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.

Oribotritia: Some Mites Just Want to be Left Alone

I'm aware that a lot of people would not automatically think of mites when categorising cuteness, but I dare you to look at the animal in the photo above (taken by Tom Murray) and tell me it's not adorable. This is a box mite of the genus Oribotritia, a cosmopolitan genus found on all the continents except Australia. More than 80 species of Oribotritia have been described so far, and there's probably more to come. Box mites are armoured mites with a body form known as 'ptychoid'; as described in an earlier post, this means that the legs can be drawn back close to the body, and the prodorsum (the articulated shield at the front of the body covering the mouthparts folded over to cover the soft parts. The figure below from Schmelzle et al. (2009) shows how it works. The mite on the left is Oribotritia banksi, the one on the right is a different ptychoid, Rhysotritia ardua:

This figure also shows some of the distinguishing features of Oribotritia. There are a number of families of ptychoid mites; interestingly, indications are that not all ptychoids are directly related to each other. Instead, the ptychoid morphology has evolved a number of times. Oribotritia belongs to the 'true' box mites, in which the notogaster (the main 'body' of the mite) is a single undivided dorsal plate, while the genital and anal plates are long and together occupy the entire length of the underside. The three main families of true box mites are the Phthiracaridae, Oribotritiidae and Euphthiracaridae. Phthiracarids have the ventral plates broad and the venter overall more or less U-shaped; the other two families have the venter narrow and triangular. In oribotritiids like Oribotritia, the genital and anal plates have another elongate pair of plates running outside them, but in euphthiracarids (like Rhysotritia ardua in the figure above), all the ventral plates have become fused into a single plate pair.

When fully withdrawn into its sclerotised shell, the ptychoid mite is pretty effectively sealed away from would-be predators. As well as this mechanical defense, glandular openings on the side of the notogaster secrete defensive oils to repel predators chemically. The overall aim is that the mite should simply be left unmolested to pursue its own interests: feeding on decaying vegetation.

REFERENCES

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

Schmelzle, S., L. Helfen, R. A. Norton & M. Heethoff. 2009. The ptychoid defensive mechanism in Euphthiracaroidea (Acari: Oribatida): a comparison of muscular elements with functional considerations. Arthropod Structure and Development 38: 461-472.

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.

An Introduction to Malaconothrus

Specimen of Malaconothrus monodactylus, from the Biodiversity Institute of Ontario (M. mollisetosus was listed as a synonym of M. monodactylus by Subías 2004).

Malaconothrus is a genus of about sixty species of oribatid mites found almost worldwide. The only continent from which Malaconothrus species have not yet been recorded is Antarctica, though M. translamellatus is known from Île Amsterdam in the subantarctic Indian Ocean (Subías 2004). Malaconothrus species specialise in damp habitats, often found among moss or in marshes. They are small yellowish mites, often covered with an ornamented cerotegument (a thick waxy cuticle) (Luxton 1987). They are also parthenogenetic, with females laying unfertilised eggs that hatch into more females.

Schematic drawing of Malaconothrus monodactylus (minus legs) from Luxton (1987).

Malaconothrus belongs to a group of oribatids called the Crotonioidea (often also referred to as nothroids). Because crotonioids are long-lived, slow-breeding and poor dispersers, they have received a certain amount of attention as potential indicators of environment health. In the context of the post linked to above, crotonioids are part of the Desmonomata, so outside the large oribatid clade of the Circumdehiscentiae or Brachypylina*. They have broad genital and anal plates that take up the greater part of the underside behind the legs (Balogh & Balogh 1992). Malaconothrus and its most closely related genus, Trimalaconothrus, differ from other crotonioids in having a band of soft cuticle across the underside between the levels of the second and third legs, i. e. they are dichoid rather than holoid (Norton 2001). They also lack bothridia, specialised enlarged sensory setae that are present at the rear of the prodorsum in the majority of oribatids. Malaconothrus and Trimalaconothrus are distinguished from each other by Malaconothrus having one claw at the end of each leg, while Trimalaconothrus has three. Subías (2004) divided Malaconothrus between two subgenera: in Cristonothrus, the dorsum is divided by a pair of longitudinal ridges, but in Malaconothrus sensu stricto there are no dorsal ridges.

*For some reason, oribatids seem to suffer something of an embarrassment of higher taxon names.

Dorsal and ventral view of Malaconothrus rohri from Balogh (1997). Note the pattern of ridges on the dorsum characteristic of Cristonothrus.

Malaconothrus has suffered a certain degree of confusion about its type status (Luxton 1987). When he first established Malaconothrus in 1904 (as a subgenus of Lohmannia), Berlese only listed one name in explicit combination, Lohmannia (Malaconothrus) egregia. However, in his discussion of this species, Berlese compared it to the pre-existing Nothrus monodactylus in a manner that implied the latter should also be included in his new subgenus. Subsequent authors have disagreed over whether L. egregia or N. monodactylus should be regarded as the type species of Malaconothrus, though more recent authors have settled on the latter.

REFERENCES

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

Balogh, P. 1997. New species of oribatids (Acari) from the neotropical region. Opusc. Zool. Budapest 29-30: 21-30.

