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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.

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The insects differ from other arthropods in that the body is divided into three distinct regions,—the head, thorax, and abdomen, the latter regions in certain generalized forms not always very distinctly differentiated. The body behind the head may also conveniently be called the trunk, and the segments composing it the trunk-segments.