Acariform Mites  -  Superorder:   Acariformes
      Acariform Mites are by far the most diverse group of arachnids with possibly about 60,000* species  described and estimates of total  numbers of species worldwide ranging from 500,000 to 1,000,000.  All these species are arranged in only two orders meaning that both could outnumber the Order: Coleoptera (Beetles) which has an estimated 350,000 species, by a substantial margin.  
Until recently the Acariformes was one of three superorders but the former Opiliocariformes has been made redundant and placed in the Parasitiformes as an order.   As of April 2024 the 'Catalogue of Life' records 22,601 species in 404 families making the Acariformes the most numerous of the two superorders.   
      As stated on the previous page the Acariformes are the oldest of the two superorders with the fossil record going back to the Early Devonian, some 411 My ago.  Whereas never numerous, fossils of varying ages have been found worldwide with some of the oldest found in Aberdeenshire in Scotland among other localities.   
The Acariform Mites are usually small (i.e. measured in micrometres) with only a few reaching a total bodylength of a few millimeters.  The largest known Acariform Mite is a species of Velvet Mite which can reach a length of upto 14 mm.  The largest fossil acariform was described in 2018 after having been discovered in 100 My old, Burmese amber from the Cretaceous. It was about 10 mm long.
      The current wisdom is to divided the Acariformes into two orders;  Sarcoptiform Mites and Trombidiform Mites.  However, many sources still lists the Endeostigmata as an order of the Acariformes rather than as a suborder of the Sarcoptiformes.  The Endeostigmata are a small, phylogenetically problematic  group of about 120+ species (in 10 families) of gall producing mites.  Placing them as a suborder with the Sarcoptiformes appears to be an arrangement of convenience for the sake of monophyly.  Further studies are obviously needed to determine their exact place in the Acari.
      Mites are everywhere.  Quite literally.  There is no environment on the planet where they are not found and, on top of that, they are present in numbers that sometimes are difficult to come to grips with.  One study has shown that a mere square meter of mixed, decomposing  leaf litter may hide upto  1,000,000 mites (representing as many as 200 species from 50 different families) all feeding on the detritus itself and on the bacteria and other microfauna within it.  In doing so they help regulate the process of  composting and recycling of the organic materials.  Needles to say, the mites themselves, thanks to their sheer numbers are an important food source for other predators as well.   Mites are also found living comfortably with other larger animals.  Not just in their nests but also on the animal itself and, in that regard, we are no exception.  We share our homes, and even our beds, with millions of the little creatures.  Unfortunately, most relationships between mites and their hosts are not mutually beneficial with the host , of course, coming off second best.

*  =  Estimate by editor based on a reported figure of 55,000 described species in Krantz and Walter, 2009.  It must be stated though that the 'Catalogue of Life' in April 2024, records less than 30,000 species for both Acariformes and Parasitiformes combined.
References  and  links:
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>      Arribas, P.,  Andújar, C., Moraza, M.L.,  Linard, B.,  Emerson, B.C.  and  Vogler, A.P.,  (2020).   ‘Mitochondrial Metagenomics Reveals the Ancient Origin and Phylodiversity of Soil Mites and Provides a Phylogeny of the Acari.’    Molecular Biology and Evolution,  Vol 37,  Issue 3,  March 2020,  pp 683–694.    https://doi.org/10.1093/molbev/msz255
>      Bussaman, P.,  Sa-uth, C.,  Chandrapatya, A.,  Atlihan, R.,  Gökçe, A.,  Saska, P.  and  Chi, H.,  (2017).   ‘Fast Population Growth in Physogastry Reproduction of Luciaphorus perniciosus  (Acari: Pygmephoridae) at Different Temperatures.’   Journal of Economic Entomology,  Vol. 110,  Issue 4,  August 2017, pp 1397–1403.   https://doi.org/10.1093/jee/tox102
>      Dunlop, J.,  Frahnert, K.  and  Mąkol, J.,  (2018).   ‘A giant mite in Cretaceous Burmese amber.’   Fossil Record. 21. 285-290.   DOI:10.5194/fr-21-285-2018. 
>      Krantz, G.W. and Walter, D.E. (eds.), (2009). ‘A manual of Acarology, 3rd Edition.’ Lubbock, Texas: Texas Tech University Press.
>      Proctor, H.,  (1998).    'Acariformes. The "mite-like" mites.'   V. 09 August 1998.   http://tolweb.org/Acariformes/2563/1998.08.09   in:  The Tree of Life Web Project,  http://tolweb.org/

      https://biodiversity.org.au/afd/taxa/Acari
      https://biodiversity.org.au/afd/taxa/SARCOPTIFORMES/statistics
      https://biodiversity.org.au/afd/taxa/TROMBIDIFORMES/statistics
​      https://en.wikipedia.org/wiki/Acariformes
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