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.
Abdomen consisting at the most of from ten to twelve segments. No functional abdominal legs except in the Thysanura, and in the larvæ of Lepidoptera. A pair of 1– or many-jointed cercopods on the tenth segment; and in certain forms a pair of styles on the ninth segment. In certain orders an ovipositor or sting formed of three pairs of styliform processes; in Collembola a single pair of processes forming the elater.
The genital openings opisthogoneate, usually single, but paired in Thysanura (Lepisma), Dermaptera, and Plectoptera (Ephemeridæ).
The digestive canal in the winged orders is highly differentiated, the fore-intestine being divided into an œsophagus and proventriculus, the hind-intestine into an ileum, colon, and rectum, with rectal glands.
The nervous system consists of a well-developed brain, in the more specialized orders highly complicated; no more than thirteen pairs of ganglia, which may be more or less fused in the more specialized orders. Three frontal ganglia, and a well-developed, sympathetic system present.
Stigmata confined (except possibly in Sminthurus) to the thorax and abdomen, not more than ten pairs in all, and usually but nine pairs. Tracheal system as a rule highly differentiated; invariably with tænidia.
Dorsal vessel with ostia and valvules; no arteries except the cephalic aorta; no veins. After birth there is in the more specialized pterygote orders a reduction in the number of terminal segments of the abdomen.
Development either direct (Synaptera), or with an incomplete (with nymph and winged or imaginal stages), or complete metamorphosis; in the latter case with a larval, pupal, and imago stage.
The insects may be divided into two sub-classes,—the Synaptera, and the winged orders, Pterygota, of Gegenbaur (1877), since the differences between the two groups appear on the whole to be of more than ordinal rank.