Insects respire neither by means of lungs nor of gills, and the blood is not brought to a particular part of the body for nitration, as by circulation in vertebrate and many inver tebrate animals, but the air which enters by the breathing-pores is conveyed by tubes to all parts of the body, and even through the delicate structure of the wings, so that the whole frame is rendered more light by the very means employed to maintain and increase muscular energy. Respiration is extremely active in insects; they consume a great quantity of oxygen in proportion to their size, and they display, in general, an extra ordinary degree of activity and muscular energy. The flight of very many kinds is far more rapid in proportion to their size than that of birds; others display a similar superi ority of powers in running, swimming, or digging and burrowing; whilst the leaping of ninny, as fleas and grasshoppers, and the springing of others, as cheesehoppers, prodigi ously exceeds anything of which any vertebrate animal is capable. The respiration of aquatic insects takes place in the same manner as that of other insects, and they come to the surface of the water for fresh supplies of air.
The blood of insects is thin and colorless. It is not everywhere inclosed in vessels, butis freely diffused in interstices between the muscles and other organs. and in the visceral cavity. It contains globules or corpuscles of determinate shape. How far the dorsal vessel (see AnTICITLATA) should be regarded as a heart, is not fully determined ;Iut by its contractions and dilatations, a constant motion of the blood is maintained.
The members of insects have generally a structure analogous to that of ;he trunk, in being composed of articulations, the hard and solid part of which is the external cover ing. This appears very perfectly in the legs, the antenna;, and the palpi, but not in the wings.
The legs of insects consist of two principal parts, the thigh (femur) and shank (tibia), with two smaller articulations, the coxa and troehanter, interposed between the body and the thigh, and at the extremity of the shank, a set of three, four, or five small articula tions, called the tarsus. The last segment of the tarsus in terrestrial insects is generally terminated by a pair of hooks or little claws. and many dipterous insects, as the house fly (q.v.), have disks and stickers for taking hold of smooth surfaces.
The wings of insects are often very large in proportion to the size of the body, and the rings of the thorax are soldered together. and supported by supplementary pieces, to give firm support to them, and to the powerful muscles necessary for their action. The hard covering of the body of an insect consists, like the skin of vertebrate animals, of three layers, and the membranes of the wings are filmy expansions of the outermost of these, the epidermis. The ribs or nervures in the wings of insects are tubes, of which
one of the uses is the conveying of air even to the extremities of the wings. The forms of the wings are very various; some of the more important diversities being character istic of different orders. The bodies of insects are often very much covered with hairs, which are often very long and thick in proportion to the size of the animal, and on the wings of butterflies and other lepidoptera are flattened and expanded so as to form scales (see Burrmtimy) often richly colored, and also, by reason of very fine parallel strive, with which they are marked, displaying an admirable iridescence or reflection of evanescent prismatic colors in changing light. The first pair of wings in colcopterous insects tr beetles is represented by a pair of hard chitinous elytra (Gr. coverings), or wing-covers. Orthopterous insects have softer leathery or parchment-like elytra.
Insects feed on very different kinds of food; some prey on other insects, sonic devour animal, and some vegetable substances, some suck the juices of animals, some the juices of plants or the honey of their flowers. The structure of the mouth varies accordingly, and the digestive organs also vary. The mouth is either adapted for gnawing, cutting, and tearing, or merely for sucking, or it is adapted partially for both of these purposes. The parts of a mandibulate mouth are an upper lip (labrum) and an under lip (labium). moving vertically; and an upper pair of jaws or mandibles (mandibula) and a lower pair of jaws (maxidce), moving horizontally. The upper and under lip meet when the mouth is shut. Both are as hard as the jaws. The lower lip is sometimes regarded as consist ing of two parts, called the chin (mentam), and the tongue (lingua), which is more mem braneous and fleshy, and reposes on the inside of the chin. The upper jaws or mandibles are usually powerful, and often strongly toothed and hooked, sometimes furnished with cutting edges like sharp scissors, and sometimes adapted for bruising and grinding. They are also the instruments which bees and other insects use for their wonderful operations of cutting, tearing, building, plastering, etc. The lower jaws or maxillm are gen erally less powerful. In some insects, in which the mandibles are enlarged into great organs of prehension, the maxillm alone serve for the ordinary use of jaws in eating. To the maxillm and the lower lip are attached organs called palpi or feelers, consisting of a number of minute articulations, supposed to be delicate organs of touch connected with the purposes of the mouth, and distinguished as maxillary palpi and labial palpi.