ACARI | Mesostigmata

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I will probably need to revisit this brief introduction at some point, but my limited experience to date suggests that in-the-field encounters with any of the members of this group are probably going to be rather limited. Notwithstanding the practicalities of actually finding and photographing these mites, they are extremely difficult to identify even with expert help. Determining the family alone can be a challenge and going beyond that is usually no more that a suggestion. I see little point of including photos where the individual has no identity, so have adopted a slightly different approach to the way the species that I am able to feature are presented as confirmed below.

They are mostly free-living predatory species that inhabit the soil and leaf-litter, rotting deadwood, or detrital matter such as dung and manure or rotting seaweed where they hunt various small arthropods including other mites, but there are also some that are plant-dwelling predators. And others that have an association with animal or bird nests or are parasites on various invertebrates and vertebrate species.

The majority are tiny, either too small to photograph in situ or of a size that are going to produce pretty unsatisfactory images, but there are a few species, particularly in the Parasitidae family, that are substantially larger averaging close to 1.5mm and, consequently, they are the species that are mainly featured here.

Mesostigmatans have six fully functional walking legs with articulating coxae, whereas in the other groups they're effectively fused to the body and, because they don't have eyes, they often use the front pair as sensory feelers, a bit like antennae.

The most characteristic identifying feature though is that their stigmatal openings are located on the side of the body above the two rear pairs of legs and are usually connected with an anterior elongated peritrematal groove. Another feature that could be visible is that the gnathosoma has a sclerotised ring around the bases of the chelicerae. However, the most important identifying characters are often associated with the sternal shields and ventral setae, which are obviously out of sight on the underside. Not only do they assist in providing reliable identification of many species, but they are also used to determine the sex.

The latest available data confirms that nearly 600 species have now been recorded in Britain, which are placed in just over 170 genera in 50 families that, in turn, are separated into various superfamilies and higher-ranking divisions.

At the top level, Mesostigmata is split into three suborders; Monogynaspida, Sejida and Trigynaspida. Most of the species currently represented here are in the Monogynaspida, which comprises three infraorders (cohorts); Gamasina, Microgyniina and Uropodina.

Gamasina is the primary subgroup, which includes the hyporders (subcohorts) Dermanyssiae and Parasitiae, as well as Epicriiae with just two families that are not currently represented. Any member of the group can safely be referred to as a gamasid mite even if the actual identity is unknown.

Dermanyssiae is the largest division encompassing most of the gamasid families that are separately split into six superfamilies. Parasitiae on the other hand purely acts as a higher-ranking taxon for Parasitoidea, which includes a single, albeit very large family Parasitidae.

The Microgyniina comprises a single family and just one species, and can effectively be ignored. The Uropodina on the other hand includes just over 100 species in 19 families that are split into three superfamiles. The majority are in the Uropodoidea and, although I initially thought that they wouldn't be included, I'm pleased that I can represent them to some degree.

Sejida is a very small suborder that only includes two species.

The families and species I'm able to represent are shown below in their systematic (hierarchal alphabetical) listed order - see the final introductory paragraph on the Oribatida page for further information - but, for convenience, are split into the following four sections:

1) Monogynaspida > Gamasina > Dermanyssiae (hyporder)

2) Monogynaspida > Gamasina > Parasitiae > Parasitidae (family)

3) Monogynaspida > Uropodina > Uropodoidea (superfamily)

4) Sejida > Sejoidea > Sejidae (family)

PARASITIFORMES > Mesostigmata

Monogynaspida > Gamasina > Dermanyssiae

Gamasina is the major taxonomic division of the Mesostigmata with around 490 species in 25 families. The largest is Parasitidae, which is placed in its own hyporder as described in the following section. Two small families Epicriidae and Zerconidae, that are not represented here, are also separated.

All of the remaining families are grouped together within the Dermanyssiae which, in many respects, is little more than a convenient taxonomic subdivision that provides clear separation from the Parasitidae. It's a diverse, species-rich group that includes most of the familiar families but, unfortunately, the majority of species are very small offering limited opportunities for casual observation.

