CIRCULATION IN INSECTS. 



245 



The discovery of the circulation in insects, and 

 of its varying energy at different periods of 



whose drawings these 6gures have been engraved, and to whom 

 I am indebted also for the description which follows : — 



The dorsal vessel of this insect is an elongated and gradually 

 tapering vessel, extending from the hinder part of the abdomen, 

 along the back, towards the head ; and furnished with valves, 



339 



which correspond very nearly in their situation to the incisions of 

 the body. During the changes of the insect from the larva to the 

 imago state, it undergoes a slight modification of form. In 

 every state it may be distinguished into two portions, a dorsal and 

 an aortal. The dorsal portion, which is the one in which a pulsa- 

 tion is chiefly observable, is furnished with distinct valves, is at- 

 tached along the dorsal part of the body by lateral muscles, and 

 has vessels which enter it laterally, pouring into it the circulating 

 fluid, which is returning from the sides and inferior portions of 

 the body. In the caterpillar, this portion of the dorsal vessel ex- 

 tends from the twelfth to the anterior part of the fifth segment. 

 It is furnished with eight double valves, which are formed, as 

 Mr. Bowerbank has correctly described them in the Ephemera 

 marginata ; namely, the upper valve " by a reflecting inwards 



