Chelicerata (Arachnids Including Spiders, Mites and Scorpions)

Abstract

Abstract Chelicerata is the group of arthropods that includes extant marine sea spiders (Pycnogonida) and horseshoe crabs (Xiphosurida), mainly terrestrial Arachnida (spiders, mites, scorpions, etc.), and extinct fossil groups, including sea scorpions. Most chelicerates have two body regions, a prosoma bearing feeding and leg‐like appendages and an opisthosoma bearing respiratory structures and other organ systems. The vast majority are predators that can have significant impacts on prey populations. Arachnids are notable for ingesting primarily liquified food and for a variety of predatory adaptations, including venoms and silken webs. Chelicerates probably first evolved in the Cambrian, but the earliest fossils are known from the Ordovician. Among extant arthropods, they form the sister group to the mandibulates (insects, crustaceans and myriapods). Within Chelicerata, sea spiders are the sister group to the horseshoe crabs and arachnids. Relationships among fossil groups and among the 11 extant arachnid orders are uncertain. The chelicerates form one of the most numerous and species‐rich groups of animals and are rivaled in diversity only by the insects. The chelicerate body consists of two regions: an anterior prosoma (or cephalothorax) that bears the eyes, mouthparts and six pairs of appendages and a posterior opisthosoma (or abdomen) that houses most of the respiratory, digestive and reproductive organs. The extant chelicerates include two marine groups, Pycnogonida (sea spiders) and Xiphosurida (horseshoe crabs), and the terrestrial Arachnida (spiders, mites, scorpions, whipscorpions and several other orders). The vast majority of chelicerates prey on other animals and are ecologically important in terrestrial systems for their effect on insect numbers, but many mites feed on plants or are parasitic. The chelicerates are extremely ancient, with the earliest fossils of several living and extinct forms found in Silurian and Devonian strata. Among the extant arthropods, Chelicerata is the sister group to Mandibulata, the clade that includes myriapods, crustaceans and insects. The phylogenetic relationships of many fossil groups and most the arachnid orders remain uncertain.