They are separated into six superfamilies; Ascoidea (with 5 families), Dermanyssoidea (7), Eviphidoidea (4), Phytoseioidea (1), Rhodacaroidea (4) and Veigaiidae (1). I'll keep an open mind, but I think that from a photographer's point of view only six or seven of the 22 families are likely to be encountered:

Asoidea > Ascidae and Blattisociidae

Dermanyssoidea > Laelapidae, Eviphididae and Macrochelidae

Rhodacaroidea > Ologamasidae

At present, I can only represent four of these families and only with a single species, which is why I've lumped them all together. Experts may be able to determine the family, possibly even the genus simply from the look or general appearance, but at the moment I can't. The important identification features are either too small to see or, more likely, associated with the ventral shields and setae, which obviously makes any assessed identification from photos uncertain. My approach after initially trying to ascertain the family is to seek help from one of the FB groups - Acarology or Soil Biodiversity UK. Dependent on the feedback I receive, I will research further.

The following notes specifically relate to the individuals featured here.

Ascidae - 20 species in 9 genera; sometimes split into subfamilies Arctoseiinae and Ascinae based on the extent of the leg chaetotaxy and whether the opisthonotal region of the dorsal shield has 4 or 5 pairs of lateral setae. The individual featured here is considered to be in the former and probably Zerconopsis sp. from the paddle-shaped (spathulate Z5) setae on the rear, which is characteristic of the genus. If that assessment is correct, it could well be Zerconopsis remiger (350-630µm) although that's no more than a tentative suggestion. In woodland habitat, the genus is mostly associated with both dry and damp forest litter, and loose bark, but is also known to occur on bracket fungi.

Ascidae > cf.Zerconopsis sp.

Blattisociidae > cf.Zerconopsis spathuliger

Macrochelidae > cf.Macrocheles sp. (teneral male)

Blattisociidae - a diverse family of around 30 species in 5 genera; shown here in the Asoidea in accordance with the current British checklist, but will probably be reassigned to the Phytoseioidea. I found a number of these tiny mites amongst a small colony of Silvanus bidentatus flat-bark beetles under the loose bark of an old fallen oak tree. They are around 2.5mm long, but at least four-times the size of the mites. In fact, it took me a while to realise the mites were there. They were identified as Blattisociidae, possibly Zerconopsis spathuliger, or certainly similar. The diagnostic characters of the genera are mainly associated with the dorsal seta, so obviously impossible to determine from photos.

Macrochelidae - over 40 species in 5 genera, mostly placed in the largest genus Macrocheles. Macrochelids are typically yellowish-brown to dark brown coloured mites with entire (holodorsal) dorsal shields usually with well-developed recurved petritremes, which loop over the stigma to join at the rear. Most species are predaceous on nematodes and the eggs of larvae of other micro-invertebrates occurring in a wide range of decomposing organic matter, such as dung or manure, or even beach wrack, whereas others, notably in the Dissoloncha and Geholaspis genera are more likely to be encountered in soil and leaf-litter. Identification is mainly associated with checking the setae on the ventral shields, but the long needle-like central dorsal setae evident in the adult featured here suggests Geholaspis sp. The pale, near white individual in the previous photo could be a deutonymph, but is considered more likely a teneral male Macrocheles and given the size and habitat, possibly one of the common woodland forms such as Macrocheles submotus but, again, that's purely an uneducated guess.

Macrochelidae > cf.Geholaspis sp.

Ologamasidae - 11 species in 6 genera; predatory woodland species occurring in soil and litter. The fast-moving individual that I just about managed to photograph in Garston Wood was positively identified as a male Euryparasitus emarginatus and described as a 'formidable predator built like a tank'. I thought it was Parastidae > Parasitinae and I'm still not sure if I could tell them apart, although I guess it would be possible if better information regarding the dorsal seta, which I don't. And, just

to help illustrate the point, I've added a photo of a not dissimilar undetermined species for comparison.

Ologamasidae > Euryparasitus emarginatus (confirmed)

Superficially similar undetermined species

Location / habitat notes: Apart from Euryparasitus emarginatus, which I found under a cut log in Garston Wood and the similar 'undetermined species' that I spotted in our back garden, the remaining individuals were all photographed either under the bark of fallen trees or on large pieces of deadwood at Barrow Moor in the New Forest.

PARASITIFORMES > Mesostigmata

Monogynaspida > Gamasina > Parasitiae > Parasitoidea > Parasitidae

As previously noted, the Parasitidae is the only family in the Parasitiae hyporder and, therefore, completely separated from other members of the Gamasina, which makes sense as it's clearly the largest family with close on 100 recognised species. They are split into two groups utilising the rarely adopted subfamily ranking; Parasitinae with 42 species in 12 genera, and Pergamasinae with 57 species that are also placed in 12 genera.

