Hymenoptera

Hymenoptera is a major insect order that includes sawflies, wasps, bees, and ants. More than 150,000 living species have been described, along with over 2,000 extinct ones. Many of these insects are parasitic. Females usually possess a specialized ovipositor used to deposit eggs into hosts or hard-to-reach places; this structure is often adapted into a stinger. Young undergo complete metamorphosis (holometabolism), passing through a wormlike larval stage and an inactive pupal stage before becoming adults.

The name Hymenoptera comes from the Ancient Greek words for "membrane" and "wing." Molecular evidence shows that Hymenoptera is the earliest branching group within Holometabola. The order originated in the Triassic period, with the oldest fossils belonging to the family Xyelidae. Social hymenopterans appeared during the Cretaceous. The group’s evolution has been extensively studied by researchers such as Alex Rasnitsyn and Michael S. Engel. Since 2000, phylogenetic relationships within Hymenoptera have been intensively investigated using both morphology and molecular data. A 2023 molecular study using ultra-conserved elements confirmed many earlier findings and produced a relatively robust phylogeny for the entire order, though relationships among Symphyta lineages remain an active area of research.

Hymenopterans range from very small to large insects. Their mouthparts are adapted for chewing, with well-developed mandibles (ectognathous mouthparts). Many species have evolved a long proboscis for drinking liquids like nectar. Most have two pairs of wings, though some solitary wasps and worker ants lack them. They typically have large compound eyes and three simple eyes (ocelli). The hind wing’s forward edge has hooked bristles called hamuli that lock onto the fore wing, keeping the wings together. Smaller species may have only two or three hamuli per side, while larger wasps can have many, ensuring a tight grip. Hymenopteran wings have relatively few veins compared to many other insects, especially in smaller species.

In more ancestral hymenopterans, the ovipositor is blade-like and used for slicing plant tissues. In most species, however, it is modified for piercing, sometimes reaching several times the body length. In some, the ovipositor has become a stinger, with eggs laid from the base rather than the tip, which is used only to inject venom. The sting typically immobilizes prey but may also serve in defense for some wasps and bees.

Hymenopteran larvae generally have a distinct head, three thoracic segments, and usually nine or ten abdominal segments. In the suborder Symphyta, the eruciform larvae resemble caterpillars and typically feed on leaves. They have large chewing mandibles, three pairs of thoracic legs, and usually six or eight abdominal prolegs. Unlike caterpillars, their prolegs lack grasping spines, and their antennae are reduced to stubs. Symphytan larvae that are wood or stem borers have no abdominal legs and smaller thoracic legs. With rare exceptions, larvae of the suborder Apocrita are legless and maggot-like, adapted to a protected environment—either inside a host organism or within a nest cell cared for by adults. In parasitic forms, the head is often greatly reduced and partially withdrawn into the prothorax; sense organs are poorly developed, with no ocelli, very small or absent antennae, and toothlike, sicklelike, or spinelike mandibles. These larvae cannot defecate until adulthood because their digestive tract is a blind sac, likely to avoid contaminating their surroundings. Larvae of stinging forms (Aculeata) usually have ten pairs of spiracles, while parasitic forms typically have nine.

In most or all hymenopterans, sex is determined by chromosome number. Fertilized eggs, with two sets of chromosomes, develop into diploid females; unfertilized eggs, with one set, become haploid males. The female controls fertilization voluntarily, allowing her to determine the sex of her offspring—a phenomenon called haplodiploidy. The genetic mechanism can be more complex: in many species, sex is determined by a single gene locus with many alleles. Haploids are male, and diploids heterozygous at that locus are female. However, a diploid homozygous at the sex locus develops as a male instead. This often occurs when parents are siblings or close relatives. Diploid males, produced by inbreeding, are known in many ant, bee, and wasp species; they are usually sterile, though a few species have fertile diploid males. A consequence of haplodiploidy is that females share more genes with their sisters than with their daughters, making cooperation among related females unusually advantageous. This is thought to have contributed to the multiple origins of eusociality within the order. In many colonies of bees, ants, and wasps, worker females will remove eggs laid by other workers.