Arthropods own more than eighty percent of animal species
Phylum Arthropoda—jointed-leg animals with chitinous exoskeletons, segmented bodies, and paired limbs—holds estimates from about one point seventeen million to five or ten million species. That is over eighty percent of known animals. Insects alone outnumber every other class of described life.
Growth demands moulting: the cuticle is shed so a larger shell can harden. Inside, an open circulatory system pushes haemolymph through a haemocoel, and ladder-like ventral nerve cords serve repeated segments. Vision mixes compound eyes with simple ocelli; spiders flip the script by using image-forming ocelli as main eyes, sometimes able to track prey. Greek árthron and poús yield jointed foot; Dumortier used arthropodes in eighteen thirty-two, and von Siebold's Arthropoda appeared in eighteen forty-three.
Cambrian fossils already show the clade. Most analyses nest arthropods within Ecdysozoa beside cycloneuralian moulting worms, though basal animal relationships stay unsettled. They conquered marine, freshwater, terrestrial, and aerial habitats and, with amniotes, are one of only two major animal groups fully at home in dry environments. Size extremes both belong to crustaceans: tiny tantulocaridans versus the largest marine forms. Compound eyes deliver the main visual stream for most species, while simple ocelli mostly report light direction—except in image-forming spider eyes.
Without arthropods, soils, reefs, crops, and disease ecology collapse. Their success is architectural—armor that must be shed, limbs that hinge, and nervous systems built like segmented ladders. Counting species is still guesswork at the high end, yet even the low estimate crowns them Earth's dominant animal blueprint. Every moult is a gamble between growth and vulnerability, yet that cycle underwrites reefs, crops, and disease vectors alike. When people speak of animal diversity they are usually speaking, statistically, of arthropods.
Source: Arthropod
