Chelicerates are a subphylum of arthropods, comprising between 10% and 30% of all arthropod species. They are named after their chelicerae, used for feeding. The five major classes include Arachnida and sea spiders, with arachnids being the largest and most diverse group. Tick and mite species are likely the most numerous, but have yet to be properly classified. Other groups of arachnids include cave spiders, harvestmen, whip scorpions, scorpions, and solifuges.
Chelicerates, also called subphylum Chelicerata, are a major subphylum of the phylum Arthropoda, also known as arthropods, the largest phylum in the animal kingdom, with over one million species recognized by science. Chelicerata comprises between 10% and 30% of all arthropod species: mainly because mites are chelicerates and the estimated total number of mite species is huge, in the hundreds of thousands, perhaps even more than a million.
Chelicerates get their name from their chelicerae, specialized appendages used to feed and manipulate food, in place of the masticatory mandibles employed by other subphyla of arthopods, such as Hexapoda (which includes insects). There are five major classes of chelicerates: Arachnida (spiders, scorpions, mites, etc.), Xiphosura (horseshoe crabs), Eurypterida (sea scorpions), Chasmataspidida (chasmataspids), and Pycnogonida (sea spiders). Of these, Eurypterida and Chasmataspidida are extinct, and Xiphosura, the horseshoe crab – considered a “living fossil” – consists of only four species.
Only the arachnids and sea spiders remain. Arachnids are a famous, primarily terrestrial and extremely diverse group consisting of over 110,000 described species and an estimated total exceeding one million, while sea spiders are an unusual group of marine chelicerates numbering only 1300 species. Sea spiders are usually small, about 1 cm across, although some of the largest deep-sea specimens can reach 9 cm (3.5 inches).
The largest and most successful of the chelicerates, by far, are the arachnids. When you hear the word “arachnid,” spiders immediately spring to mind, and indeed, spiders are the most numerous arachnid, if only the described species are counted. About 40,000 spider species have been identified, out of a total of 110,000 arachnids known to science. However, another group of arachnids – ticks and mites – is likely to be the most numerous, but due to their small size, has yet to be properly classified. Different mite species may have few external differences, requiring comparisons of DNA sequences to verify distinct species. This huge project has just begun.
Other groups of arachnids include cave spiders (not actually spiders), harvestmen (daddy long legs), whip scorpions, scorpions, solifuges, and the little (
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