Parasitids are robust, mainly yellowish to dark-brown coloured, very active predatory mites that typically have a broadly oval or teardrop to lozenge-shaped body with an average length of around 800-1200µm, although some are up to 1.5mm, which together gives them a reasonably distinctive look. Apart from a couple of exceptions noted below where the genus can be determined, they can't be identified from photos, but they can usually be separated into their respective subfamily. In this respect it's useful to know the sex as mesostigmatan keys mostly refer to adult females as they provide a consistent base

for identification. With some families the males are barely known, but in the Parasitidae they are regularly seen and, although close examination is always required for a confirmed identity, can usually be picked out as leg II is thickened with large outgrowths or spurs.

Parasitinae females are visually distinguished by having a clearly, or sometimes less obvious, divided dorsal shield with separate anterior (podonotal) and posterior (opisthonotal) plates, or very occasionally with a schizodorsal shield (some Gamasodes) where the podonotal and opisthonotal plates are fused in the centre just leaving lateral indentations.

Parasitinae > undetermined genera (female)

In the Pergamasinae the female dorsal shield is entire (holodorsal) completely covering the idiosoma. Deutonymphs can be separated in the same way, but with Parasitinae males the shield appears entire, which can make identification more difficult. In the most detailed literature that I have specifically relating to the Parasitinae it clearly states that "males of some species of Pergamasinae have a transverse suture between the podonotal and opisthonotal regions, whilst in others there's complete fusion. In the Parasitinae all males have a median transverse suture".

Pergamasinae > Pergamasus sp. (female)

Parasitinae > undetermined genera (male)

Another useful feature that might be discernible from photos is the dorsal setae, which is a further identification aid for separating the subfamilies, but also important if you were trying to determine the genus or species. There's a central area

on the podontal region called the 'dorsal hexagon' outlined by three pairs of setae (j5, z5 and j6 that need to be seen on

a diagram), which in the Parasitinae are usually of noticeable dissimilar length and with z5 often thickened, whereas in Pergamasinae they're pretty much similar in length and form.

For some reason the Parasitinae seem to be better supported in respect of illustrated keys, although I am conscious that the document I have access to is a little dated and doesn't cover more recently recognised species or genera, which is a pity. I'm under no illusions of being able to identify any of the individuals I photograph to their genus, but the most likely seem to be Parasitus where there are five or possibly six relatively common species, and a handful of others that I believe are also frequently recorded in the Parasitellus, Poecilochirus and Porrhostaspis genera. Three other genera should also be mentioned, Eugamasus, Gamasodes and Vularogamasus, but the more I read through the keys and any information I can find on the internet, the more I think that subfamily identification is about as good as it will get.

Parasitinae > undetermined genera (female)

Parasitinae > undetermined genera (male)

In the Pergamasinae on the other hand there are a couple of genera that can be identified. The most frequent is Pergamasus which have a distinctive look about them - pear (pyriform) to tear-drop shaped idiosoma, orange-brown colour and a very hairy rearend (opisthonotal region hypertrichous). The other is Holoparasitus, which has a well-sclerotized globular-shaped body unlike other members of the family.

Pergamasinae > Pergamasus sp. (female)

Pergamasinae > Holoparasitus sp.

An interesting observation that could help with identifying the likely subfamily (useful with males) although certainly not something that can be relied on, relates to where the different groups are most likely to be found. Whereas "members of the Pergamasinae primarily inhabit relatively stable forest and grassland humous, moss and soil ecosystems, the Parasitinae species more commonly occur in temporary accumulations of organic debris, including manure, vegetable compost and beach-wrack, and in the nest habitats of mammals and insects". I've quoted the wording directly from the Manual of Acarology to ensure there's no misinterpretation. It stands up reasonably well with many of the individuals I've found, but there are certainly some where I can't really see a connection. However, if I look at my photos of Parasitinae species I see that quite a lot were taken at a couple of particular locations in the New Forest. The habitat would appear more suitable for Pergamasinae until you take into account that free-roaming ponies are always grazing in these areas thereby providing a constant source of manure, and given that these ponies do not receive regular doses of wormer, which contain drugs to kill internal parasites, the composition of their dung is not toxic to the organisms that feed off it. I guess therefore, that certain Parasitinae species have worked this out and are probably well established in many areas of the forest.

