Mosquitoes and other insects do not have any nostrils nor do they breathe through any openings on the head. Along the sides of the thorax and abdomen is a series of very minute openings known as the spiracles. Through these the air passes into a system of air-tubes, the tracheæ. There are two main trunks or divisions of the tracheæ just inside the body-wall and a number of shorter connecting trunks. From these larger vessels arise a great number of smaller ones which branch and subdivide again and again until all the tissues are supplied by these minute little air-tubes that carry the oxygen to all parts of the body and carry off the waste carbon dioxid. These air-tubes are emptied and filled by the contractions of the walls of the abdomen. When the body-wall contracts the air is forced out of the thin-walled trachea through the spiracles; when the pressure is removed they are refilled by the fresh air rushing in. THE BLOOD After a mosquito has been feeding on a man or some other animal it is often so distended that the blood shows rich and red through the thin sides of the walls of the abdomen. This, however, is the blood of the victim and not of the mosquito. The blood of insects is not red but pale yellowish or greenish. It is not confined in definite vessels, but fills all the space inside the body cavity that is not occupied by some of the tissues or organs. It bathes the walls of the alimentary canal and gathers there the nourishment which it carries to all parts of the body. It does not carry oxygen or collect the carbon dioxid as does the blood of higher animals. That work, as we have just seen, is done by the air-tubes. Above the alimentary canal, extending almost the whole length of the abdomen and thorax, is a thin-walled pulsating vessel, the heart. This consists of a series of chambers each communicating with the one in front of it by an opening which is guarded by a valve. When one of these chambers contracts it forces the blood that is in it forward into the next chamber which, in its turn, sends it on. As the walls relax the valves at the sides are opened and the blood that is in the body-cavity rushes in to fill the empty chamber. As these regular rythmical pulsations recur the blood is forced forward through the heart into the head where it bathes the organs there. We shall see in another chapter that the malarial parasite escapes from the walls of the stomach of the mosquito into the blood in the body-cavity and finally reaches the salivary glands. As the heart is constantly driving blood to this part of the body the parasites readily reach the glands from which they finally escape into the new host. [Illustration: FIG. 72--Heads of Culicinæ mosquitoes; _a_, male; _b_, female. (After Manson.)] [Illustration: FIG. 73--Heads of Anophelinæ mosquitoes; _c_, male; _d_, female. (After Manson.)] [Illustration: FIG. 74--Wing of _Anopheles maculipennis_.] [Illustration: FIG. 75--Wing of _Theobaldia incidens_.]