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, but in many of their affinities, and thinks 
that it is also widely diffused amongst the Muscidce 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 
" Reaumur iv. 264. b Ibid. 260 -. 
VOL. IV. H 
