Arachnid

Arachnids Temporal range: 435–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N Early Silurian – present
Left to right: Phidippus mystaceus (Araneae), Pseudoscorpion (Pseudoscorpiones), Hottentotta tamulus (Scorpiones), Ixodes ricinus (Ixodida), Heterophrynus (Amblypygi), Aceria anthocoptes (Trombidiformes), Harvestman (Opiliones), Galeodes caspius (Solifugae), and a Whip scorpion (Thelyphonidae).
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Class:	Arachnida Lamarck, 1801
Orders
Ricinulei Opiliones – harvestmen Solifugae – camel spiders Acariformes mites Parasitiformes mites and ticks Pseudoscorpiones – pseudoscorpions Scorpiones – scorpions †Phalangiotarbi (extinct) †Trigonotarbida (extinct) †Uraraneida (extinct) Araneae – spiders †Haptopoda (extinct) Amblypygi – whip spiders Schizomida – short-tailed whipscorpions Palpigradi – micro-whipscorpions Thelyphonida – vinegaroons ?Xiphosura – horseshoe crabs

The arachnids are a class of eight-legged arthropods.^[1] They are a highly successful group of mainly terrestrial invertebrates: spiders,^[2] scorpions, harvestmen, ticks, and mites, and a number of smaller groups.^[3]

In 2019, a molecular phylogeny study put horseshoe crabs in the Arachnida.^[4]

1. ↑ Ruppert E.E. Fo, R.S. and Barnes R.D. 2004. Invertebrate zoology 7 ed, Brooks/Cole. p520 ISBN 0030259827.
2. ↑ Foelix, Rainer F. 1996. Biology of spiders. Oxford University Press. ISBN 0-19-509593-6.
3. ↑ Schultz J.W. 2000. A phylogenetic analysis of the arachnid orders based on morphological characters. Zoological Journal of the Linnean Society 150: 221–265. [1]^{[permanent dead link]}
4. ↑ Ballesteros J.A. & Sharma P.P. 2019. A critical appraisal of the placement of Xiphosura (Chelicerata) with account of known sources of phylogenetic error. Systematic Biology. 68 (6): 896–917. [2]