90 INTERNAL ANATOMY OF INSECTS. 
ture a . But, convincing as they seem, they appear to 
have been founded in error, and on the idea that a circula- 
tion, as well as a heart, necessarily implies a vascular 
* Marcel de Serres, in his Observations on the Dorsal Vessel of 
Insects l , endeavours to prove that the principal use of that vessel 
is the more perfect animalization of the chyle that, transuding through 
the pores of the intestinal canal, is imbibed by it. In insects, he ob- 
serves, that undergo metamorphoses, in which the growth or develop- 
ment of parts is often very rapid, it is requisite that a considerable 
portion of the chyle should be in reserve for this purpose. On this 
account it is that the Epiploon or adipose tissue is so abundant in 
larva? to what it is in the perfect insect. That the importance also of 
this part to insects is proved by the circumstance, that all their in- 
terior parts communicate by fibrils with this tissue, and that proba- 
bly their various organs derive the nutriment from it by their means. 
He then asks by which of the viscera is the fat elaborated, or by what 
means does the chyle which transudes from the intestinal canal pass 
to the state of fat ? Facts seem to indicate, says he, that the func- 
tion of the dorsal vessel is to pump up the chyle, and to cause it 
then to transude through the meshes of the adipose tissue, where it 
finishes by elaborating that mass of fat so abundant in larva? and 
certain perfect insects, which are thus enabled to sustain the effects 
of a long fast. So that this vessel is only a secretory organ, analo- 
gous to so many others that exist in insects ; but the secretion which 
it has to produce is the most important of all, since the support of 
the vital powers depends upon it : it is, in effect, that vessel which 
completes the function of animalization, and which itself prepares the 
nutritive fluid 3 . He observes, amongst other reasons he brings to 
support his theory, that the colour of the fluid which it contains is 
always analogous to that of the adipose tissue that surrounds it, and 
that the colour of that tissue never changes without that of the fluid 
undergoing a corresponding alteration, — that when, as in many per- 
fect insects, the quantity of fat diminishes, the dorsal vessel also di- 
minishes in size, and that the same reagents which coagulate the fat, 
coagulate equally the fluid in the dorsal vessel, which seems to indi- 
cate an identity between them 3 . 
But there are circumstances that militate against this hypothesis. 
The analysis which Lyonet has given of the fluid contained in the 
dorsal vessel of the Cossus 4 , seems to prove that it is more analogous 
' Mem. du Mas. 1810. s Ibid. 68-. 
3 Ibid. 69—. 4 See above, p. 85. 