Pergamasinae > cf. Pergamasus sp. (nymph)

Pergamasinae > cf. Pergamasus sp. (female with prey)

Location / habitat notes: There are too many photos here to detail where each one was taken but, following on from the previous note, I can confirm that none of the individuals I've captioned as Parastinae sp. have been found in any woodland location other than the New Forest. Coincidence maybe, but just from the few Pergamasinae > Pergamasus sp. featured here I can see that I've photographed them at various local woodland sites, both broadleaved and coniferous, as well as in the New Forest.

PARASITIFORMES > Mesostigmata

Monogynaspida > Uropodina > Uropodiae > Uropodoidea

The Uropodina infraorder (cohort) encompasses three superfamilies; the largest and most significant, and probably the only one that will be represented here, is Uropodoidea, which includes 14 families with 40 or so genera and around 90 species that are commonly known as 'turtle mites'.

They are readily distinguished, having a mainly yellow or reddish-brown coloured, strongly-sclerotized, distinctive disc- shaped 'turtle-like' body and short legs; variable in size from 300-1100µm; idiosoma usually oval or circular in outline, covered by a large dorsal shield that may be fused anteriorly with separate marginal shields; gnathosoma largely hidden.

Notwithstanding the fact that most of these species are too small to photograph, they are not going to be encountered that often and without expert help they're certainly not going to be identified. The family relationships are unstable and different authors may have conflicting opinions regarding the systematic arrangement and, as such, most photographed individuals should simply be referred to as uropodines or uropodine 'turtle-mites'.

Uropodoidea > Trematuridae > Oodinychus sp.

Uropodoidea > Trematuridae > Oodinychus sp. (+ Pergamasus sp.)

Uropodoidea > Trematuridae > Oodinychus sp.

However, one of the world's leading authorities on all of these species Jenö Kontschán has looked at some of these photos and has confirmed that they are in the Oodinychus genus, which would place them in the Trematuridae family. The sole British representative of that family is Oodinychus janeti. Visible features include an oval-shaped idiosoma covered with tiny pits, and short, needle-like dorsal and marginal setae.

I've also photographed some very similar looking individuals in the New Forest, but they could well be a different species.

Uropodoidea > undetermined genera / family

Uropodoids can be found in decaying organic substrates such as rotting wood, probably feeding on fungi as seen here, but also in manure, and within leaf-litter, mosses or beach wrack, or within animal and bird nests. The deutonymphs often disperse by attaching themselves to various host species via a small anal pedicle.

Uropodoidea > Trematuridae > Oodinychus sp.

Location / habitat notes: The Oodinychus sp. were found on a very small piece of dry deadwood in coniferous woodland at White Sheets Plantation, which borders Holt Heath in East Dorset. The undetermined individuals were again photographed deadwood, but this time laying in leaf-litter in primarily broadleaved woodland within the South Oakley inclosure in the New Forest near Burley.

PARASITIFORMES > Mesostigmata

Sejida > Sejina > Sejoidea > Sejidae

Sejus togatus is a predatory species that is usually found under bark of decaying timber or in knotholes, and occasionally in forest leaf-litter. They are relatively large in comparison with most mesostigmadid mites with females typically around 1mm. Although individually variable, they are distinctive and, as they are the only member of the family, are pretty much unmistakeable. They have a densely setose body with a fragmented dorsal shield, and a pair of hornlike posterolateral processes bearing distal setae. The deutonymphs are often phoretic on beetles and flies.

As well as being the only British representative of the Sejidae family it is also only one of two species within the whole Sejida infraorder. As far as I'm aware, the other species Asternolaelaps fecundus, which is in the closely related Ichthystomatogasteridae family, is very rarely recorded and unlikely to be encountered let alone photographed.

Sejidae > Sejus togatus

Location / habitat notes: There are two different individuals featured here that were found on separate visits to Barrow Moor in the New Forest but, coincidently or not, were both photographed under loose bark on the same large fallen oak tree that came down in high winds quite a few years. The bark remained intact for a long while, but is now falling away at the slightest touch in some places. It supports a lot of life although surprisingly not that many mite species that I've seen. I have however found a lot of mites in the general vicinity of the tree, but mostly on small pieces of deadwood lying in the leaf-litter.

Created May 25 | updated June 25 (v.2)