The Prostigmata: Endless Forms

Water mite, possibly Piona coccinea, photographed by Roger Key.

Some groups are just so diverse that it is difficult just to know where to start in introducing them. My topic for today, the mites of the Prostigmata, are definitely one of those groups. Even though few would doubt the coherence of the Prostigmata, their morphological diversity is such that it is difficult to identify features that characterise them all. The majority are small and/or poorly sclerotised mites, but some species are extremely large (by mite standards, at least) and others are heavily armoured. The name 'Prostigmata' refers to the presence in many species of tracheae with the spiracle openings between the cheliceral bases, but many lack tracheae. In one group, the Heterostigmatina, the males and juveniles usually lack tracheae but adult females have tracheae with the spiracles placed at the front of the sides of the body, outside the chelicerae. Prostigmatans include predators, plant-feeders and parasites; their chelicerae, accordingly, may be pincer-like like those of other mite groups, or they may be variously modified. Many groups have the chelicerae fused into a puncturing stylophore; others have the mobile finger adapted into a protruding blade or stylet. The plant-feeding spider mites of the Tetranychoidea have the mobile fingers modified into long thin whips that can be retracted right back into the body, or extended to form the two halves of a sap-sucking tube. Even such features as the number of legs can't always be relied upon: while most Prostigmata have eight legs as is usual for arachnids (though, as with other mites, the fourth pair of legs only develops in the post-larval instars), the hind pairs are reduced or lost in a number of parasitic groups. The gall-forming plant mites of the Eriophyoidea have only four legs at the very front of the body, with an anal sucker at the end of the body to hold them in place.

The labidostomatid Sellnickia, from Macromite.

The phylogeny of the Prostigmata remains poorly known. Six major groups ('cohorts' or 'supercohorts') were recognised in the Prostigmata by Walter et al. (2009), but Dabert et al. (2010) found in their (admittedly somewhat preliminary) molecular analyses that relationships between the groups were not stable with regard to analysis method. These groups are the Labidostomatidae, Eupodides, Anystina, Parasitengonina, Raphignathina and Heterostigmatina. The Labidostomatidae are a small group of heavily armoured predatory mites with chelate chelicerae, found living on or in soil or leaf litter. The other groups, in contrast, are all more diverse.

Bdellid mite feeding on a psocodean, photographed by John J. Kent.

The Eupodides are mostly soft-bodied forms with striated integument. Most have a pair of specialised sensory setae called bothridia on the prodorsum, but these are missing in the Eriphyoidea and the marine Halacaroidea. The snout mites of the Bdelloidea are predatory mites with the chelicerae extended into an elongate proboscis; other members of the Eupodides include plant-feeders, fungivores and parasites.

Rake-legged mite Microcaeculus, photographed by Walter P. Pfiegler.

The Anystina are mostly predatory mites; some species of the families Caeculidae and Anystidae are relatively large, over a millimetre in length. Most Anystina, as well as members of the Parasitengonina and Raphignathina, have the pedipalp developed into what is called a 'thumb-claw process': the tarsus of the pedipalp is offset on the tibia, which has a terminal claw-like seta (sometimes more than one). The tibial 'claw' and the tarsus work together for grasping prey. The Caeculidae, rake-legged mites, are currently particular favourites of mine as my colleagues and I are currently in the process of preparing a description of a new species of one. These heavily sclerotised mites have a double ventral row of large spine-like setae on the forelegs; they sit in place with the forelegs raised until a springtail or some other small animal walks underneath them, at which point they drop the legs like a cage.

Trombidiid velvet mite taking down a micro-wasp, photographed by Jason Green.
The Parasitengonina are most notable for their complex life cycles, with parasitic larvae and free-living predatory adults. The group includes both terrestrial and aquatic species; the aquatic Hydrachnidiae are particularly diverse and often heavily armoured. Differences between larvae and adults are so great that taxonomists have often had no choice but to establish separate classifications for both, with relatively few larval 'species' as yet connected to their corresponding adults. Some of the terrestrial species are particularly large: velvet mites of the Trombidiidae may be over a centimetre in length.

Peacock mite Tuckerella, photographed by Christopher Pooley.
The Raphignathina are another ecologically diverse group: the Tetranychoidea are plant parasites, while other species are animal parasites or free-living predators. Raphignathina may be armoured or soft-bodied; the prodorsum lacks bothridial setae. Vertebrate-associated members of the Raphignathina include the Demodex mites that many people have peacefully living in their hair follicles. Other members of the Raphignathina include the Syringophilidae, bird parasites that live inside the quills of feathers, and the Cloacaridae that can be found in the mucous membranes of a turtle's cloaca.

Broad mite Polyphagotarsonemus latus, from here. Note the oddly stick-like modified hind legs, which are used by the male to carry larval females until they moult to maturity, as also done by the Scutacaridae.

The unusual tracheal system of the Heterostigmatina has already been referred to; this group also includes both free-living and parasitic species, with many species found in association with insects. Most species have a dorsal covering of sclerotised plates, and the palps are often greatly reduced. Heterostigmate mites described in previous posts are the Pygmephoroidea and Acarophenax.

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: 222-241.

Walter, D. E., E. E. Lindquist, I. M. Smith, D. R. Cook & G. W. Krantz. 2009. Order Trombidiformes. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology, 3rd ed., pp. 233-420. Texas Tech University Press.