly ends with an open mouth. 

 The heart is lodged in a space, the pericardium, whose dorsal 

 wall is formed by the terga of the various segments, and the 

 ventral by a pericardium or pericardial membrane. The alary 



