OF INSECTS. 
149 
This leads to the consideration of the second great step 
in the process of nutrition, or what has been called 
The Circulating System . Scarcely any point in 
the anatomy or physiology of insects has excited 
so much interest and attention as the movements of 
the nutritive fluid, and the nature of the organ by 
which its motions are produced. The most opposite 
opinions on the subject have been maintained by 
different observers, and it is only of late that evi- 
dence has been obtained of a sufficiently conclusive 
nature to establish the fact, that there is a trans- 
lation of the blood, which virtually amounts to a 
kind of circulation, although it is very imperfect when 
compared with that of vertebral animals. 
The organ which gives the impulse to this circu- 
latory movement is named the dorsal vessel. It ex- 
tends along the back from the head to the anus, 
lying only a short wav beneath the integument, and 
consequently above the digestive canal, from which 
it is separated by a layer of fatty matter, (See Plate 
II. flg. 1. a 3 a 3 a.) When examined in a living 
insect, (it is best seen in a larva with a smooth 
transparent skin,) it is found to have a regular ex- 
pansive and contractive motion, by means of which 
a fluctuating movement is communicated to the con- 
tained fluid. The "whole organ, therefore, somewhat 
resembles an artery, although it is in fact the repre- 
sentative of the heart in this class of animals, and 
is frequently called by that name. In its general 
shape it is commonly more or less fusiform, widest in 
the abdomen, and diminishing towards the head. As 
it necessarily follows the contour of the body, it is 
