_Excretory System._--Nitrogenous waste-matter is removed from the body by the Malpighian tubes which open into the food-canal, usually where the hind-gut joins the stomach. These tubes vary in number from four to over a hundred in different orders of insects. The cells which line them and also the cavities of the tubes contain urates, which are excreted from the blood in the surrounding body-cavity. This cavity contains an irregular mass of whitish tissue, the fat-body, consisting of fat-cells which undergo degradation and become more or less filled with urates. When the worn-out cells are broken down, the urates are carried dissolved in the blood to the Malpighian tubes for excretion. The fat-body is therefore the seat of important metabolic processes in the hexapod body. _Reproductive System._--All the Hexapoda are of separate sexes. The ovaries (fig. 13) in the female are paired, each ovary consisting of a variable number of tubes (one in the bristle-tail _Campodea_ and fifteen hundred in a queen termite) in which the eggs are developed. From each ovary an oviduct (fig. 13, od) leads, and in some of the more primitive insects (bristle-tails, earwigs, may-flies) the two oviducts open separately direct to the exterior. Usually they open into a median vagina, formed by an ectodermal inpushing and lined with chitin. The vagina usually opens in front of the eighth abdominal sternite. Behind it is situated a spermatheca (fig. 14, sp) and the ovipositor previously mentioned, with its three pairs of processes (Fig. 14, G, g). [Illustration: From Miall and Denny, _The Cockroach_, Lovell Reeve & Co. FIG. 13.--Ovaries of Cockroach, with Oviducts Od and Colleterial Glands CG.] [Illustration: From Miall and Denny, _The Cockroach_, Lovell Reeve & Co. FIG. 14.--Hinder Abdominal Segment and Ovipositor of Female Cockroach. Magnified. T^8 &c. Tergites. S^7, 7th Sternite. S^8, Sclerite between 7th and 8th sterna. S^9, 8th Sclerite. Od, Vagina. sp, Spermatheca. G, Anterior, and g, posterior gonapophyses.] The paired testes of the male consist of a variable number of seminal tubes, those of each testis opening into a _vas deferens_. In some bristle-tails and may-flies, the two _vasa deferentia_ open separately, but usually they lead into a sperm-reservoir, whence issues a median ejaculatory duet. The male opening is on the ninth abdominal segment, to which belong the processes that form the claspers or genital armature. Accessory glands are commonly present in connexion both with the male and the female reproductive organs. The poison-glands of the sting in wasps and bees are well-known examples of these. EMBRYOLOGY