Hexapods are a diverse group of arthropods that includes insects and some close relatives. They have three pairs of legs and share many characteristics, but their relationships are not fully known. They diverged from fairy shrimp about 440 million years ago and are found on all continents, with over a million described species. Beetles make up about 40% of insects and 25% of all known life forms.
The hexapods, subphylum Hexapoda, are a group of arthropods that includes insects and some close relatives, including springtails, proturans, and dipluras. The hexapods are probably monophyletic, meaning they all descend from a common ancestor and include no members not descended from that ancestor. However, there is some evidence that springtails may have evolved their characteristics independently, in an example of convergent evolution, although this is not the majority view. The relationships between members of Hexapoda are not fully known: there are three main cladistic hypotheses. Most other arthropods, such as spiders and crustaceans, have more than six legs.
The distinguishing feature of hexapods is three pairs of legs, from which they get their name (hexa means “six”), but they share many other characteristics in common, including a consolidated thorax and genetic similarity. The hexapods diverged from their sister group, the fairy shrimp, about 440 million years ago, during the Silurian period, shortly after the emergence of terrestrial vascular plants. Little is known about how the hexapods got away from the fairy shrimp and started walking on dry land. Much more is known about the early history of tetrapods. Before the arrival of vascular plants and hexapods on land, it was largely sterile.
Hexapods are found on all continents on Earth, including Antarctica. In particular, springtails have been discovered in some of the harshest environments on Earth, such as the McMurdo Dry Valleys in Antarctica, which contain so little moisture and life that the large animals that die there dry out and are mummified instead of decomposing, like cute a lot everywhere.
The most familiar hexapods to humans would be insects: flies, bees, mosquitoes, beetles, mites, etc. In terms of species, the hexapods are the most diverse subphylum on the planet, with over a million described species and total species estimates of between 5 and 8 million. Notably, there are over a million beetle species, with new species being discovered all the time. In all, beetles consist of about 40% insects and about 25% of all known life forms. If you randomly select an animal species from the earth’s surface, it is most likely a beetle.
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