This is a large and diverse order of endopterygote insects, including sawflies, horntails, wasps, bees, ants and relatives. Most species carry well developed antennae and two pairs of membranous wings (the second pair is small), which are frequently connected by hooks (hamuli) on the hind wing. The venation of the wings is often greatly reduced. Mandibles are used primarily for biting, but also serve for lapping or nectar sucking (as in bumble bees). The antennae are well-developed. The abdomen is usually basally constricted, and its first segment is fused with the metathorax. There is a marked constriction between segments 1 and 2 of the abdomen in all but the most primitive species and a few secondarily specialized ones. In addition to egg-laying females may use their ovipositor for sawing (sawflies), piercing, or stinging.
Metamorphosis is complete and endopterygote. The typical larva has a well-developed head, and 3 thoracic and usually 9 or 10 abdominal segments. Among the Symphyta, the head is strongly sclerotized and there are powerful biting parts; the larva are eruciform. Among the Apocrita, the larvae are apodous, maggot-like in form. A cocoon is generally present.
There are various gradations in offspring care, from simple oviposition to progressive provisioning of larvae. Bees, ants and wasps are often eusocial.
Two suborders are recognized: the primitive, relatively unobtrusive, phytophagous Symphyta (= Chalastogastra) and the much more specialized and abundant Apocrita (= Clistogastra), with numerous adaptations to predation, parasitism, exploitation of nectar and pollen resources of the flowering plants and sophisticated, organized forms of social life. The order includes over 130.000 described species. There is no order of insects as beneficial to mankind as this one, with the exception of a few species considered to be a pest.