190 PHYSIOLOGY OF INSECTS. 



at the same time, cavities passing down each side of the 

 body convey another portion of the blood to its lower ex- 

 tremity. These are properly cavities or portions of the 

 great abdominal cavity, their boundaries not being clearly 

 definable. They communicate at each junction of the sec- 

 tions of the body with the great abdominal cavity, as a 

 portion of the blood they convey is discharged at these 

 points, and supplies the place of other portions received 

 through its valves into the great dorsal vessel. 



" The circulation is also strikingly and beautifully exhi- 

 bited in the tail. Here the ascending and descending 

 canals, like vein and artery, accompany each other ; and, 

 at the same instant that the blood is seen to pass up the 

 one, with the usual pulsatory motion, it descends in the 

 other in a similar manner. This is the more apparent, as 

 the sides of the canals are well denned, and each perfectly 

 distinct from the other. 



"Although the blood passes with the same pulsatory 

 motion through these minute canals as it does in other 

 parts of the body, yet no pulsation of either the ascending 

 or descending canals themselves can be detected. The 

 motion, therefore, seems to be entirely dependant on the 

 action of the great dorsal vessel, which evidently performs 

 in the insect, the same functions that the heart does in 

 vertebrated animals. 



" Upon fixing the insect so as to obtain a side view, the 

 great dorsal vessel presents a very interesting appearance. 

 It is seen continually and regularly oscillating backwards 

 and forwards, upwards and downwards, and at the same 

 time the main current of the blood in the great abdominal 

 cavity winds its way in all directions towards the hinder 

 extremity of the insect. Scarcely any larvae exhibit the 

 circulation of the blood in so beautiful a manner as the 

 one described, although there are few in which it is not 



