=Haase, Erich.= Die Abdominalanhänge der Insekten mit Berücksichtigung der Myriapoden. (Morph. Jahrbuch, xv, pp. 331–435, 2 Pls., 1889.) =Schmidt, Peter.= Beiträge zur Kenntnis der niederen Myriapoden. (Zeits. f. wissen. Zool., lix, pp. 436–510, 2 Pls., 1895.) INSECTA (HEXAPODA) We are now prepared to discuss the fundamental or essential characters of the insects, including the wingless subclass (Synaptera), and the winged (Pterygota). =Diagnostic characters of insects.=—_Body consisting of not more than twenty-one segments, which are usually heteronomous or of unequal size and shape, arranged in three usually well-defined regions_; i.e. _a head, thorax, and hind-body or abdomen. Head small and flattened or rounded, composed of not less than six segments, and bearing, besides the eyes, at least four pairs of jointed appendages_; i.e. _one pair of antennæ, and three pairs of masticatory appendages, the distal or molar portion of which is primarily divided into three divisions, supported on a stipes and cardo, and in certain orders modified into piercing or sucking structures. The head is composed of an epicranium, bearing a distinct clypeus and labrum, with the epipharynx. Mandibles 1–jointed, without a palpus and very generally with no, or uncertain, traces of a lacinia and a stipes. Two pairs of maxillæ; the first pair separate, usually 3–lobed, comprising a lacinia, galea, and palpifer, with a palpus which is never more than 6–jointed. The second pair united to form the labium or under lip, composed of two laciniæ fused together; in the generalized forms with a rudimentary galea; bearing a pair of palpi, never more than 4–jointed; with paraglossæ sometimes present._ (_A third pair of mouth-appendages situated between the antennæ and mandibles in the embryo of Anurida, and Apis, and adult Campodea._) _The epipharynx forming the roof of the mouth, and bearing gustatory organs. Hypopharynx usually well developed, lying on the under side of the mouth, just above the labium, and receiving the end of the salivary duct._ _Eyes of two kinds: a pair of compound, and from two to three simple eyes (ocelli)._ _The thorax consisting of three segments, the two latter segments in the winged orders highly differentiated into numerous tergal and lateral pieces and a single sternum; in the Synaptera the segments are undivided. (In the higher Hymenoptera the basal abdominal segment coalesced with the thorax.) Three pairs of legs, each foot ending in a pair of claws. Two pairs of wings (except in the Synaptera), a pair to each of the two hinder thoracic segments; the wings occasionally reduced or wanting in certain adaptive forms, which, however, had winged ancestors._