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.

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.

Neostrinatina mixoppia

Dorsum of Neostrinatina mixoppia, from Mahunka (1978).

Time for another oribatid. This is Neostrinatina mixoppia, a species described as the only member of its genus by S. Mahunka in 1978. It was described on the basis of two specimens from near Coban in the highlands of Guatemala. Neostrinatina belongs to the family Oppiidae, a group of often smaller oribatids with moniliform legs, and is a bit over a quarter of a millimetre in length. N. mixoppia noticeably differs from other oppiids in its long pectinate sensillus on either side of the prodorsum. The other dorsal setae are also particularly long and barbed. Other distinctive features of this species, according to Mahunka, are a pair of lateral teeth on the dorsosejugal suture (the junction between the prodorsum and the notogaster, or what one might think of as the 'head' and 'body' regions of the dorsum) that jut towards the sensilli, and an 'enormous, spiniform excrescence' projecting forwards from the anogenital region. I must admit, though, I've been trying to interpret Mahunka's illustration of the ventral region of N. mixoppia and I'm still not entirely sure what this latter feature looks like. Like other oppiids, the prodorsum does not have the projecting lamellae found in many oribatid families; instead, N. mixoppia has a pair of branching costulae (thickened ridges). The legs each end in a single claw.

Venter of Neostrinatina mixoppia, from Mahunka (1978).

Oppiids are currently recognised as the most diverse family of oribatids with over 1000 known species, the greater number of these found in the tropics. Though the ecology of N. mixoppia itself is unknown, other oppiids feed on fungi. The single claws on the legs suggest a terrestrial habitat. As with many (if not most) oribatid groups, the relationships of oppiids are in great need of revision with many genera being arranged on the basis of potentially convergent characters. Mahunka himself recognised this in his description of N. mixoppia, expressing the opinion that it represented '? mixture of at least three present day " genera"'. The number of dorsal setae suggested one genus, the dorsosejugal teeth suggested another. Perhaps one day we'll know which is which.

REFERENCE

Mahunka, S. 1978. Neue und interessante Milben aus dem Genfer Museum XXV. On some oribatids collected by Dr. P. Strinati in Guatemala (Acari: Oribatida). Acarologia 20 (3): 133–142.

Nosybelba: A Uniquely Madagascan Mite

Dorsal and ventral views of the main body of Nosybelba oppiana, from Mahunka (1994).

Why yes, it's another random oribatid! Nosybelba oppiana was described from Madagascar by Sándor Mahunka in 1994; Mahunka regarded it as distinct enough from other oribatids that he placed it in its own monospecific family. To date, the original description appears to be the sum total of our knowledge of Nosybelba oppiana. Subías et al. (2012) transferred it to a separate subfamily within the larger family Oppiidae, and transferred a second Madagascan species 'Oppia spinipes' Balogh 1964 to Nosybelba, but this was in the context of a species checklist only without supporting discussion (also, the name Oppia spinipes was used for an oribatid species by Banks in 1906, so whatever the status of Balogh's species it needs a new name).

Leg I of Nosybelba oppiana, from Mahunka (1994). Femur and genu of the right, tibia and tarsus on the left.

So what can we tell about Nosybelba from its description? One of the first things that attracts attention is that it has rather weird legs. The tarsi (the terminal segments) of the legs are really short, shorter on all legs than the adjoining tibia. On the first pair of legs, the tarsus is also compressed longitudinally, and a dorsal process on the tibia (that bears a large sensory seta) overhangs the tarsus. To my admittedly uneducated eyes, the overall structure does not give an impression of mobility. I'm guessing that Nosybelba is not the most agile of oribatids. At the end of each leg is a single large claw; as mentioned in a previous post, the number of claws on an oribatid's legs tends to correlate with habitat, with single claws suggesting a terrestrial lifestyle.

Lateral view of front end of Nosybelba oppiana (minus legs), from Mahunka (1994).

Another noteworthy feature of Nosybelba can be found in its mouthparts. The mentum, the 'under-head' shelf that underlies the chelicerae, does not have a basal articulation, so the chelicerae are limited in their range of movement. The chelicerae themselves do not have any teeth, so Nosybelba is not feeding on anything that requires a great deal of processing before swallowing. In another oribatid family, the Suctobelbidae, similar chelicerae are related to a diet of plant matter that is in an advanced state of decay; Nosybelba is presumably also a connoiseur of the rotten and the liquefied.

REFERENCES

Banks, N. 1906. New Oribatidae from the United States. Proceedings of the Academy of Natural Sciences of Philadelphia 58 (3): 490-500.

Mahunka, S. 1994. Oribatids from Madagascar II. (Acari: Oribatida). Revue Suisse de Zoologie 101 (1): 47-88.

