Mites  and  Ticks  -   Subclass:  Acari
      Mites and Ticks together form a subclass of organisms that may well be obsolete, as a subclass, before too long.  The taxon has, in recent studies been proven to be of polyphyletic origins and the relationships of ticks and mites with other arachnids places them at opposing branches on the phylogenetic tree.  The Superorder: Parasitiformes (Ticks and 'some' Mites) is related to the Pseudoscorpions and the Superorder: Acariformes (Mites) is related to the Solifuges or Camel Spiders.  It should be noted that there have also been some studies indicating that both superorders are indeed related but it is often suggested that these outcomes may have been due to a particular type of error called 'long branch attraction'.  In this a shared characteristic may be seen as having been the result of a common ancestor even though it may well have been the outcome of 'convergent evolution'.     
      The two superorders currently still placed within the subclass are taken as valid taxons then and they provide us not only with a clearcut separation with regards to their relationships with other arachnids but, also, with regards to their origins.  The Acariformes appear in the fossil records some 400+ My ago whereas the Parasitiformes took another 300+ My to arrive.  And this despite the fact that the Acariformes, thanks to their smaller size in general, have severely less fossilisation potential than the Parasitiformes.
For now the taxon: Subclass remains in use in most resources and so as not to cause unnecessary confusion, it will be retained here. However, in keeping with the notion that both superorders are sufficiently different, further description will be limited to their respective pages. 
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      The study of Mites and and Ticks is called 'acarology' and it is one branch of 'arachnology' that seems to be frought with the most controversy particularly with regards to taxonomic classification as has been demonstrated above already.  As we progress further down the taxonomic ranks, more and more discord is encountered and the 'who belongs where?' becomes even more convoluted.  Whereas numerous resources on the subject are available, there is definitely no concensus among them and some even contradict themselves.
Picture
Image 1. Various mites from C.L. Koch, (1839). 'Uebersicht des Arachnidensystems'
      More of this taxonomic confusion will come to the fore on the following pages and it does beg the question whether or not, in the grand scheme of things, the place of Mites and Ticks is in need of a rethink:  Are they arthropods? Yes, definitely.  Arachnids?  Yeah, sort of?
After all, there are some considerable differences between the Acari and other arachnids. Not just morphologically as adults but, also developmentally, in the earlier stages of their life-cycle. 

      It is said by some that 'mites are acarids that are not ticks' and, by inference, the reverse is then true as well.  Others would hold that ticks are 'mite-like arachnids' or, that ticks are simply 'large parasitic mites'.   Regardless of how you view them they are definitely different.  Contrary to other arachnids which have two segmented body components, the cephalothorax (or prosoma) and the abdomen (or ophisthosoma), the Acari do not have segments (some traces of primary segmentation may remain) and the prosoma and opisthosoma are 'fused'.  The front region of the body is called the 'capitulum' and to this the pedipalps and chelicerae are attached. Internally, it contains the salivary glands.  There is no brain nor are there eyes located on or in the capitulum and so it is not a head, but rather, more like an extended mouth.  The remainder of the body is called the 'idiosoma' and from this the legs protrude underneath, it contains the vital organs and, if present, the eyes or other sensory structures.   The two parts are joined together by a flexible section of 'cuticle'.  This bodyplan is unique to mites.     

      Like other arachnids, most adult mite (and ticks) have four pairs of segmented legs joined to the underside of the body by means of a coxa and coxal plate.  The legs carry setae (=bristles) along their length and the tarsus (the last segment) has claws.  Aquatic mites may have some or all of the legs modified for swimming.   Larval mites however, have at most three pairs of legs with the fourth pair not emerging until after their final moult into nymphs. There are also mites with less than three pairs of legs, some gall mites have only two pairs and some parasitic mites have between one and three pairs.  
      Most mites reproduce sexually, either directly or indirectly, and the females lay their eggs in or on the substrate they live on.  Hatching of the eggs may take about six weeks depending on the species, and the larvae often resemble the adults but with only three pairs of legs instead of four.  After three moults the larvae become nymphs and after another three oults they are adults.  There are quite a few mites that are capable of reproduction by means of 'parthenogenesis' with or without undergoing 'automixis' (= a type of self-fertilization that retains meiosis) as well.
N.B.  The taxonomic classification of Mites and Ticks has always proven to be problematic as the evolution of these organisms is not at all well understood.   Even modern methods such as phylogenetic research has only been able to shine no more than flickering light on the subject.   The system used in the following pages has been derived from that used on the 'iNaturalist' website which, often, with regards the Acari, clashes with other websites such as 'Catalogue of Life' or 'COL' and many other such datasets.  
References  and  links:

>      Dabert, M., Witalinski, W., Kazmierski, A., Olszanowski, Z. and Dabert, J., (2010). ‘Molecular phylogeny of acariform mites (Acari, Arachnida): strong conflict between phylogenetic signal and long-branch attraction artifacts.’  Molecular Phylogenetics and Evolution.  56 (1): 222–241.  doi:10.1016/j.ympev.2009.12.020.
>      Halliday, B.,  (2007).   ‘Mites, Order Acari.’   Australasian Arachnological Society,  Australasian Arachnology 77, April 2007.
>      Koch, C.L.,  (1850).   'Ubersicht des Arachnidensystems.'  Verlag van J.L. Lotzbeck, Nurnberg, Germany.
>      Krantz, G.W. and Walter, D.E. (eds.), (2009). ‘A manual of Acarology, 3rd Edition.’ Lubbock, Texas: Texas Tech University Press.
>      Sidorchuk, E.A., (2018).  ‘Mites as fossils: forever small?’  International Journal of Acarology. 44 (8): 349–359.  doi:10.1080/01647954.2018.1497085. 

      https://en.wikipedia.org/wiki/Automixis
      https://en.wikipedia.org/wiki/Convergent_evolution
      https://en.wikipedia.org/wiki/Long_branch_attraction
      https://en.wikipedia.org/wiki/Parthenogenesis
      https://eol.org/pages/2586535/articles
      https://www.mindat.org/taxon-367.html

Header photo:

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