Page:Jardine Naturalist's library Entomology.djvu/156

150 usually straightest in larvæ, (particularly in caterpillars,) and in the imago it curves downwards between the thorax and abdomen, to enable it to pass through the narrow isthmus by which these parts are united. On close examination it is found that the coats of this vessel are composed of two membranes, the exterior of which is dense and muscular. Such at least appeared their composition to Strauss, but Mr. Newport detected a third membrane of an extremely thin and delicate texture.

The greater portion of the vessel contained in the abdomen, consists of a consecutive series of chambers or cells, separated from each other either by single or double valvules, which permit the transmission of the blood from behind forwards, or in the direction of the head, but prevent its retrograde motion. "A little behind these valvules, on both sides of each chamber, there is a transverse opening, likewise having a semilunar valvule internally, which allows the blood to enter the organ and opposes its exit. The number of the cells varies according to the species, or rather the families. M. Strauss found eight in the Melolontha vulgaris, M. Burmeister five in the larva of Calosoma Sycophanta, while M. J. Müller discovered only one in Phasma; but in this he was probably mistaken. However this may be, the organization in question explains at once the progress of the blood in the dorsal vessel, and the reason of it being always filled. For when the hinder cell, which is usually shorter than the others, and which we shall suppose full of blood, contracts, the fluid, pressed on all sides by the containing vessel, is forced