Subías, L S., U. Ya. Shtanchaeva & A. Arillo. 2012. Listado de los ácaros oribátidos (Acariformes, Oribatida) de las diferentes regiones biogeográficas del mundo. Monografías electrónicas S.E.A. 4.

The Mites of the Incas

Diagnostic views of Haplozetes similis, from Gil & Subías (1993) as 'Lauritzenia (Incabates) sinuatus'.

The oribatid mite genus Incabates was first established by Marie Hammer in 1961 for a species from Peru (not surprisingly, with that name). Since then, species have been assigned to Incabates from tropical and subtropical regions almost throughout the world (though not, as yet, from the Ethiopian bioregion—Subías 2004). Incabates belongs to the Haplozetidae, an oribatid family distinguished by their possession of well-developed pteromorphs that are often, though not always, mobile, and jointed chelate-dentate chelicerae (Norton & Behan-Pelletier 2009; the nature of oribatid 'pteromorphs' has been explained in an earlier post). Incabates has been distinguished from other haplozetid genera by having a series of dorsal glandular openings on the body developed as tubular saccules, ten pairs of setae on the notogaster (the dorsum of the main body), four pairs of setae around the genital opening, and three claws at the end of each leg (Gil & Subías 1993; Weigmann & Monson 2004). The presence of three claws rather than one claw on each leg appears to be correlated with an arboreal lifestyle in oribatids (Karasawa & Hijii 2008), and Incabates species do appear to be mostly associated with forest habitats.

There is a lot of taxonomic instability within (and indeed, around) the Haplozetidae, and it remains uncertain at this point whether Incabates should be recognised as a distinct taxon. Genera of Haplozetidae have often been distinguished by combinations of characters, often simply numerical (such as numbers of setae) and it may be debatable to what extent these characters reflect actual relationships. The checklist of the world oribatid fauna by Subías (2004) lists Incabates as a subgenus of Lauritzenia. Gil & Subías (1993) separated Lauritzenia (including Incabates) from the related genus Haplozetes by the number of genital setae (four pairs in Lauritzenia vs five in Haplozetes) and then divided each of these genera into subgenera on the basis of claw number (tridactyl Incabates vs monodactyl Lauritzenia subgenus Lauritzenia). However, Weigmann (2010) argued that these characters might be too plastic to warrant generic distinction, and supported treating all as a single undivided genus Haplozetes until the relationships within the group were better established. As yet, that's something we're still waiting on.

REFERENCES

Gil, J., & L. S. Subías. 1993. La familia Haplozetidae Grandjean, 1936 (Acari, Oribatida) en la Península Ibérica. Mediterránea Ser. Biol. 14: 23-30.

Karasawa, S., & N. Hijii. 2008. Vertical stratification of oribatid (Acari: Oribatida) communities in relation to their morphological and life-history traits and tree structures in a subtropical forest in Japan. Ecological Research 23 (1): 57-69.

Norton, R. A., & V. M. Behan-Pelletier. 2009. Suborder Oribatida. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology, 3rd ed., pp. 430-564. Texas Tech University Press.

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.

Weigmann, G. 2010. Oribatid mites (Acari: Oribatida) from the coastal region of Portugal. IV. The genera Coronoquadroppia, Scheloribates, Haplozetes and Pilobates. Soil Organisms 82 (3): 383-406.

Weigmann, G., & F. D. Monson. 2004. A new genus and species of Haplozetidae (Arachnida: Acari) from Great Britain with a key to the European genera. Journal of Natural History 38 (11): 1415-1420.

South American Mites

Diagnostic views of Charassobates cavernosus (clockwise from left: dorsum; prodorsum with covering lamellae removed; venter) from Schuster (1969).

For this week's random taxon, I drew the oribatid mite family Charassobatidae. This has proved to be something of a challenge: info on the charassobatids seems a little hard to come by. It doesn't help that the 'Charassobatidae' has haemorrhaged taxa somewhat. Balogh & Balogh (1992), in their invaluable (if not entirely unproblematic) identification guide to oribatids, listed three genera in the Charassobatidae: Charassobates, Topalia and Ametroproctus. The main reason for associating these genera appears to have been the presence of massively expanded lamellae on the prodorsum (the 'head' part of the mite). However, Behan-Pelletier (1988), in transferring Ametroproctus to the family Cymbaeremaeidae, argued that this feature had probably arisen convergently as similar large lamellae are known from other oribatid families. Topalia has also since been removed from Charassobatidae, which is now redundant with Charassobates. Norton & Behan-Pelletier (2009) placed Charassobates in the superfamily Licneremaeoidea, but the monophyly of that group has been questioned (Schäffer et al. 2010).

