24 ON T.HE AX ATOMY OF THE FLY. 



in the visceral cavity and bathes all the internal organs. A single 

 vessel runs the whole length of the back, commencing near the 

 apex of the abdomen and terminating in the head, called the 

 dorsal vessel. It is open at either extremity, and serves the 

 purpose of a heart pulsating rythmically, and pumping the 

 circulating fluid from the posterior extremity of the insect, 

 into the head, from which it returns amongst the visc- 

 era. The pulsation of this vessel may be observed in the fly 

 just after the insect emerges from the pupa case, before its 

 integuments become hardened and opaque ; it makes about 1 80 

 pulsations in a minute. 



The dorsal vessel being small in comparison to the cavity oi 

 the body, the return of the blood is necessarily slow. This gives 

 rise to a peculiar distribution of the viscera as well as to a very 

 important modification of the respiratory organs and function, 

 adapting the insect to pass a life of great vital activity, with a 

 comparatively slow circulation. 



Thus the salivary glands not only extend the whole length of 

 the thorax in complicated convolutions, but pass into the abdo- 

 men and only terminate at its posterior extremity. This brings 

 them into relation with the blood throughout a large part of the 

 circulation. So the ductless glands, called fat bodies or corps 

 graisseux, which seem to be analogous to the ductless glands of 

 animals, and to be concerned in the elaboration of the circulating 

 fluid itself, are found largely developed in the abdomen, and 

 also, though more sparingly, in the head. The analogues 

 of the kidneys, the rectal papillae, pulsating organs having a 

 central cavity alternately filled and emptied of blood by a 

 rhythmic muscular act, are placed near the posterior extremity 

 of the dorsal vessel, so that they excrete the efiete matter from 

 the circulation just before the return of the blood to the head 

 and thorax. The same may be said with regard to the dis- 

 position of the liver tubes, only to a less degree. 



