OF INSECTS. 
155 
was a secreting organ, although he did not determine 
the nature of the alleged secretion. Other opinions 
have been advanced on the subject ; but probably M. 
Leon Dufour is the only eminent entomotomist, of the 
present day, who denies the existence of any kind of 
circulation in insects.* 
Respiratory System . — The necessity for the blood 
being placed in communication with atmospheric air 
before it is adapted for assimilation with the organic 
mass, is as indispensable among insects as any other 
class of animals. W e accordingly find an intricate 
and highly developed system of vessels, pervading, in 
a multitude of ramifications, every portion of their 
frame, in a manner very similar to the distribution of 
the blood-vessels in quadrupeds. The relations, in- 
deed, between these two systems, as they subsist in 
the vertebrata, seem, as has been well remarked, to 
do completely reversed in the case of insects — in the 
former the blood is the moving and pervasive element 
— in the latter it is the air. The sanguineous fluid 
bathes almost every part of the cavity of the body ; 
and being too languid to repair, with sufficient rapi- 
dity, to a given point to receive its vital principle, it 
is provided that the latter should be conveyed towards 
it ; and this is done so effectually, that it can be im- 
parted, with equal facility, wherever there is a mole- 
cule of the blood to be decarbonised. 
The organs of respiration may be conveniently 
considered, as has been done by Kirby and Spence, 
* Lacord. Intro. II. 69. note (3.) 
