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Water Mites and Allies - Infraorder: Hydrachnidia
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Water Mites are found on every continent except Antarctica with the vast majority of species found in fresh water. The number of species is hard to determine as most of the 6000+ species described as of 2015 are found in North America and Europe. Such a number skewed to locality is likely to fall short of the true number world wide. Further convoluting the matter is the fact that on-line resources do not agree on the taxonomic arrangements involving Water Mites, making it difficult to ascertain who belongs where and why. The classification used here is the same as that used on the web-site, 'iNaturalist' which currently, July 2024, appears to be the most up-to-date.
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The Hydrachnidia are the only subgroup of the Class: Arachnida that have succesfully adapted to life in the water. Only a small handful of spiders are known to hunt on the surface of the water and there are a few that even venture under the surface but, Water Mites are different; they have made the water their habitat. Most are larval parasites on other arthropods that also have predatory nymphal and adult life stages. The parasite hosts are most commonly the aquatic larval stages of certain flies, dragonflies, caddisflies and mosquitoes. Because of the latter they are considered a possible means of biological control of mosquito numbers in areas where mosquito-borne disease is an issue.
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As nymphs and adults, Water Mites are predators and their diet consists mostly of small crustaceans, other Water Mites, as well as the eggs, larvae and even pupae of aquatic insects. Despite their small size and lack of physical defenses they are not commonly preyed upon themselves by other invertebrates nor by fish as it is thought they may secrete noxious substances from glands in their skin. The presence of large, diverse numbers of Water Mites is considered to be a sign of a healthy waterway so they may well be useful as pollution indicators.
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References and links:
> Proctor, H.C., (2009). ‘Hydrachnida (Water Mites)’ in: Likens, G.E. (Editor), ‘Encyclopedia of Inland Waters’, pp. 335 – 345. Academic Press. doi:10.1016/B978-012370626-3.00176-9.
> Proctor, H.C., Smith, I.M., Cook, D.R. and Smith, B.P., (2015). ‘Subphylum Chelicerata, Class Arachnida’ in: 'Thorp and Covich's Freshwater Invertebrates', pp. 599 – 660. Academic Press. doi:10.1016/b978-0-12-385026-3.00025-5
> Smith, I.M., Cook, D.R. and Smith, B.P., (2010). ‘Water Mites (Hydrachnidiae) and Other Arachnids’ In: Thorp, J.H. and Covich, A.P., (Editors), ‘Ecology and Classification of North American Freshwater Invertebrates.’, pp.485 – 586. Academic Press. doi:10.1016/b978-0-12-374855-3.00015-7
https://en.wikipedia.org/wiki/Hydrachnidia
> Proctor, H.C., (2009). ‘Hydrachnida (Water Mites)’ in: Likens, G.E. (Editor), ‘Encyclopedia of Inland Waters’, pp. 335 – 345. Academic Press. doi:10.1016/B978-012370626-3.00176-9.
> Proctor, H.C., Smith, I.M., Cook, D.R. and Smith, B.P., (2015). ‘Subphylum Chelicerata, Class Arachnida’ in: 'Thorp and Covich's Freshwater Invertebrates', pp. 599 – 660. Academic Press. doi:10.1016/b978-0-12-385026-3.00025-5
> Smith, I.M., Cook, D.R. and Smith, B.P., (2010). ‘Water Mites (Hydrachnidiae) and Other Arachnids’ In: Thorp, J.H. and Covich, A.P., (Editors), ‘Ecology and Classification of North American Freshwater Invertebrates.’, pp.485 – 586. Academic Press. doi:10.1016/b978-0-12-374855-3.00015-7
https://en.wikipedia.org/wiki/Hydrachnidia
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