Field of Science

These Ants Must Be Crazy

Black or longhorn crazy ant Paratrechina longicornis, copyright Efram Goldberg.


I have to admit that my ant-identifying skills are fairly rudimentary. I can recognise some of the more distinctive and/or common varieties—meat ants, bull ants, strobe ants, maybe even green-headed ants—but that's about as far as it goes. One ant species that I would have a decent chance of recognising right off the bat, however, is the black crazy ant Paratrechina longicornis.

Black crazy ants are an excellent example of what ant experts refer to as 'tramp species'—generalist species that have spread over a wide range in association with humans. Indeed, the black crazy ant is believed to be the most widespread of all ant species (Wetterer 2008): in tropical regions, it is nigh-on ubiquitous, and in cooler regions it lives within buildings and other warm structures built by humans. So widespread is it, and so readily does it spread, that we can't say for absolute certain where it originally came from: most likely it originated somewhere in south-east Asia, but other possibilities have been considered over the years.

Black crazy ants belong to the ant subfamily Formicinae; as such, they lack the sting carried by ants of other subfamilies and instead have a nozzle-like pore in its place that they use to spray formic acid at perceived threats. They are distinguished from other ants by their slender appearance, with numerous upright bristles on the body, and long legs and antennae. The antennae are most distinctive, with a particularly long scape (the first antennal segment, before the sharp 'elbow'). Paratrechina longicornis are known as 'crazy' ants because of their erratic mode of foraging, wandering about seemingly aimlessly and not following clear trails. Other ants with similar modes of behaviour have also been dubbed crazy ants, such as the yellow crazy ant Anoplolepis gracilipes, but they are not close relatives.

Effectiveness in numbers: black crazy ants bring down a Florida carpenter ant Camponotus floridanus, from AntWeb.


Black crazy ants may form large or small colonies as circumstances allow; part of the secret of their success is that these colonies can be found in man-made marginal habitats such as on ships at sea. Crazy ant colonies may reach plague proportions; this website relates an account of students at a Florida primary school being so beset by crazy ants that food and other possessions had to be kept in sealed bags on tables at all times with the table legs set in bowls of water to prevent the ants crawling up them. Black crazy ants produced winged reproductives like other ants, but the new queens remove their wings before they expand and emerge from the nest already wingless. While at first glance this seems counter-productive, I can see this behaviour being another factor in their success as a tramp. Colonies living in isolated habitatssuch as the aforementioned ships and buildings in cold climates will tend to persist in that location, rather than losing all their reproductive potential in fruitless exploratory nuptial flights.

In recent times, P. longicornis has been recognised as one of a number of species in the genus Paratrechina (of which it is the effective type). However, a phylogenetic study by LaPolla et al. (2010) of the group of genera to which Paratrechina belongs has found that the genus as then recognised was polyphyletic. Rather than being directly related to other 'Paratrechnina', P. longicornis was most closely related to two south-east Asian genera Euprenolepis and Pseudolasius. This lead to the resurrection of two older generic names, Nylanderia and the cringe-inducingly named Paraparatrechina, into which all Paratrechina species other than P. longicornis were transferred.

REFERENCES

LaPolla, J. S., S. G. Brady & S. O. Shattuck. 2010. Phylogeny and taxonomy of the Prenolepis genus-group of ants (Hymenoptera: Formicidae). Systematic Entomology 35: 118–131.

Wetterer, J. K. 2008. Worldwide spread of the longhorn crazy ant, Paratrechina longicornis (Hymenoptera: Formicidae). Myrmecological News 11: 137–149.

Phytoseiids and the Importance of Taxonomy

Gupta's (1975) original figures for Amblyseius syzygii.


For this week's semi-random post, I drew the mite species Amblyseius syzygii. Or perhaps that should be Typhlodromips syzygii, as it's more likely to be designated now. Typhlodromips syzygii is a member of the Phytoseiidae, a diverse family of about 1600 known species of predatory mite. Despite, or perhaps because of, their being an economically significant group (more on that in a bit), phytoseiids have been somewhat plagued by competing nomenclatural systems. Until relatively recently, they were predominately classified into just a few, very large genera. In the last couple of decades, however, there has been a move towards a much more finely divided classification, but you will still find many sources that will continue to use the more conservative system, particularly among those with more of an economic interest in the group than a taxonomic one.

