Insects were the first organisms on Earth to evolve active flight. It is likely that the ability to fly arose only once in insects, over 300 million years ago in the Carboniferous period. About 70 million years later active flight arose within vertebrates (pterosaurs), followed by separate origins in birds and bats.
Winged insects, or pterygotes, have radiated into over 100 times more species than all of the winged vertebrate lineages combined. They include all of the commonly encountered insects; the most common non-pterygote insect lineage are the Thysanura (silverfish and firebrats).
Many groups of insects have subsequently lost the ability to fly. Some of these (lice and fleas, for example) have lost all remnants of wings. It is only through their evident relationship with particular groups of winged insects (lice are closely related to psocids [book lice]; fleas are related to flies) that it is clear that their ancestors had wings.
Paleopterous and neopterous insects
Two groups of pterygotes evolved the ability to fold their wings back flat against their abdomens. The largest of these is the clade Neoptera ("new wing"). Remaining insects (all of the taxa in the above tree except for Neoptera) are sometimes referred to as Paleoptera ("old wing"), because they lack the sophisticated wing-folding mechanism of neopterous insects. Members of paleopterous insects cannot fold their wings back over their abdomens, with the exception of the extinct order Diaphonopterodea, which could fold their wings using a mechanism different than Neoptera. (Some neopterous insects, such as butterflies, can no longer fold their wings against their abdomen, but this clearly represents a secondary loss.)
The primary derived characteristic of pterygotes is the presence of veined wings on the second (meso-) and third (meta-) thoracic segment.
Evolution and Systematics
Discussion of Phylogenetic Relationships
While the relationships of the extinct paleodictyopteroid orders (Diaphanopterodea, Paleodictyoptera, Megasecoptera, and Permothemistida) to each other and to other orders have not been extensively examined, the relationships of the three living lineages (Ephemeroptera, Odonata, and Neoptera) to each other have been the subject of much debate. Each of the three possible relationships are supported by various authors.
A monophyletic Paleoptera was advocated by Hennig (1969, 1981) and Hovmöller et al. (2002):
=== Odonata ==Paleoptera=| =====| === Ephemeroptera | ================ Neoptera
Boudreaux (1979) favored monophyly of Ephemeroptera + Neoptera:
====== Odonata | =====| === Ephemeroptera ===| === Neoptera
Kristensen (1975, 1981, 1991) and Wheeler et al. (2001) present evidence in support of monophyly of Odonata + Neoptera:
====== Ephemeroptera | =====| === Odonata ===| === Neoptera
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Pterygota has a pantropical distribution.
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