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 way beneath the integument, and 

 consequently above the digestive canal, from which 

 it is separated by a layer of fatty matter, (See Plate 

 II. fig. 1. a,j a, a.) When examined in a living 

 aisect, (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 m 

 the abdomen, and diminishing towards the head. As 

 it necessarily follows the contour of the body, it is 