It is as a result of this taxonomic changing of the guard that syzygii, once nestled in the broad genus Amblyseius, has been separated as part of a genus Typhlodromips that was placed by Chant & McMurtry (2005) in an entirely separate tribe from its former host. The genera are separated by features such as the number, shape and proportions of the dorsal setae, and the arrangement of macrosetae on the legs. In fact, counting setae seems to be a major part of taxonomy in the Mesostigmata (the major mite group to which the phyoseiids belong) as a whole, which is part of why this is one group of mites I've so far had difficulty in coming to terms with. Counting setae sounds like it should be easy, but in my experience it's usually not. Especially when the animal is slide-mounted, requiring you to move to focus on the microscope up and down in order to see all the setae, leading to confusion about whether a given visible setae is one you've already counted or not.

Anywho, even after the split, T. syzygii is one of about sixty species in its genus. It was first described in 1975 from West Bengal, from a specimen collected on a jambul tree Syzygium cumini (Gupta 1975; hence the species name). Since then, it has been recorded all around southern and eastern Asia, and from a wide variety of different plants. Distinguishing features of the species include (again) proportions of the dorsal setae, with T. syzygii possessing features such as a pair of large posterolateral serrate setae, as well as details such as the shape of the ventral plates on the body.

A related amblyseiid, Amblyseius swirskii, attacking a thrips. Copyright Steven Arthurs.


I referred before to the economic significance of phytoseiids. This is because a number of species in this family have been utilised as biocontrol agents for plant-feeding mites and other minute pests such as thrips. In some places, you can even buy commercially-produced satchels containing colonies of phytoseiids that can be hung in an orchard and allow the mites to disperse among your trees (arachnids in a bag, people, arachnids in a bag). Alternatively, industrial-size blowers may be used to fire clouds of mites across a crop. I have come across reference to Amblyseius syzygii as a predator of the tea red spider mite Oligonychus coffeae, a significant crop pest. The use of phytoseiids in pest control has, in turn, lead to a massive amount of research on phytoseiid distribution, prey preferences, and pesticide resistances (some phytoseiids can be applied in combination with pesticides that affect the target pest but not the phytoseiids). However, this has also required a lot of attention to phytoseiid taxonomy. For all that phytoseiid species may be obscenely difficult to distinguish, even closely related species may vary significantly in each of the aforementioned factors. For instance, Beard (1999) refers to a number of morphologically all-but-indistinguishable but behaviourally distinct species/populations/whatever that have been identified as the biocontrol 'species' Neoseiulus cucumeris. Some strains might be found only on low-growing plants and never on trees, others may be quite high above the ground. Some may cluster around the flowers of their host plants, others may prefer the fruits and/or young leaves. And they might differ in prey preferences: of two strains found in Britain, one would munch quite happily on the broad mite Polyphagotarsonemus latus, the other would refuse to touch it. Obviously, introducing the wrong strain in a pest control effort could lead to a lot of money being spent on a futile attempt.

REFERENCES

Beard, J. J. 1999. Taxonomy and biological control: Neoseiulus cucumeris (Acari: Phytoseiidae), a case study. Australian Journal of Entomology 38: 51–59.

Chant, D. A., &. J. A. McMurtry. 2005. A review of the subfamily Amblyseiinae Muma (Acari: Phytoseiidae): part VII. Typhlodromipsini n. tribe. International Journal of Acarology 31 (4): 315–340.

Gupta, S. K. 1975. Mites of the genus Amblyseius (Acarina: Phytoseiidae) from India with descriptions of eight new species. International Journal of Acarology 1 (2): 26–45.

Why is an Oak like a Cassowary?

Beach casuarina Casuarina equisetifolia bearing flowers and cones, copyright Atamari.


When one thinks of the Australian vegetation, one might think of towering eucalypts or hardy acacias. One might contemplate unwelcoming spinifex or vibrant grevilleas. But perhaps few groups of plants are so distinctively Australian as the Casuarinaceae, the casuarinas or she-oaks. Members of this family are also found in south-east Asia and the Pacific Islands, but it is in Australia that they reach their highest diversity.

Casuarinas are also unmistakable. They are flowering plants, but they are wind-pollinated and the flowers are highly reduced, being borne in small clusters or spikes. The clusters of fruits, when mature, look more like a miniature pine cone than anything else. The trees that bear these cones also look a bit like pines themselves, with their narrow photosynthetic branches (cladodes) bearing a superficial resemblance to pine needles. The leaves proper are reduced to tiny teeth arranged around nodes or joints on the branches. The outer layer of the cladodes is composed of a thick cortex which together with the needle-like morphology helps resist desiccation. The name of the family refers to the resemblance of their branches to the hair-like feathers of a cassowary Casuarius. Casuarinas are so distinct from other flowering plants that their affinities were long uncertain, though more recent studies have suggested a relationship to other wind-pollinated trees in families such as the Betulaceae (Steane et al. 2003).

