The Microzetid Enigma

The armoured mites of the Oribatida include their fair share of ornately ornamented species but perhaps the most grotesque of all are to be found under members of the family Microzetidae. These typically fairly small oribatids (the average size is about a third of a millimetre) are primarily found in soil and litter deposits around the world. They include a handful of species found in the far north but are primarily found in warmer regions with the greatest known diversity in the Neotropics (Woas 2002).

Dorsal, ventral and lateral views of Acaroceras galapagoensis, from Heinrich Schatz & Jose Palacios-Vargas.

The microzetids are primarily distinguished by elaborate outgrowths of the cuticle around the front of the body. In many oribatids, a pair of thin lamellae run down either side of the prodorsum (the part of a mite that might at first glance be taken for the 'head'). In microzetids, these lamellae have become massively enlarged and detached from the prodorsum over much of their length. As a result, they form a kind of hood over the front of the body. They are flanked on either side by similar lateral extensions called tutoria. The prodorsum as a whole is often remarkably large compared to the rear part of the dorsum, the notogaster. Indeed, the notogaster is often as wide as or wider than it is long. A pair of wing-like extensions, pteromorphs, extend on either side of the front of the notogaster; in microzetids, the pteromorphs are typically sharply pointed. To top all these excrescences off, the insertions of the first pair of legs are also shielded by well-developed flanges called pedotecta.

What, if anything, is the purpose of all these anatomical extravagances is a question I am unable to answer: whether they are related in some way to defense or water retention, for instance. They also make it difficult to understand the position of microzetids relative to other oribatids. The presence of pteromorphs has commonly been thought characteristic of a group of oribatids that have been referred to as the Poronoticae. However, microzetids lack any sign of another distinctive feature of poronotic oribatids: the array of glandular openings on the cuticle known as the octotaxic system. Some oribatids are known to have reduced octotaxic systems, and microzetids do bear a certain resemblance to a definitely poronotic family in the Oribatellidae, so it is possible they represent poronotic mites in which the octotaxic system has been lost. However, other features of microzetids further support affinities outside the Poronoticae. In particular, nymphs of microzetids carry scalps. As they moult from one instar to the next, the shed cuticle of the notogaster is retained in place like a cap. Over successive instars, this cap becomes a stack of scalps that potentially assist in defence (a would-be predator attempting to grab onto the notogaster finds itself holding only an empty scalp). This is generally thought to be a primitive bahaviour that was lost in the ancestor of the poronotics. So are the microzetids primitive relatives of the poronotics, descended from ancestors that had acquired pteromorphs but not yet lost the scalp-carrying habit? Are they derived poronotics that eschewed the octotaxic system and taken up their scalps once more? Further research into oribatid phylogeny is needed to know.

REFERENCE

Woas, S. 2002. Acari: Oribatida. In: Adis, J. (ed.) Amazonian Arachnida and Myriapoda: Identification keys to all classes, orders, families, some genera, and lists of known terrestrial species pp. 21–291. Pensoft: Sofia.

Atropacarus

The little guy pictured above (photo copyright Scott Justis) is a representative of the box mite genus Atropacarus, members of which can be found in most parts of the world. Atropacarus is a genus of the Phthiracaroidea, a group of box mites characterised by the plates on the underside of body being relatively wide, in contrast to the narrow ventral plates of its sister group, the Euphthiracaroidea (members of which have featured on this site before: here and here). The difference in configuration of these plates reflects a difference in the way that the body is contracted to allow legs and prosoma to be withdrawn beneath the protective cover of the notogaster. In euphthiracaroids, the sides of the notogaster are contracted inwards; in phthiracaroids, the ventral plates of the body are lifted upwards (Schmelzle et al. 2015).

