These are winged or secondarily apterous, terrestrial or aquatic insects. The adult stage is characterized by the following characters: one or two pairs of veined wings on the second or on the second and third thoracic somites; thoracic segments usually large; mesothorax and metathorax generally more or less united; pleural sclerites well-developed; abdomen with 8 to 11 distinguishable segments; pregenital segments without appendages, terminalia often modified in relation to habits of mating or oviposition; and spiracles with regulating apparatus provided with muscles (secondary lost in Diptera). Mating always takes place by infernal fertilization, either with the male intromitting sperm or with a spermatophore, directly into the female genital tract.
In insects, the development of wings was probably a single phyletic event. It was perhaps the most important general adaptive event in insect evolution, because it provided a means to escape from enemies and facilitated dispersal, opening up new environmental niches for the pterygotes to exploit. During evolution, all the wings may have been lost in one or both sexes, or one pair may have been modified to form shieldlike covers (Coleoptera), flight balancers (Diptera), or other structures with specialized functions.
There is no general agreement on the relationships between the three living lineages (Ephemeroptera (mayflies), Odonata (dragonflies and damselflies) and Neoptera) of the Pterygota (see Pterygota for different views on phylogenetic relationships). The Pterygota can be divided into two infraclasses: the Paleoptera (the most characterizing feature is that the fore and hind wings articulate to the thorax by humeral and axillary plates, which are fused with the veins and do not permit the wings to be folded against the body at rest) and the Neoptera (the most characterizing feature is that the wings articulate by discrete axillaries that enable them to be folded back along the body, except in secondarily specialized forms).
Formerly, the Pterygota were divided into the Exopterygota and the Endopterygota. The Exopterygotes constitute insects with a simple, incomplete (hemimetabolous) metamorphosis. There is usually no pupal instar, the wings develop externally, and the immature stages usually resemble the adults in structure and habits. The Endopterygotes are insects with a complete (homometabolous) metamorphosis, accompanied by a pupal instar. The wings develop internally, and the larvae differ from the adults in structure and habits.