The silk glands or vessels consist of two long thick-walled sacs running along the sides of the body, which open by a common orifice—the spinneret or seripositor—on the under lip of the larva. As the larva approaches maturity these vessels become gorged with a clear viscous fluid, which, upon being exposed to the air immediately hardens to a solid mass. Advantage is taken of this peculiarity to prepare from fully developed larvae silk worm gut used for casting lines in rod-fishing, and for numerous other purposes where lightness, tenacity, flexibility and strength are essential. The larvae are killed and hardened by steeping some hours in strong acetic acid ; the silk glands are then separated from the bodies, and the viscous fluid drawn out to the condition of a fine uniform line, which is stretched between pins at the ex tremity of a board. The board is then exposed to the sunlight till the lines dry and harden into the condition of gut. The prepara tion of gut is, however, merely an unimportant collateral manu facture. When the larva is fully mature, and ready to change into the pupa condition, it proceeds to spin its cocoon, in which oper ation it ejects from both glands simultaneously a continuous and reelable thread of Boo to 1,2o0yd. in length, moving its head around in regular order continuously for three days or thereabouts. The thread so ejected forms the cocoon, and consists of filaments seriposited from two separate glands. Two other glands, named after their discoverer the glands of Filippi, secrete a glutinous or resinous substance, which is believed to serve the double purpose of helping the thin viscous threads through the spinneret and causing the two filaments to adhere when brought into contact with the atmosphere.
Under the microscope cocoon silk presents the appearance of a somewhat flattened combination of two filaments placed side by side, being on an average ,,zoo in. in thickness. The cocoons are white or yellow in colour, oviform in shape, and often with a con striction in the middle. According to race, etc., they vary con siderably in size and weight, but on an average they measure from an inch to an inch and a half in length, and from half an inch to an inch in diameter. They form hard, firm and compact shells with some straggling flossy filaments on the exterior, and the interior layers are so closely and densely agglutinated as to constitute a parchment-like mass which resists all attempts at unwinding. The whole cocoon with its enclosed pupa weighs from 15 grains for the smaller races to about 5o grains for the breeds which spin large cocoons. From io to days after the completion Of the co coon the enclosed insect is ready to escape ; it moistens one end of its self-made prison, thereby enabling itself to push aside the fibres and make an opening by which the perfect moth comes forth. The sexes almost immediately couple ; the female in from four to six days lays her eggs, numbering 500 and upwards; and, with that the life cycle of the moth being complete, both sexes soon die.