The classification of phthiracaroids is subject to conflict with two main systems in the recent literature. In one, championed by the Polish acarologist Wojciech Niedbała, the phthiracaroids are divided between two families with Atropacarus in the Steganacaridae. Species of Atropacarus have the surface of the notogaster extensively covered with dimples. The dorsal seta on the tibia of the fourth leg is short and closely associated with a solenidion (a type of specialised sensory hair). The setae of the genital plate are arranged in a more or less straight row along the inner margin of the plate with the fifth and sixth setae further apart than the fourth and fifth (Niedbała 1986). Niedbała divides Atropacarus between two subgenera. In Atropacarus sensu stricto, there are sixteen or more pairs of setae on the notogaster and the second adanal seta is moved inwards on the ano-adanal plate to form a more or less straight line with the anal setae. In Hoplophorella, there are fifteen pairs of setae on the notogaster and the second adanal seta is distinctly laterally placed relative to the anal setae.

The super-hairy Atropacarus niedbalai, from Liu & Zhang (2013). Scale bar = 100 µm.

In the competing system, used for instance by Subías (2019), Atropacarus and Hoplophorella are treated as distinct genera and each is in turn divided into subgenera by the number of setae on the ano-adanal plate. To a certain extent, of course, the question of whether to treat Atropacarus and Hoplophorella as genera or subgenera is arbitrary. Nevertheless, this arguably cosmetic distinction does relate to an underlying difference in theory. The classification of phthiracaroids used by Subías (2019) is a largely diagnostic one, inspired by a desire to facilitate specimen identifications. Niedbała's classification, in contrast, is intended to reflect phylogenetic relationships. Simple setal counts may be convenient when composing keys but one might question its overall phylogenetic significance. Neotrichy (increases in setal count by multiplication of the original setae) is not uncommon in phthiracaroids, particularly on the notogaster. Setal counts may vary between individuals of a single species and overall neotrichy reaches an extreme in the New Zealand species Atropacarus niedbalai. In this species, the basic count of fifteen or sixteen pairs of notogastral setae has been increased to 109 or 115 pairs, with further neotrichy on the prodorsum and ventral plates (Liu & Zhang 2013). Subías (2019) defends his choice of classification by arguing that Niedbała's key features are often difficult to discern. I sympathise with the difficulty but, as a wise man once said, species are under no obligation to evolve with regard to the convenience of taxonomists.

REFERENCES

Liu, D., & Z.-Q. Zhang. 2013. Atropacarus (Atropacarus) niedbalai sp. nov., an extreme case of neotrichy in oribatid mites (Acari: Oribatida: Phthiracaridae). International Journal of Acarology 39 (6): 507–512.

Niedbała, W. 1986. Système des Phthiracaroidea (Oribatida, Euptyctima). Acarologia 27 (1): 61–84.

Schmelzle, S., R. A. Norton & M. Heethoff. 2015. Mechanics of the ptychoid defense mechanism in Ptyctima (Acari, Oribatida): one problem, two solutions. Zoologischer Anzeiger 2015: 27–40.

Subías, L. S. 2019. Nuevas adiciones al listado mundial de ácaros oribátidos (Acari, Oribatida) (14a actualización). Revista Ibérica de Aracnología 34: 76–80.

Caloppiidae

The concept of ranks in taxonomy is ultimately an arbitrary one. There is no real definition of what constitutes an 'order', a 'family' or a 'subfamily'. What determines the rank that a given taxon is recognised at is a combination of tradition, convenience, and the taxon's relationships to other recognised taxa. As such, the question of whether a given classification is overly 'split' or 'lumped' is a meaningless one and arguing the point is a complete waste of time. That said, the classification of the 'higher' oribatid mites is massively oversplit.

A big part of the reason why oribatid classification seems such a mess, with large numbers of small families containing only a handful of genera and/or species apiece, can be attributed to simple ignorance. We simply do not have a good handle on how many oribatid taxa are related to each other and as a result we find ourselves with a great many orphan taxa still hunting for a good home. The Caloppiidae may be regarded as one such taxon.

Dorsal view of Luissubiasia microporosa, from Ermilov (2016). Scale bar = 100 µm; labels with 'A' indicate areae porosae.

