Hexapoda

Hexapoda, meaning "six legs" in Greek, is a subphylum of arthropods that holds the title of the largest group within the phylum, containing the vast majority of living arthropod species. This group includes the crown class Insecta—what we call true insects—alongside the much smaller class Entognatha. Entognatha itself is made up of three wingless groups that were once lumped in with insects: Collembola (springtails), Protura (coneheads), and Diplura (two-pronged bristletails). Among these, insects and springtails are especially numerous and play key roles on land as pollinators, primary consumers, scavengers, detritivores, and tiny predators.

The name hexapod comes from their most obvious trait: a body split into three parts, with a consolidated thorax that carries three pairs of legs. Most other arthropods have more than three leg pairs. Recent research generally places Hexapoda as a subgroup within Pancrustacea.

In terms of body shape, hexapods range from 0.5 mm to over 300 mm long, with a clear division into a head, thorax, and abdomen. The head starts with a presegmental acron that usually has eyes—though Protura and Diplura lack them—followed by six tightly fused segments. Their appendages break down like this: segment I has none; segment II has antennae for sensing (missing in Protura); segment III has none; segment IV has mandibles for crushing; segment V has maxillae for chewing; and segment VI has a labium, or lower lip. The mouth sits between segments four and five, covered by a projection from segment six called the labrum, or upper lip. In true insects, the mouthparts are exposed (ectognathous), while in other hexapods they are tucked away (endognathous). Similar head appendages show up in Myriapoda and Crustacea, though crustaceans have an extra set of antennae.

Collembola and Diplura have segmented antennae, with each segment having its own muscles. Insect antennae, by contrast, have only three segments: the scape, pedicel, and flagellum. Muscles are limited to the first two; the flagellum is muscle-free and made up of many annuli, giving it the name "annulated antenna." Johnston's organ, located on the pedicel, is absent in Entognatha.

The thorax consists of three segments, each bearing one pair of legs. Like most land-adapted arthropods, each leg has a single walking branch made of five segments, without the gill branches seen in some other arthropods. In most insects, the second and third thoracic segments also support wings. These wings might be related to the gill branches of crustaceans, or they could have evolved from outgrowths of the segments themselves.

The abdomen develops in an epimorphic pattern—meaning all segments are present by the end of embryonic development—in all hexapod groups except Protura. Protura follow an anamorphic pattern, where juveniles hatch with fewer segments and add more with each molt until they reach the adult count. True insects have eleven abdominal segments (often reduced in many species), while Protura have twelve and Collembola have only six (sometimes just four). Abdominal appendages are very reduced, limited to external genitalia and, in some cases, a pair of sensory cerci on the last segment.

Historically, myriapods were thought to be hexapods' closest relatives, based on looks, and both were placed in a group called Uniramia or Atelocerata. But early in the 21st century, that idea was challenged, and it now seems crustaceans may be the hexapods' nearest kin. The non-insect hexapods have been treated either as a single evolutionary line (Class Entognatha) or as several separate lines with different ties to Insecta. Diplura, for instance, might be closer to insects than to springtails.

A 2002 molecular study suggests hexapods split from their sister group, the Anostraca (fairy shrimps), around the start of the Silurian period, 440 million years ago—right when vascular plants first appeared on land. Since then, remipedians have been identified as the closest living relatives of hexapods. Several internal relationship hypotheses have been proposed over the years, including: Entognatha (proturans, collembolans, diplurans) versus Ectognatha (insects); Ellipura (proturans and collembolans) versus Cercophora (diplurans and insects); collembolans, Nonoculata (proturans and diplurans), and insects; and proturans, Antennomusculata (collembolans and diplurans), and insects. A 2016 cladogram from Kjer et al. outlines one such arrangement.

An incomplete possible insect fossil, *Strudiella devonica*, comes from the Devonian period and might help fill a gap in the arthropod fossil record from 385 million to 325 million years ago. However, some researchers argue it could be a decomposed crustacean or something else. In 2023, a hexapod-like fossil from the Ordovician marine site Castle Bank was reported, but more study is needed to confirm what it is.