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Insects. A class of air-breathing invertebrate animals, in which the body is divided into a variable number of segments, which usually become modified to form three distinct regions, known as the head, the thorax, and the abdomen. The total number of segments never exceeds twenty. Of these, five, and probably six, become completely united to form the head. On the front of the head between the eyes, or in front of them, is a pair of jointed organs called antennae. The mouth parts consist of an upper lip or labrum, an under lip or labium, and between them two pairs of jaws opening sidewise. The upper jaws are called mandibles, the lower curved pair are called the maxillae. There may be also inside the mouth an organ resembling a tongue. There is generally a pair of compound eyes on the sides of the head, and sometimes simple eyes, or ocelli, also. The thorax always consists of three segments, which are termed respectively the pro-thorax, the meso-thorax, and the meta-thorax. Each of these carries a pair of jointed legs, and the possession of these six legs is characteristic of the whole class of insects. In the adult state there are two (sometimes one) pairs of wings which are attached to the meta-thorax or meso-thorax or to both. The remaining segments constitute the abdomen; they have no appendages except in the final segment, which in the female is sometimes prolonged to form an ovipositor. The organs of the mouth in insects are of two principal types, viz., masticatory (beetles, dragon flies, ants, etc.), and suctorial (butterflies, moths, fleas, gnats, ,etc.). The digestive apparatus, or alimentary canal, usually consists of an esophagus, a crop, a gizzard, a stomach, a small intestine, a large intestine, and a rectum, together with organs playing the part of salivary glands, liver, and kidneys. There is no definite and regular course of circulation in insects. The heart is represented by a contractile tube, situated on the back, and termed the dorsal vessel. Respiration is effected by means of branching air tubes, or tracheae, which ramify through the entire body, and open on the exterior by lateral apertures, known as stigmata, or spiracles. The nervous system consists of a ganglion above the mouth known as the brain, and a chain of ganglia placed on the ventral interior, and connected by a series. of double cords. The sexes of insects are in different individuals, and most are oviparous. Most insects in the course of their lives pass through a series of changes, which constitute the metamorphosis, before attaining maturity.