Caloppiids are a pantropical group of about thirty species of poronotic oribatids (the group of oribatids exhibiting the octotaxic system, an arrangement of glandular openings on the notogaster), with three genera recognised in the family by Ermilov (2016): Zetorchella, Brassiella and Luissubiasia. Zetorchella, which includes the majority of the family's species, is also pantropical in distribution. Brassiella is known from the Indo-Pacific region and Liussubiasia is known from a single species from Cuba. Past authors have often referred to Zetorchella and the Caloppiidae by the names Chaunoproctus and Chaunoproctidae, respectively, but as the name Chaunoproctus had already had dibs called on it before the mite was named (by a bird, the now-extinct Bonin grosbeak Chaunoproctus ferreorostris), their respective most senior synonyms have to take over. Caloppiids are more or less egg-shaped in dorsal view. They lack the distinct pteromorphs of most other poronotics though they may have quadrangular projections in the humeral region (the 'shoulders'). The integument is usually heavily sculpted and foveate. The legs end in three claws apiece. The most characteristic feature of the group is that the openings of the octotaxic system on the notogaster, of which five pairs are present, are extremely small. The octotaxic system can take two forms, recessed saccules or porose patches. Those of caloppiids have usually been described as saccules but Ermilov (2016) states that, at least in some species, they are very small porose areas.

Going by their overall appearance, caloppiids are classified within the superfamily Oripodoidea. However, one of the most characteristic features of the Oripodoidea as an evolutionary group is that their nymphs have notogastral setae borne on individual off-centred sclerites (oribatid nymphs often look very different from their adults and are often more soft-bodied). At this point in time, we simply do not know what the nymphs of caloppiids look like so we cannot say whether they possess this crucial feature. Conversely, with their lack of pteromorphs, caloppiids bear a distinct similarity to the more diverse oripodoid family Oribatulidae. The two families have mostly been separated on the basis of caloppiids supposedly having an octotaxic system of saccules rather than porose areas, a distinction that I've already noted may not hold up. There's also something of an open question whether the distinction between saccules and porose areas is really as significant as it has been thought in the past. So, at present, we can't say with confidence whether caloppiids are true oripodoids... or whether they are not only oripodoids but don't even warrant recognition as a distinct family from oribatulids.

REFERENCE

Ermilov, S. G. 2016. Luissubiasia microporosa gen. nov., sp. nov. (Acari, Oribatida, Caloppiidae) from Cuba. International Journal of Acarology 42 (2): 127–134.

Oribatid Time Again

The oribatid mite genus Neogymnobates was first recognised from Illinois in 1917. Since then, the genus has been found to be more widespread in North America and has also been described from Korea and Tibet. Species of Neogymnobates are known from arboreal habitats or in association with fallen wood, and live as grazers of micro-vegetation such as lichens.

Neogymnobates luteus, copyright Monica Young.

Neogymnobates belongs to the Ceratozetidae, a diverse family of oribatids whose characteristic features include a tutorium (a projecting tooth-like structure) on the side of the prodorsum and immovable pteromorphs on either side of the front of the notogaster. Neogymnobates has the lamellae on either side of the prodorsum widely separated from each other and connected by a transverse translamella at the front. There are thirteen pairs of setae on the notogaster and four pairs of porose areas (Balogh & Balogh 1992). One species, N. marilynae of British Columbia and Washington State, is known to have an extra unpaired porose area on the midline near the rear of the notogaster (Behan-Pelletier 2000), an unusual feature among oribatids but one whose significance is uncertain). Their legs end in three claws, a feature that (as I've commented before) correlates with their arboreal habits.

Half a dozen species of Neogymnobates have been recognised to date (Subías 2004). The species are distinguished by features such as the size and appearance of the setae, and the development of the prodorsal lamellae and translamella. One Korean species, N. parvisetiger, has been awarded its own subgenus Koreozetes due to its particularly small, almost indiscernable notogastral setae and its anteriorly notched rather than rounded rostrum (Aoki 1974). Most species are only known from limited ranges except one, N. luteus, for which separate subspecies have been recognised in northern North America and in Korea. Rather unexpectedly, this last species has also recently been recorded from Zanzibar (Ermilov & Khaustov 2018). This is a remarkable range increase, both geographically and ecologically (enough so that I can't help feeling it would benefit from double-checking) that raises the possibility that we may yet have a lot to learn about this oribatid genus.

