Arachnids (Arachnida) belong to the arthropods (Arthropoda), which in almost all cases exhibit a segmented body with a chitinous exosceleton and in all cases segmented legs. Most extant animal species are arthropods, including the extremely species-rich insects (Hexapoda), crustaceans, centi- and millipedes as well as arachnids.
The arachnids are comprised of spiders (Araneae) as well as a number of other more or less known animal groups, like harvestmen, scorpions and mites.
All arachnids share the following characters: a bi-partite body consisting of a prosoma (a highly chitinized front body with four pairs of legs) and an opisthosoma, which is often covered by a soft integument.
The relatedness and ancestry (phylogeny) within the arachnids is not yet fully understood. Using different sets of characters and fossil finds results in slightly differing phylogenetic trees and even molecular studies have so far not been able to clearify this question. The current state of knowledge on the evolution of arachnida is regularly presented by Jason Dunlop.
Blick, T. & Harvey, M.S. (2011) Worldwide catalogues and species numbers of the arachnid orders (Arachnida). - Arachnologische Mitteilungen 41: 41–43. - doi: 10.5431/aramit4108
Fet, V., Sissom, W., Lowe, G. & Braunwalder, M. (2000) Catalog of the scorpions of the world (1758-1998). New York Entomological Society, New York. 1-690 pp.
Giribet, G. (2000) Catalogue of the Cyphophthalmi of the World (Arachnida, Opiliones) [Catálogo mundial de Cyphophthalmi (Arachnida, Opiliones)]. - Revista Ibérica de Aracnología 2: 49–76.
Kury, A.B. (2004) Annotated catalogue of the Laniatores of the New World (Arachnida, Opiliones). - Revista Ibérica de Aracnología 1: 1–337.
Harvey, M.S. (1991) Catalogue of the Pseudoscorpionida. Manchester University Press, Manchester. 1-726 pp. Online
Small Orders (Amblypygi, Palpigradi, Schizomida, Solifugae, Thelyphonida)
Harvey, M.S. (2003) Catalogue of the smaller arachnid orders of the world. CSIRO Publishing, Collingwood. 1-385 pp.
World Spider Catalog (2018). World Spider Catalog. Natural History Museum Bern, online at http://wsc.nmbe.ch, version 19.0, accessed on 30.1.2018.