Mecoptera (from the Greek: mecos = "long", ptera = "wings") is an order of insects in the superorder Holometabola with about six hundred species in nine families worldwide.

Mecopterans are sometimes called scorpionflies after their largest family, Panorpidae, in which the males have enlarged genitals raised over the body that look similar to the stingers of scorpions, and long beaklike rostra.

They are somewhat fly-like in appearance, being small to medium-sized insects with long slender bodies and narrow membranous wings.

Most breed in moist environments such as leaf litter or moss, and the eggs may not hatch until the wet season arrives.

The larvae are caterpillar-like and mostly feed on vegetable matter, and the non-feeding pupae may pass through a diapause until weather conditions are favorable.

These were mainly wind-pollinated plants, but fossil mecopterans had siphon-feeding apparatus that could have fertilized these early gymnosperms by feeding on their nectar and pollen.

[20][21] The Aneuretopsychina were the most diverse group of mecopterans in the Latest Permian, taking the place of the Permochoristidae, to the Middle Triassic.

Two of the most important insect orders, Lepidoptera (butterflies and moths) and Diptera (true flies), along with Trichoptera (caddisflies), probably evolved from ancestors belonging to, or strictly related to, the Mecoptera.

Evidence includes anatomical and biochemical similarities as well as transitional fossils, such as Permotanyderus and Choristotanyderus, which lie between the Mecoptera and Diptera.

The two possible trees are shown below:[26] (a) Mecoptera (clades in boldface) is paraphyletic, containing Siphonaptera:[26][27] Diptera (true flies) Pistillifera (scorpionflies, hangingflies, 400 spp.)

(b) Mecoptera is monophyletic, sister to Siphonaptera:[26] Diptera (true flies) Pistillifera (scorpionflies, hangingflies, 400 spp.)

[28] Nannochoristidae[a] Boreidae (snow scorpionflies) Siphonaptera (fleas) Eomeropidae (mainly fossil (Triassic to present), 1 extant sp.)

[23][29] The fore and hind wings are similar in shape, being long and narrow, with numerous cross-veins, and somewhat resembling those of primitive insects such as mayflies.

The abdomen typically curves upwards in the male, superficially resembling the tail of a scorpion, the tip containing an enlarged structure called the genital bulb.

Scorpionflies, family Panorpidae, generally live in broad-leaf woodlands with plentiful damp leaf litter.

Snow scorpionflies, family Boreidae, appear in winter and are to be seen on snowfields and on moss; the larvae being able to jump like fleas.

They mostly breed among mosses, in leaf litter and other moist places, but their reproductive habits have been little studied, and at least one species, Nannochorista philpotti, has aquatic larvae.

[10] Most mecopterans live in moist environments; in hotter climates, the adults may therefore be active and visible only for short periods of the year.

[10] Hangingflies (Bittacidae) provide a nuptial meal in the form of a captured insect prey, such as a caterpillar, bug, or fly.

In species that live in hot conditions, the eggs may not hatch for several months, the larvae only emerging when the dry season has finished.

In areas where the family Panorpidae occurs, such as the eastern United States, these scorpionflies can be the first insects to arrive at a donated human cadaver, and remain on a corpse for one or two days.