REFERENCES

Aoki, J. 1974. Oribatid mites from Korea. I. Acta Zoologica Academiae Scientiarum Hungaricae 20 (3–4): 233–241.

Balogh, J., & P. Balogh. 1992. The Oribatid Mites Genera of the World vol. 1. Hungarian Natural History Museum: Budapest.

Behan-Pelletier, V. M. 2000. Ceratozetidae (Acari: Oribatida) of arboreal habitats. Canadian Entomologist 132: 153–182.

Ermilov, S. G., & A. A. Khaustov. 2018. A contribution to the knowledge of oribatid mites (Acari, Oribatida) of Zanzibar. Acarina 26 (2): 151–159.

Subías, L. S. 2004. Listado sistemático, sinonímico y biogeográfico de los ácaros oribátidos (Acariformes, Oribatida) del mundo (1758–2002). Graellsia 60 (número extraordinario): 3–305.

Scheloribates

Over the years, I've put up several posts about the diversity of oribatid mites. It's time for another one.

Scheloribates laevigatus, copyright R. Penttinen.

One of the largest genera of oribatids out there is the genus Scheloribates, for which well over 200 species have been described. Their distribution is pretty much worldwide; they are found in a range of microhabitats, such as in leaf litter, in pastures or marshes, or among rocks. Distinguishing features of the genus from other oribatids include well-developed, immobile pteromorphs, tridactylous (three-clawed) legs, and a notogaster with ten pairs of setae and three pairs of sacculi (little sac-shaped glandular openings) (Ermilov & Anichkin 2014).

Considering their abundance in soil habitats, Scheloribates probably have a significant role to play in decomposition and nutrient cycling. Studies on the diet of one of the better-known species, S. laevigatus, have found that it will eat almost any type of vegetable or fungal matter, though its preferred diet is microscopic algae (Hubert et al. 1999). Indeed, they are most abundant in damper habitats that would provide good conditions for the growth of such algae.

Scheloribates species may impact on human lives in other ways too. They are an intermediate host for the larvae of anoplocephalid tapeworms that infect livestock when the mites are accidentally ingested during grazing. S. laevigatus is a known host for at least eight tapeworm species in North America. Rates of tapeworm infestation in Scheloribates can be quite high; over 60% of the individuals of one species at a particular locality in Australia were infected (Lee & Pajak 1990). Scheloribates species are also noteworthy as a likely source of the toxic alkaloids found in the skin of arrow-poison frogs. The alkaloids are likely to be synthesised by the mites (as suggested by their presence in adults but not in juveniles, despite no known difference in diet between the two life stages) and then sequestered by the frogs after they eat the mites (Saporito et al. 2011). And if they eat enough mites, they end up becoming dangerous even to something the size of a human.

REFERENCES

Ermilov, S. G., & A. E. Anichkin. 2014. A new species of Scheloribates (Scheloribates) from Vietnam, with notes on taxonomic status of some taxa in Scheloribatidae (Acari, Oribatida). International Journal of Acarology 40 (1): 109–116.

Robert, J., V. Šostr & J. Smrž. 1999. Feeding of the oribatid mite Scheloribates laevigatus (Acari: Oribatida) in laboratory experiments. Pedobiologia 43: 328–339.

Lee, D. C., & G. A. Pajak. 1990. Scheloribates Berlese and Megascheloribates gen. nov. from southeastern Australia, with comments on Scheloribatidae (Acarida: Cryptostigmata: Oriopodoidea). Invertebrate Taxonomy 4: 205–246.

Saporito, R. A., R. A. Norton, N. R. Andriamaharavo, H. M. Garraffo & T. F. Spande. 2011. Alkaloids in the mite Scheloribates laevigatus: further alkaloids common to oribatid mites and poison frogs. Journal of Chemical Ecology 37: 213–218.

Lasiobelba: the Oppiid Way

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Lateral view (minus legs) of Lasiobelba pontica, from Vasiliu & Ivan (2011).

