Thus, another plausible etymology involves Hymen, the Ancient Greek god of marriage, as these insects have "married" wings in flight.

[citation needed] Molecular analysis finds that Hymenoptera is the earliest branching group of Holometabola.

[7] Hymenoptera (sawflies, wasps) Coleoptera (beetles) Strepsiptera (twisted-wing parasites) Raphidioptera (snakeflies) Megaloptera (alderflies and allies) Neuroptera (Lacewings and allies) Lepidoptera (butterflies, moths) Trichoptera (caddisflies) Diptera Mecoptera (scorpionflies) Siphonaptera (fleas) Hymenoptera originated in the Triassic, with the oldest fossils belonging to the family Xyelidae.

[10] In 2023, a molecular study[10] based on the analysis of ultra-conserved elements confirmed many previous findings and produced a relatively robust phylogeny of the whole Order.

The smaller species may have only two or three hamuli on each side, but the largest wasps may have a considerable number, keeping the wings gripped together especially tightly.

In some species, the ovipositor has become modified as a stinger, and the eggs are laid from the base of the structure, rather than from the tip, which is used only to inject venom.

With rare exceptions, larvae of the suborder Apocrita have no legs and are maggotlike in form, and are adapted to life in a protected environment.

In parasitic forms, the head is often greatly reduced and partially withdrawn into the prothorax (anterior part of the thorax).

Sense organs appear to be poorly developed, with no ocelli, very small or absent antennae, and toothlike, sicklelike, or spinelike mandibles.

They are also unable to defecate until they reach adulthood due to having an incomplete digestive tract (a blind sac), presumably to avoid contaminating their environment.

Because of this, cooperation among kindred females may be unusually advantageous and has been hypothesized to contribute to the multiple origins of eusociality within this order.

[21] Oocytes that undergo automixis with central fusion often have a reduced rate of crossover recombination, which helps to maintain heterozygosity and avoid inbreeding depression.

Species that display central fusion with reduced recombination include the ants Platythyrea punctata[19] and Wasmannia auropunctata[20] and the Cape honey bee Apis mellifera capensis.

[20] Single queen colonies of the narrow headed ant Formica exsecta illustrate the possible deleterious effects of increased homozygosity.

Stinging wasps are predators, and will provision their larvae with immobilised prey, while bees feed on nectar and pollen.

Habits intermediate between those of the herbivorous and parasitoid forms are shown in some hymenopterans, which inhabit the galls or nests of other insects, stealing their food, and eventually killing and eating the occupant.

The larvae are herbivorous, free-living, and eruciform, usually with three pairs of true legs, prolegs (on every segment, unlike Lepidoptera) and ocelli.

[4] Hymenoptera as a group are highly susceptible to habitat loss, which can lead to substantial decreases in species richness and have major ecological implications due to their pivotal role as plant pollinators.