Mites are amazing. They've been around in one form or another at least since the Devonian, around 400 million years and are able to live in more places than pretty much anything else on the planet except perhaps nematodes. However, many attack and eat my collembolan friends, so this important and numerous animal was going to be ignored and hidden somewhere on the site as punishment. Of course, I then felt bad, so I relented and gave them their own page. But I'll just leave this here- a Labidostomatidae mite eating its own young.
Mites are the most numerous of the mesofauna, closely but cautiously followed by Collembola. Even Astigmatina, the group of mites not usually associated with the soil, are numerously covering much of everything else.
Knemidocoptes gallinae, the depluming mite, a parasite of chickens, gives birth to live young.
In fact, almost every animal and plant so far studied appears to have at least one species of mite associated with it.
It's estimated that each adult human being might be supporting over a million Demodex follicle mites, with a little colony of mites able to live in a single hair follicle. They have no anus, so instead of excreting, they eventually explode and die in a tiny shower of faeces.
Mites are often dismissed for looking a little off-putting. Not as cute or as pretty as Collembola for example. I used to think this myself. But I changed my mind. I've realised that you just need to get to know them and they become beautiful, like this member of the Prostigmata, Anystidae family, from New Zealand, an Erythracarus species.
And this Trombididae, or velvet mite, already unusual enough by being white rather than red, is arrestingly beautiful too.
Mites don't have heads. They have a structure called
the gnathsoma, housing the mouth and feeding parts.
Any inkling of a brain, or eyes, if present, are on
or in the idiosoma, the big, body-like structure behind it.
Mites are incredibly ancient and diverse and are notoriously difficult to make sense of, taxonomically. Superorders get made and then dissolved, orders get remade, removed, families get shuffled, promoted, demoted or merged... Only the very basics have been decided and agreed upon. There are so many potential traps and confusion, especially when talking about orders and super orders, cohorts and clades, that I'm just going to try and avoid the whole thing as much as I can and hope it all goes away.
So, in what is a constantly changing and evolving family tree, here is an attempt at a hopefully reasonably up to date and roughly accurate overview at least. That should be safe.
Mites, or Acari divide into two lineages. These are the Parasitiformes and Acariformes. Until very recently, Opilioacarida still ranked as a third lineage, but now resides in Parasitiformes.
Ixodida- the hard and soft ticks.
Holothyrida- rare, Gondwanan distributed mites that are carrion feeders, a kind of missing link between the ticks and other mites.
Mesostigmata- the mainly free-living, soil-loving, often predatory mites, described in more detail below.
Opilioacarida- thought to be amongst the most primitive of all mites. They retain segmentation of the abdomen, as well as often having six eyes and strong pigmentation. They are described in more detail below.
A large percentage of the world's species of mites are members of the Acariformes, whether they know it or not.
Within Acariformes, the Sarcoptiformes and Trombidiformes form two rough halves, splitting not quite down the middle. But each of the titles hides, as one internet mite site cautions, a 'morass of subdivisions.' The brief descriptions following will hopefully act as a mite-friendly Sweet Track over the top.
The Sarcoptiformes consist of-
Most of Endeostigmata- an uncommon and primitive group of mites, often found living in surprisingly extreme environments.
Oribatida- the world-wide and very common mites, including the Astimatina. They are described in more detail below.
The Trombidiformes consist of-
Sphaerolichida- containing two Endeostigmata families left over from the Sarcoptiformes.
Prostigmata- containing many 'pest' mites like velvet mites, spider mites and the infamous pre-adult mites commonly known as chiggers. Prostigmata are described in more detail below.
Mesostigmatans tend to be well sclerotised, and are fluid feeders, like their cousins, the Prostigmata. Some mesostigmatan families dedicate themselves to parasitic and phoretic encounters with birds, mammals, reptiles and some social insects, such as the infamous varroa mite that parasitises the honey bee. But most are free-living soil predators, merciless and efficient hunters of anything and everything with a cuticle. They are usually fast moving, as befits a top predator, and not particularly coloured, sticking to muted browns and creams.
As the name might suggest, the stigmata or spiracles of the Mesostigmata are positioned midway on the side of the body, usually associated with a distinctive perotrematal groove.
Like the prostigmatan Eupodidae, the first pair of legs of the Laelapidae have elongated and developed sensory setae to act as feelers. They use them while hunting, to feel for prey like Collembola and guide them towards their chelicerae. Some species even have a glue-like substance on them to help further entrap their hapless quarry.
Because of their unrelenting and efficient attack capabilities, some families in Mesostigmata are used as biocontrol agents. Phytoeiulsus persimilis is a specific predator of the two-spotted mite, a common pest on house plants and in greenhouses. One adult can eat around seven of its prostigmatan cousins a day.