The animal illustrated above is a typical representative of Lasiobelba, a cosmopolitan genus of oribatid mites. Lasiobelba includes over thirty species of the family Oppiidae (Ermilov et al. 2014), commonly recognised as the most diverse family of oribatids. Oppiids are inhabitants of soils, where they primarily feed on fungi. Distinctive features of Lasiobelba within the Oppiidae include the absence of costulae (thickened ridges) on the prodorsum, and the presence of nine to ten pairs of setae on the notogaster that are inserted in two or four subparallel rows. The bothridial setae (the large sensory setae near the corners of the prodorsum) may be spindle-shaped at the ends or linearly hair-like; the two bothridial morphologies are used to distinguish two subgenera Lasiobelba and Antennoppia, respectively.

As is common for oribatids, there doesn't seem to be much information available for this genus beyond taxonomic studies. Lasiobelba species are most diverse in tropical and subtropical regions, with few reaching colder parts of the world. When they described the species L. pontica from the Movile Cave in Romania, Vasiliu & Ivan (2011) noted that this genus was otherwise unknown from the country. They suggested that this species might represent a relict of a warmer era that had managed to survive in the stable environment of the cave system after inclement conditions had driven it from the surface.

REFERENCES

Ermilov, S. G., U. Ya. Shtanchaeva, L. S. Subías & J. Martens. 2014. Two new species of oribatid mites of Lasiobelba (Acari, Oribatida, Oppiidae) from Nepal, including a key to all species of the genus. ZooKeys 424: 1–17.

Vasiliu, N. A., & O. Ivan. 2011. New oppiid species (Acari, Oribatida, Oppiidae) from Romanian caves. Trav. Inst. Spéol. "Émile Racovitza" 50: 3–14.

Mites from a Land of Ice and Snow

Mycobates sarekensis, from Siepel & Dimmers (2010).

The Arctic tundra is not an inviting place. Cold winds sweep through a forebidding landscape, unhindered by forest. In places, patches of bare rock can be seen, with no vegetation able to retain a foothold other than hardy lichens. And yet even here you can find an entire ecosystem in place if you look closely enough.

The animal shown at the top of this post is an oribatid mite of the genus Mycobates, a group of about 35 species belonging to the family Punctoribatidae (sometimes referred to as Mycobatidae). These are sturdy, stocky oribatids with a body that is oval in cross-section, with a length generally around the half-millimetre mark (Seniczak et al. 2015). Characteristic features of Mycobates include pteromorphs (triangular outgrowths of the body wall that hang down over the bases of the legs) that are hinged by a line of weaker cuticle so they can be moved up and down, a convex pedotectum I (another protruding shelf, this time on the underside of the body shielding the base of the first pair of legs) and overlapping lobes at the back of the body where the dorsal shield (the notogaster) overhangs the edge of the venter (Behan-Pelletier 1994). Their legs are curved inwards towards the body, and the dorsal setae are usually smooth, without barbs, and flexible (Seniczak et al. 2015).

SEM of Mycobates beringianus from Behan-Pelletier (1994). Note the cluster of pores visible as a lighter patch on the side of the notogaster; these are secretory or respiratory structures.

Many of these features are suited to the preferred habitat of a number of species in the genus: burrowing through the thalli of lichens (which are both home and food). Mycobates species are found in cooler boreal and alpine habitats. Species found in more Arctic habitats, such as the tundra-dwelling M. sarekensis, are found close to ground level (in the tundra, there's not many other levels to be found at). In more temperate regions, they are often arboreal, crawling about on the trunks and branches of trees. Some species have been found in association with mosses as well as lichens; many of the northern species are able, snuggled as they are in their moss or lichen hosts, to live in microhabitats too dry for many other invertebrates.

REFERENCES

Behan-Pelletier, V. M. 1994. Mycobates (Acari: Oribatida: Mycobatidae) of America north of Mexico. Canadian Entomologist 126: 1301–1361.

Seniczak, S., A. Seniczak & S. J. Coulson. 2015. Morphology, distribution and biology of Mycobates sarekensis (Acari: Oribatida: Punctoribatidae). International Journal of Acarology 41 (8): 663–675.