Mites in Red Velvet

Adult Platytrombidium fasciatum, copyright Walter Pfliegler.

Mites in red velvet,
decorated with stripes.
Completing their diet,
hunting down eggs from flies*.

*With apologies to Justin Hayward.

Among the mites most likely to be seen by the casual observer are the various species of active predators known as red velvet mites. They grow to relatively large sizes for a mite (the species in the photo above can get up to 2.5 mm long), they are brightly coloured and they can often be seen moving about in search of food. As well as the colour, the name 'red velvet mite' refers to their dense covering of setae giving them almost a teddy-bearish appearance. There aren't many mites that could be described as cuddly, but these are arguably among them (at least as adults, as explained below).

Red velvet mites form a number of families in the mite clade Parasitengonina. Earlier posts on this site (here and here) have already described the somewhat complicated life cycles of parasitengonines, but to recap briefly: parasitengonines start their lives as parasitic larvae, followed by a dormant 'pupa-like' stage, followed by an active predatory nymph, then another dormant 'pupa', and finally the active predatory adult. Whereas differences between the active nymphs and adults are slight (kind of raising the question as to why the intervening dormant phase), differences between adults and larvae are significant. From their appearance alone, there is no way of telling whether a given larval form corresponds to a given adult, and connecting the two requires challenging indirect methods such as brood-rearing. Nevertheless, both forms are commonly encountered: not only are adults significant micro-predators, the larvae are often found attachned to insects and other arthropods. Some larval species, commonly known as chiggers, attack vertebrates such as humans and so are even more well-studied. Because of the resulting need to classify both adults and larvae without an easy way to connect the two, a kind of double taxonomy has developed with many parasitengonines. Adults and larvae are treated as if they were separate 'genera' and 'species', with separate names for each. Sometimes a larval 'species' may be successfully connected to an adult 'species' and the two can be synonymised, but many taxa remain that are known only from one or the other.

The genus Platytrombidium, belonging to the velvet mite family Microtrombidiidae, was established in 1936 on the basis of adults, but its larval form was not described until 2005. A number of species have been assigned to this genus from various parts of the world but, as a result of obtaining better descriptions of both adult and larva, Gabryś et al. (2005) restricted it to three species known from the Palaearctic region (Europe and northern Asia). Adult Platytrombidium are characterised by an even covering of stout, uniform setae covered with delicate setules; when alive, they are even more readily recognised from their transverse white stripes across the body. As adults and active nymphs, Platytrombidium fasciatum (the best-known species in the genus and the only one with known larvae) feed on fly eggs. Their larvae are also parasites on drosophilids and similar small flies, most often found attached to the dorsal surface of the abdomen (Gabryś et al. recorded one larva found attached to its host's eye).

Most of the confusion about the taxonomy of Platytrombidium has revolved around its relationship to the very similar genus Atractothrombium. For a long time, the only recognised difference between the two was whether the setae on the body were pointed (Platytrombidium) or blunt (Atractothrombium). Needless to say, this was not a very clear character, and might appear to vary even over the surface of a single individual. Nevertheless, Gabryś et al. (2005) found that they were able to distinguish the type species of the two genera by features of the adult palps and larval claws (Atractothrombium sylvaticum is also evenly dark red, lacking the white stripes of Platytrombidium fasciatum). They also differ in habits: both are predators and parasites of flies but whereas P. fasciatum is found in drier habitats such as gardens and parks, A. sylvaticum prefers damp habitats that flood regularly, such as reed beds and salt marshes.

REFERENCES

Gabryś, G., A. Wohltmann & J. Mąkol. 2005. A redescription of Platytrombidium fasciatum (C. L. Koch, 1836) and Atractothrombium sylvaticum (C. L. Koch, 1835) (Acari: Parasitengona: Microtrombidiidae) with notes on synonymy, biology and life cycle. Annales Zoologici 55 (3): 477–496.