The middle part of the body, called the thorax, is really a strong box with heavy walls for the attachment of the powerful wing and leg muscles. The three pairs of legs are covered with hairs and scales, and their tips are provided with a pair of claws which vary somewhat in the different species. The wings (Fig. 68) are long and narrow with a characteristic venation. Along the veins and the margin of the wings are the scales which readily enable one to distinguish mosquitoes from other insects that may look much like them. In some species these scales are long and narrow, almost hair-like, in others they are quite broad and flat (Fig. 69). Just back of the wings is a pair of balancers, short thread-like processes knobbed at the end. These probably represent the second pair of wings with which most insects are provided, and seem to serve as balancers or orienting organs when the insect is flying. On the sides of the thorax are two small slit-like openings, the breathing-pores. These are the openings into the tracheal or respiratory system. THE ABDOMEN The long cylindrical abdomen is composed of eight segments. These are rather strongly chitinized above and below, but a narrow strip along the side is unchitinized. In this strip are situated the abdominal breathing-pores. The tip of the abdomen is furnished with a pair of movable organs, which in the male are variously modified and serve as clasping organs at mating time. THE DIGESTIVE SYSTEM The mouth-parts of the mosquito have just been described. It will be remembered that the labrum is provided with a groove. Through this the blood or other food is sucked up by means of a strong-walled pumping organ, the pharynx, situated in the head (Fig. 70). Just back of the pharynx is the esophagus which leads to the beginning of the stomach. Close to its posterior end the esophagus gives off three food reservoirs, two above and a single larger one below. In dissections these will often be seen to be filled with minute bubbles. The stomach reaches from the middle of the thorax to beyond the middle of the abdomen. At its posterior end are given off five long slender processes, the Malpighian tubules which are organs of excretion, acting like the kidneys of higher animals. The hindgut is that portion of the intestine from the stomach to the end of the body. [Illustration: FIG. 68--Wing of Mosquito (_O. lativittatus_).] [Illustration: FIG. 69--End of mosquito wing highly magnified to show the scales on the veins.] [Illustration: FIG. 70--Diagram to show the alimentary canal and salivary glands of a mosquito.] [Illustration: FIG. 71--Salivary glands of _Culex_ at right. _Anopheles_ at left. (After Christophers.)] THE SALIVARY GLANDS