The Hemiptera are characterized particularly by their highly specialized sucking mouth-parts, no other of the sucking insects having the proboscis composed in the same manner. The palpi of both maxillæ and labium are wholly wanting in Hemiptera and the flexible needle-like maxillæ and mandibles are enclosed in the tubular labium. This order is a large one and includes many well-known injurious species, as the chinch-bug (_Blissus leucopterus_), which occurs in immense numbers in the grain-fields of the Mississippi valley, sucking the juices from the leaves of corn and wheat, the grape Phylloxera (_Phylloxera vastatrix_), so destructive to the vines of Europe and California, the scale insects (_Coccidæ_) (figs. 62 and 63), the worst insect pests of oranges, the squash-bugs and cabbage-bug and a host of others. Some of the Hemiptera, for example, the lice and bed-bugs, are predaceous, sucking the blood of other animals. [Illustration: FIG. 64.--A water-strider, _Hygrotrechus_ sp. (From Jenkins and Kellogg.)] The water-striders (fig. 64) catch other insects, both those that live in the water and those which fall on to its surface, and holding the prey with their seizing fore legs they pierce its body with their sharp beak and suck its blood. They lay their eggs in the spring glued fast to water-plants. The young water-striders are shorter and stouter in shape than the adults. [Illustration: FIG. 65.--A water-boatman, _Corisa_ sp. (From Jenkins and Kellogg.)] The water-boatmen (fig. 65) and back-swimmers swim and dive about in the water, coming more or less frequently to the surface to get a supply of air. This air they hold under the wings, or on the sides and under part of the body entangled in the fine hairs on the surface. The insects appear to have silvery spots on the body, due to the presence of this air. The "rowing" legs of the water-boatmen (_Corisa_) are the hindmost pair; in the back-swimmers (_Notonecta_) they are the middle legs. The cicadas (fig. 66) are the familiar insects of summer which sing so shrilly from the trees, the seventeen-year cicada (_Cicada septendecim_) (oftentimes called locust) being the best known of this family. Its eggs are laid in slits cut by the female in live twigs. The young, which hatch in about six weeks, do not feed on the green foliage, but fall to the ground, burrow down to the roots of the tree and there live, sucking the juices from the roots, for sixteen years and ten or eleven months. When about to become adult, the young cicada crawls up out of the ground and clinging to the tree-trunk molts for the last time, and flies to the tree-tops. [Illustration: FIG. 66.--The seventeen-year cicada, _Cicada septendecim_; the specimen at left showing sound-making organ, _v. p._, ventral plate; _t_, tympanum. (From specimen.)]