Chelicerater

Chelicerater, more commonly spelled chelicerate, refers to members of the subphylum Chelicerata, a major lineage of arthropods defined by the presence of chelicerae, the first pair of feeding appendages near the mouth. The term chelicerater is less common in contemporary taxonomy; chelicerate is the standard singular form and chelicerates the plural.

Living chelicerates include several major lineages: arachnids (such as spiders, scorpions, harvestmen, mites, and ticks), sea spiders (Pycnogonida), and horseshoe crabs (Xiphosura). Eurypterids, the extinct sea scorpions, are also chelicerates. Among these, arachnids are the most diverse and widespread on land, while Pycnogonida and Xiphosura are primarily marine.

A unifying feature of chelicerates is the absence of antennae and mandibles, with feeding typically accomplished by the chelicerae. Most chelicerates display a two-part body plan consisting of a prosoma (cephalothorax) and an opisthosoma (abdomen or abdomen-like region). In most species, the appendage complement includes chelicerae, paired pedipalps, and four pairs of walking legs. Respiratory structures vary: many arachnids use book lungs or tracheae, while horseshoe crabs possess book gills and a protective carapace.

Ecology and life histories are diverse. Spiders, scorpions, mites, and ticks occupy a wide range of terrestrial and freshwater habitats, often as predators or parasites, whereas Pycnogonida and Xiphosura are marine. Reproduction generally involves internal fertilization in many groups, with development ranging from direct growth to juvenile stages resembling adults.

The chelicerate lineage dates back to the early Paleozoic, with a rich fossil record that illuminates the early evolution of arthropods. The enduring chelicerate body plan—especially the chelicerae and the lack of antennae—has helped sustain its members across hundreds of millions of years.