Traditionally, the Oribatida has described the box/moss/beetle mites. Most are fungivores and detritivores, with a scattering of predatory species. Most adults are sclerotised.
Many families in Oribatida have very distinctive chitinous flaps or pteromorphae. These are perfectly described in Wiley's online glossary of mite terminology, as
'...a pair of thin, lateral roof-like processes that overhang and flank the legs of some (Oribatid) mites. They may be hinged or wing-like'.
Many of the families have the ability to tuck their legs underneath their protective armour, rendering them immune from most predation, apart from being eating wholesale.
While no families of the Oribatid/moss mites are parasitic, they can be intermediate hosts to many larval tapeworm species, passing into the guts of vertebrates through ingestion.
Astigmata has recently been renamed Astigmatina, and has been placed as a cohort of Oribatida. This has slightly complicated the description of Oribatida. The Astigmatina are not so much associated with decomposition as they are with parasitising vertebrates and invertebrates alike, as well as some families following another interesting and possibly retro-evolutionary path as dust mites, together with many food and stored product mites. However, they can also be present in large amounts in some soils, where they are usually soft-bodied, tiny and white, with long setae.
As with Collembola, Oribatid mites are an important indicator of soil health in different environments, indicated by their abundance and diversity. It takes time for an Oribatid community to build due to their low fecundity. Along with many other slow-metabolising animals with similar fecundity, the oribatidan's life span can be over two years, sometimes up to seven years in colder climates. A very long time for an animal no more than 1.5mm big.
Oribatid mites are an important part of the diet of some poison frogs in Central and South America, contributing alkaloids which the frogs exude as poison.
Prostigmata includes the many families of spider mites, velvet mites, gall mites and snout-nosed mites that terrorise plants and other invertebrates. Prostigmatans are one of the most diverse groups of mites, and the most colourful. Apart from a few families, they have very little sclerotisation and usually have eyes. As the name suggests, the stigmata or spiracles are hidden at the front of the head, in the middle of the mouthparts or to the side.
All prostigs are fluid feeders and some families can be major plant pests as well as occasionally predatory.
A prostigmatan mite, Paratarsotomus macropalpis, is one of the fastest terrestrial animals alive, relative to its size. At 0.7mm big, it can run at three hundred and twenty two body lengths per second, that's 0.225 metres per second. An equivalent cheetah would need to be running at 2052 km/h . That's really, really fast.
Some, like the Eupodidae are fungivorous and have distinctive, ever-moving, elongated front legs that act as feelers, as well as serving as a possible deterrent to predators. They spend their days wandering amongst the fruiting bodies of micro fungi, all laid out like a smorgasbord in front of their chelicerae. When evening comes, they stand together in fungal devotional groups, worshipfully flicking their front legs in the stillness as the sun goes down. It's something I've noted numerous times in the UK, Canada, Australia and New Zealand. It's incredibly charming.
Some members are parasitic in their larval form- the protelean parasites of Parasitengona, like chiggers/harvest mites. Yet others are fully fledged parasites, like Demodex, the exploding follicle mite, mentioned earlier.
Parasitengona is a cohort of Prostigmata and contains nearly all of the protelean mites, including the velvet mites, the largest of the mites. It also includes Hydrachnidia, the group of mites that are aquatic. They pester fresh and salt water creatures, in case they felt left out from the fun and games.
Parasitengona has one of the most complex life cycle of all the mites.
The egg hatches into an immobile pre-larva, which, after a few hours or longer, moults, emerging usually as a parasitic, six-legged larva.
The larva then ambles off in search of a host, and once found, attaches itself, sometimes for months, before finally dropping off. It then hides in the soil, becoming a calyptostatic protonymph. This could be seen as an equivalent of a insect's chrysalis, from which the deutonymph emerges transformed, an active, free-living predator. It now also has a lovely pair of extra legs.
After a little while longer, the mite then enters a secondary calyptostase, transforming yet again into an adult form, ready to lightly and happily suck on plants.. Like I said, complex.
The Opilioacaridae are an incredibly unusual and primitive single family of mites. They have, as the name suggests, a certain Opiliones/Harvestmen feel about them. How unusual these animals are can be guessed by the fact that there is only one family, and so far, only around twelve genera in the entire order.
They are the only mites to have retained six eyes across most of the genera, as well as having a segmented abdomen, perhaps a throwback to some ancient arachnid-type ancestor. They are also brightly coloured, in stunning pinks and blues. Apart from the species found in caves, they all also have striped legs. Unlike other mites, they also continue to moult throughout their lives, enabling full regeneration of lost legs in just a few weeks.