Luxton, M. 1987. Mites of the genus Malaconothrus (Acari: Cryptostigmata) from the British Isles. Journal of Natural History 21 (1): 199-206.

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.

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.

Life in the Fast Lane (Taxon of the Week: Astigmata)

Amongst the bewildering diversity of mites inhabiting this world, the Astigmata include some of the most significance to humans. This group of 5000+ species (with doubtless many more waiting to be described) has become specialised for rapid development and high fecundity. Originally scavengers on decomposing organic matter, members of some lineages have become parasites on vertebrates.


Dust mites Dermatophagoides pteronyssinus on a bedsheet. Dust mites are common inhabitants of human houses where they feed on particles of organic matter such as flaked skin. For the majority of people, their presence in the house is of no consequence; an unfortunate minority suffer allergies to dust mite waste products. Photo from Time.

Curiously, the soft-bodied, fast-living astigmates are most closely related among other mites to the heavily-armoured, long-lived Oribatida. In fact, both morphological and molecular phylogenetic studies have indicated that astigmates are derived from within oribatids (though recovering this result in molecular analyses is dependent on the analytical method used due to the much faster evolutionary rate of astigmates; Dabert et al., 2010*). Astigmates have been derived from oribatids by a process of neoteny where the characters of nymphal oribatids have been carried over to the adult astigmate (OConnor, 2009). Astigmates have also developed a highly modified deutonymph (the second nymphal stage of development) that is specialised for dispersal through phoresy (hitching a lift on some flying insect). The astigmate deutonymph (referred to by many authors as a hypopus) is generally non-feeding and the well-developed mouthparts present in the earlier protonymph become rudimentary, only to reappear when the mite moults through to the next stage, the tritonymph. In many species, if conditions are favourable and dispersal unnecessary, a protonymph may moult directly into a tritonymph, bypassing the deutonymph stage. Other species will only develop into deutonymphs if a suitable host for dispersal is available. The Psoroptidia, the main vertebrate-associated lineage of astigmates, have dropped the deutonymph from their life cycle entirely.

*And a thank-you to Macromite for notifying me of this paper).


Deutonymphs of Chaetodactylus micheneri on a specimen of the bee Osmia californica. Though it may not look pretty, most phoretic organisms do not actually parasitise their hosts, only using them for transport. Photo from here.

While some phoretic astigmates will attach themselves to any old host, others may be very specialised. The members of the subfamily Ensliniellinae (family Winterschmidtiidae) associate solely with nest-building wasps and bees. The early stages of the enslinielline life cycle occur in the host's brood cell and the mites reaches their phoretic stage when the host larva has matured and is ready to leave the brood cell as an adult wasp (or bee). At that point, the mites cluster in specialised pockets on the host's body called acarinaria. In the wasp Ancistrocerus antilope, only the male wasps emerge from the cell carrying mites in acarinaria behind the wings (the females kill any mites in their brood cell while larvae); when the male mates with a female, its mite passengers abandon him to enter acarinaria around the female's genitalia (Houck & OConnor 2001). In other wasp species, the females carry mites in acarinaria from when they emerge. When the female lays its eggs, the mites leave the acarinaria to be sealed in the new brood cells where they will mate and lay their own eggs.


The scabies mite Sarcoptes scabiei. The Sarcoptidae are a family of parasitic mites that burrow into the skin of mammals. Most species are specialists on a small range of hosts, most commonly bats (for some reason, bats carry an extraordinary diversity of parasites), but S. scabiei is a generalist species that has been found on a wide range of hosts, from humans to wombats. Photo by Louis De Vos.

You might be wondering what the wasp gets out of this arrangement as it is hard to see why it would have developed specialised structures to transport the mites if it was not benefiting somehow. And yet, at best, the mites seem to have no significant effect on their hosts; at worst, they are actively harmful, feeding on the food stores left for the developing larva or on the larva itself (though no parasitic ensliniellines have been known to cause the death of their host). Klimov et al. (2007) have suggested that acarinaria have developed not to facilitate the mites' development but to contain them. The mites cannot break through the walls of the brood cells themselves; they can only be carried by an emerging host. If the mites cluster into acarinaria before the host emerges, they remain with already-infected individuals rather than spreading to their potentially mite-free siblings. Perhaps adaptation is not always a matter of achieving an optimum; perhaps it is sometimes simply a form of damage control.

REFERENCES

Dabert, M., W. Witalinski, A. Kazmierski, Z. Olszanowski & J. Dabert. 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.

Houck, M. A., & B. M. OConnor. 1991. Ecological and evolutionary significance of phoresy in the Astigmata. Annual Review of Entomology 36: 611-636.

Klimov, P. B., S. B. Vinson & B. M. OConnor. 2007. Acarinaria in associations of apid bees (Hymenoptera) and chaetodactylid mites (Acari). Invertebrate Systematics 21 (2): 109-136.

OConnor, B. M. 2009. Cohort Astigmata. In: Krantz, G. W., & D. E. Walter (eds). A Manual of Acarology, 3rd ed., pp. 565-657. Texas Tech University Press.