Charassobates is a strictly South American group of mites (with an outlying species in the Galapagos). They have pelopsiform (very long and slender) chelicerae, possibly indicating a liquid diet, that are also somewhat unusual among oribatids in lacking an articulation between the chelicerae and the underside of the 'head'. Some species, such as the type Charassobates cavernosus, also possess deep fossae on the dorsal surface of the main body. The nymphs are wrinkly (unlike humans, some oribatids become less wrinkly as they get older) and apheredermous (i.e. they do not carry the shed skins of former instars as protective scalps) according to Norton & Behan-Pelletier (2009) and Behan-Pelletier & Walter (2007), indicating that their description as eupheredermous (carrying scalps) by Behan-Pelletier (1988) was probably an error.

REFERENCES

Balogh, J., & P. Balogh. 1992. The Oribatid Mites Genera of the World. 2 vols. Hungarian Natural History Museum.

Behan-Pelletier, V. M. 1988. Systematic relationships of Ametroproctus, with modified definition of Cymbaeremaeidae (Acari: Oribatida). In: Channabasavanna, G. P., & C. A. Viraktamath (eds) Progress in Acarology vol. 1 pp. 301-308. E. J. Brill: Leiden.

Behan-Pelletier, V. M., & D. E. Walter. 2007. Phylleremus n. gen., from leaves of deciduous trees in eastern Australia (Oribatida: Licneremaeoidea). Zootaxa 1386: 1-17.

Norton, R. A., & V. M. Behan-Pelletier. 2009. Suborder Oribatida. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology pp. 430-564. Texas Tech University Press.

Schäffer, S., S. Koblmüller, T. Pfingstl, C. Sturmbauer & G. Krisper. 2010. Ancestral state reconstruction reveals multiple independent evolution of diagnostic morphological characters in the “Higher Oribatida” (Acari), conflicting with current classification schemes. BMC Evolutionary Biology 10: 246.

Schuster, R. 1969. Die terrestrische Milbenfauna Südamerikas in zoogeographischer Sicht. In: Fittkau, E. J. (ed.) Biogeography and Ecology in South America vol. 2 pp. 741-763. Dr W. Junk N. V.: The Hague.

On a Wing and a Mite

Dorsal and ventral views of Monogalumnella neotricha, with legs removed. Figures from Balogh & Balogh (1992).

In a previous post, I introduced you all to the oribatid mites. Oribatids come in a wide range of varieties, and the animal in the figures above is a member of the oribatid family Galumnellidae. Galumnellids belong to a group of oribatids, the galumnoids, marked by their well-developed pteromorphs: the roughly triangular structures at either side of the front of the body. Many oribatids have pteromorphs developed to a greater or lesser degree, but the pteromorphs of galumnoids are particularly noteworthy for their size and for the development of a hinge between the pteromorph and the main body, so that the pteromorph can be folded down to cover the legs for protection (other species have the pteromorphs as fixed outgrowths of the body). The name 'pteromorph', of course, means 'wing-shaped', and you can readily find cases where galumnoids have been referred to as 'winged mites' (especially in older publications). Woodring (1962) even suggested that galumnoids might provide a useful analogy for the evolution of wings in insects. However, pteromorphs are not actually wings like those of insects, being used only for protection, not flight. In animals as small as oribatids, the relative viscosity of the air becomes very high, not to mention the relative force of small air movements. Vary small arthropods that move aerially either develop long hairs or similar structures so that they can be passively lifted and carried by the breeze (like the line of silk produced by ballooning spiders) or have reduced wings with long fringes of hairs to maintain wing surface area while minimising air resistance (such as mymarid wasps, thrips or ptiliid beetles). A solid plate like the galumnoid pteromorph would be to difficult to move*.

*Similar issues affect suggestions that the absent fossil record of the earliest winged insects may indicate that flight evolved at small sizes. It seems almost certain that the first flying insects were relatively large.

The Galumnellidae can be distinguished from other galumnoid mites by the lack of protruding lamellae on the prodorsum (the top of the 'head'), the pointed rather than rounded rostrum, and the shape of their chelicerae. The chelicerae of galumnellids are long and slender, compared to the shorter, stronger chelicerae of their relatives in the Galumnidae. Galumnella has been shown in the laboratory to be panphytophagous (Badejo & Akinwole 2007)—that is, it will accept any type of plant or algal food, both living and dead.

REFERENCES

Badejo, M. A., & P. O. Akinwole. 2007. Preliminary study of the feeding habits of seven species of oribatid mites from Nigeria. Systematic and Applied Acarology 12: 121-125.

Balogh, J., & P. Balogh. 1992. The Oribatid Mites Genera of the World, 2 vols. Hungarian Natural History Museum: Budapest.

Woodring, J. P. 1962. Oribatid (Acari) pteromorphs, pterogasterine phylogeny, and evolution of wings. Annals of the Entomological Society of America 55 (4): 394-403.