Page:Jardine Naturalist's library Entomology.djvu/155

Rh 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 evidence has been obtained of a sufficiently conclusive nature to establish the fact, that there is a translation 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 circulatory movement is named the dorsal vessel. It extends 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, a, 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 expansive and contractive motion, by means of which a fluctuating movement is communicated to the contained fluid. The whole organ, therefore, somewhat resembles an artery, although it is in fact the representative 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