Mite-in-a-Box

Nothrus truncatus, a member of the holoid Desmonomata. Photo by Agriculture and Agri-Food Canada.

Mites are pretty remarkable creatures. I don't know if any other group of animals can rival mites for ecological diversity. There are mites burrowing in the leaf litter of forests, there are mites living in the sediment at the bottom of the sea, there are mites living off the secretions in your hair follicles, there are mites that live as parasites of other animals. Whatever freakish thing you can think of an animal doing, odds are that there is a mite doing it right now. Mites also include one of the few groups of terrestrial arthropods to have developed a mineralised exoskeleton - the oribatids or beetle mites. Our newest Taxon of the Week is one of the subgroups of oribatids, the Holonota.

The Holonota include the most heavily armoured of the oribatids, with most of the body encased in hardened plates. Morphologically, Holonota are distinguished from other oribatids by having the entire body covered by two dorsal plates, with the division between the two plates between the positions of the second and third pairs of legs. Technically speaking, various Holonota may be dichoid, holoid or ptychoid (Norton, 2001). In dichoid forms, a non-hardened zone runs around the body between the second and third pairs of legs, allowing the body to bend at that position. Holoid forms only have this articulation dorsally, with the ventral surface fused to a solid plate. Ptychoid forms, on the other hand, have reduced the ventral hardening and are actually able to withdraw the legs and close the anterior dorsal plate over them - the mites' answer to ostracods.

Norton (2001) suggests that the development of heavy armour in the oribatids may be related to their lifestyle. Most oribatids are long-lived (at least for mites) and have low reproductive rates, a situation that may result from their usual diet of low-nutrient decaying vegetation and fungi. Slow growth and replacement rates may result in the selective favouring of features that extend the life expectancies of individuals.


The ptychoid mite Atropacarus striculus (Phthiracaridae), showing how the legs can be withdrawn and covered over. Photo from Oribatid Mites of Alberta.

Morphologically, Holonota have been divided between three groups, the Mixonomata, Desmonomata and Circumdehiscentiae (Maraun et al., 2004). The Mixonomata includes dichoid and ptychoid forms, and has been suggested to be paraphyletic to a holoid clade formed by the other two taxa. In turn, the Desmonomata are probably paraphyletic with regards to the Circumdehiscentiae. The holoid Circumdehiscentiae are one of most speciose groups of oribatids, and show the highest degree of plate fusion, with the armour of almost the entire underside fused with the anterior dorsal plate (Norton, 2001). Molecular analyses, in contrast, have been divided in their support for this arrangement. Maraun et al. (2004) failed to support the morphological view, and did not even recover monophyly for the Holonota as a whole or for the Circumdehiscentiae. More recently, however, the morphological phylogeny with monophyletic Holonota and serially paraphyletic Mixonomata and Desmonomata was supported by the results of Domes et al. (2007).

In light of the repeated evolution of ever-greater degrees of sclerotisation within the Holonota, it might seem surprising if one lineage was to do a complete volte-face and lose all trace of armour, but exactly this possibility has been suggested (Norton, 2001). The Astigmata are a lineage of mites related to the Oribatida, but ecologically distinct. While oribatids are armoured, slow-living, litter feeders, astigmatans are unarmoured, fast-breeding and mostly live in close association with other animals, often as parasites. Astigmata include such luminaries as Sarcoptes scabiei, the skin-burrowing monstrosity that causes scabies*. Despite these differences, it has been suggested that Astigmata are actually derived from oribatids through paedomorphosis (retention of juvenile characters as adults) - and specifically from Desmonomata, one of the most heavily armoured groups of oribatids. However, support for Astigmata as derived desmonomates remains equivocal. While supported by some morphological characters and gland chemistry, the suggestion has not garnered molecular support. Mauran et al. (2004) supported an oribatid ancestry for Astigmata, though not necessarily from Desmonomata (they also only included a single species of Astigmata in their analysis). Domes et al. (2007), analysing a larger selection of astigmates, rejected a position for Astigmata within Oribatida.

*Offhand, if you had to invent a name for a revolting skin condition, could you ever come up with a more appropriate-sounding term than 'scabies'?

REFERENCES

Domes, K., M. Althammer, R. A. Norton, S. Scheu & M. Maraun. 2007. The phylogenetic relationship between Astigmata and Oribatida (Acari) as indicated by molecular markers. Experimental and Applied Acarology 42 (3): 159-171.

Maraun, M., M. Heethoff, K. Schneider, S. Scheu, G. Weigmann, J. Cianciolo, R. H. Thomas & R. A. Norton. 2004. Molecular phylogeny of oribatid mites (Oribatida, Acari): evidence for multiple radiations of parthenogenetic lineages. Experimental and Applied Acarology 33 (3): 183-201.

Norton, R. A. 2001. Systematic relationships of Nothrolohmanniidae, and the evolutionary plasticity of body form in Enarthronota (Acari: Oribatida). In Acarology: Proceedings of the 10th International Congress (R. B. Halliday, D. E. Walter, H. C. Proctor, R. A. Norton & M. J. Colloff, eds.) pp. 58-75. CSIRO Publishing: Melbourne.