OF INSECTS. 



151 



to escape, and finds no other passage but that made 

 for it in front by the yielding of the valvules which 

 separate it from the second cell. Into this, therefore, 

 it passes ; hut, at the same time, the preceding dilates, 

 and the blood contained in the intestinal cavity presses 

 against the lateral valvules, which yield and permit it 

 thus to enter by the openings which they protected. 

 The same process is repeated by the second cell, then 

 by the third, and so on ; the blood thus traverses 

 them all by regular jerks, without any of them being 

 ever left completely empty/'* 



This process will be 

 better understood from 

 an inspection of the an- 

 nexed figures, after a 

 drawing by Mr. Bower- 

 bank, with his accom- 

 panying explanation, t 

 Fig. 1st, a, a, represents 

 two chambers of the 



dorsal vessel in their 



greatest state of col- c ~ 



lapse, when the point 



of the lower valve is 



seen closely compressed 



within the upper one. 



At the commencement 



of the expansion, the 



blood is seen flowing ia 



from the lateral aper- 



* Lacord. Intro. II. 72. f See Entomol. Mag. I. PI. II. 240. 



