INTERNAL ANATOMY OF INSECTS. 97 



vorous flies, this dorsal vessel, instead of the usual form 

 which it had in the larva, assumes a very peculiar ap- 

 pearance. If, taking one of these flies by the head and 

 wings and holding it up to the light, you survey under a 

 lens the base of the lower part of its abdomen, you will 

 see through its transparent skin, which exactly forms 

 such a window as physicians have sometimes wished for 

 in order to view the interior of their patients, a flask- 

 shaped vessel having its long end directed towards the 

 trunk, in which there is a manifest pulsation and trans- 

 mission of some fluid. This vessel extends in length 

 from the junction of the trunk with the abdomen to 

 about the termination of the second segment. The in- 

 cluded fluid does not run in the dorsal vessel in a regu- 

 lar course, but is propelled at intervals by drops, as if 

 from a syringe, first from the wide end towards the trunk, 

 and then in the contrary direction, forming a very in- 

 teresting and agreeable spectacle. One circumstance led 

 Reaumur to conjecture that the neck of this vessel, which 

 he at first regarded as simple, is in fact composed of two 

 or more approximated tubes, and that the blood is con- 

 veyed forward by the outward ones, and backward by the 

 intermediate one a : he even thinks that he saw a kind 

 of secondary heart, at the extremity next the trunk, for 

 the purpose of causing the reflux. This illustrious au- 

 thor observed the above remarkable structure not only 

 in the Syrphi 9 but in many of their affinities, and thinks 

 that it is also widely diffused amongst the Muscidte b . 



I must now say something upon what I conceive to be 

 the real blood of insects ; for I think no one will object 

 to that name being given to their nutritive fluid, especially 



a Reaumur iv. 264. " Ibid, 260 -. 



VOL. IV. H 



