Six legs define Earth's largest arthropod dynasty
Hexapoda—Greek for "six legs"—is the biggest arthropod clade, uniting true insects with the smaller wingless Entognatha: springtails, coneheads, and diplurans. A consolidated thorax that bears exactly three pairs of legs sets them apart from nearly every other arthropod lineage.
Bodies from 0.5 mm to over 300 mm divide into head, thorax, and abdomen. The head fuses an acron (usually with eyes—absent in Protura and Diplura) and six segments; antennae arise on segment II (missing in Protura). True insects expose ectognathous mouthparts; other hexapods tuck endognathous ones inside. Insect antennae are annulated—only scape and pedicel are muscled, with Johnston's organ on the pedicel, absent in Entognatha—whereas collembolan and dipluran antennae keep muscles in every segment.
Each of three thoracic segments carries one five-segment walking leg without gill branches; wings, when present, spring from the second and third segments and may relate to crustacean gill branches or segmental outgrowths. Abdomens are epimorphic (all segments present at hatch) except in Protura, which add segments after molting to reach twelve; insects typically start from eleven (often reduced), collembolans six or fewer. Abdominal appendages shrink to genitalia and sometimes cerci.
Myriapods were long cast as closest kin inside Uniramia/Atelocerata; twenty-first-century evidence instead nests hexapods among pancrustaceans, with remipedians as closest living relatives and a 2002 molecular estimate placing a split from fairy-shrimp relatives near the Silurian, about 440 million years ago as vascular plants colonized land. Rival internal trees group Entognatha versus insects, or shuffle proturans, springtails, and diplurans. Fossils remain sparse: disputed Devonian Strudiella, and a 2023 Ordovician Castle Bank candidate still needing study.
Six legs, three tagmata—and most of the animal species you will ever meet.
Source: Hexapoda
