Page:Jardine Naturalist's library Entomology.djvu/172

166 Physiologists maintain that the fatty mass is analogous to the liver; but its structure, alternate increase and decrease, and the presence elsewhere of what may, in certain cases, be considered as true biliary vessels, appear sufficient reasons for rejecting this opinion. Certain secretions having a direct influence on digestion have already claimed our notice while considering that function; but there are others, which may be regarded as the produce of digestion, since extracted from the blood, to which we have yet to refer; and their importance will be judged of when it is mentioned, that it is almost entirely from them that we derive all the insect products which we have converted to our own use. Of these Silk may well be regarded as the most valuable, since it has become nearly as essential to our own purposes, as it is to the economy of the animals which produce it. The vessels which secrete it resemble the biliary vessels in shape, but are usually much larger, (in the silk-worm they are about a foot in length,) consisting of two tubes, which unite at the extremity, and open into a small perforated filiform organ, commonly placed between the palpi on the under lip. This is named the spinneret, and the size of its aperture determines the thickness of the thread. The fluid, before it comes in contact with the air, is viscous and transparent in young larvæ, but thick and opaque in mature ones. It is found by chemical analysis to be chiefly composed of a gummy matter, a small portion of another substance resembling wax,