16 - Water mites (Hydrachnida) and other arachnids

Abstract

Members of the class Arachnida are the most familiar representatives of the arthropodan subphylum Chelicerata, an ancient lineage characterized by distinctive mouthparts with two pairs of appendages, the anterior chelicerae and posterior pedipalps, and four pairs of walking legs. Chelicerates first appear definitively in the fossil record as giant water scorpions (Merostomata, subclass Eurypterida) in freshwater deposits from the Silurian. By the Devonian, eurypterids had apparently disappeared but other chelicerate lineages were proliferating, such as horseshoe crabs (Merostomata, subclass Xiphosura) in marine environments, and early derivative arachnid groups, including mites, on land. Arachnids are the only group of chelicerates exhibiting extensive modern diversity. The higher classification is still being debated, but arachnids are usually considered to be a taxonomic class comprising 11 extant subclasses, mostly terrestrial groups of highly specialized predators of other arthropods. Only two subclasses of modern arachnids are represented in freshwater habitats: mites which have diversified extensively to become arguably the most ubiquitous and adaptable clade of arthropods on Earth, and spiders which have maintained a conservative way of life as essentially terrestrial predators. This chapter presents a brief summary of information as an introduction to the literature on the biology and ecology of these spiders.