In line with their drought-resistant mien, casuarinas are most often found growing in arid and/or coastal regions. The most widespread species, the beach she-oak Casuarina equisetifolia, is found along coastlines from the Bay of Bengal to Polynesia. Their persistance in harsh conditions is also assisted by the presence of nodules on their roots containing bacteria of the genus Frankia, that function like the Rhizobium in root nodules on legumes to fix nitrogen from the atmosphere. Casuarinas also resemble pine trees in forming a mat around their base of fallen cladodes that restricts the growth of competing vegetation.

Desert oaks Allocasuarina decaisneana, copyright Cgoodwin.


Until relatively recently, casuarinas were all classed in a single genus but most authors now recognise four genera in the family. The most distinctive, whose position as sister to the remaining genera is confirmed by molecular analyses (Steane et al. 2003), is Gymnostoma, which contains eighteen species found from south-east Asia to Queensland and Fiji. Whereas other genera of Casuarinaceae have the stomata on the cladodes hidden within deep longitudinal grooves, Gymnostoma has much shallower grooves on the cladodes and the stomata more or less exposed. As such, it is less resistant to desiccation than the other genera. Gymnostoma has four of these grooves on each cladode, corresponding to four leaf-teeth around each node, so the cladodes also tend to have a squarish cross-section.

The second-most divergent genus, Ceuthostoma, contains just two species found from Palawan and Borneo to New Guinea. Ceuthostoma resembles Gymnostoma in having four teeth around each node, but resembles the remaining two genera, Casuarina and Allocasuarina, in having the stomata hidden within deep grooves. In Casuarina and Allocasuarina, the number of teeth around each node is generally increased (up to twenty in Casuarina), meaning that the cladodes are more rounded than square. As noted by Steane et al. (2003), rounder cladodes with more grooves mean that the opening of each groove is narrower, further improving desiccation resistance. Casuarina and Allocasuarina are most readily distinguished by the appearance of their seeds, which are paler and dull in Casuarina but dark brown or black and shiny in Allocasuarina. Allocasuarina is the most diverse genus of the family, with over fifty species endemic to Australia. Casuarina contains fewer species but is more widespread. Steane et al.'s molecular analysis suggested a division within Casuarina between two main clades, one of which was restricted to Australia while the other was primarily composed of Indomalesian species (as well as C. equisetifolia which, as noted above, is found damn near everywhere).

Borneo ru Gymnostoma nobile, from natureloveyou.sg.


Fossils of Casuarinaceae date back to the Palaeocene epoch, and indicate that the family was more widespread in the past with species known from the Eocene of South America and the Miocene of New Zealand. Casuarinaceae-like pollen is also known from the Palaeogene of southern Africa and Antarctica. The South American species have been assigned to the living genus Gymnostoma; the New Zealand species, though originally assigned to Casuarina, is probably also more closely related to Gymnostoma (Zamaloa et al. 2006). Though dominant in the modern flora, the drought-resistant clade of the other three genera is probably of more recent origin, and has probably only ever been unique to the Australasian region.

And I've just realised that I haven't answered the question in the title to this post. As I noted above, an alternate vernacular name for these trees to 'casuarina' is 'she-oak'. I used to wonder why this should be, seeing as casuarinas look about as unlike oaks as you might care to imagine. A good summary of the solution can be found in this newspaper column from the Western Mail of 1914. Though some have suggested that 'she-oak' may be a corruption of an Aboriginal word (despite no such word having been put on record), the more simple explanation is that even if the tree itself doesn't look like an oak, the wood that comes out of it does.

REFERENCES

Steane, D. A., K. L. Wilson & R. S. Hill. 2003. Using matK sequence data to unravel the phylogeny of Casuarinaceae. Molecular Phylogenetics and Evolution 28: 47–59.

Zamaloa, M. del C., M. A. Gandolfo, C. C. González, E. J. Romero, N. R. Cúneo & Peter Wilf. 2006. Casuarinaceae from the Eocene of Patagonia, Argentina. International Journal of Plant Sciences 167 (6): 1279–1289.

Fruit Bats in Africa

A western Woermann's fruit bat Megaloglossus azagnyi clings to the underside of a palm frond, copyright Jakob Fahr.


The diversity of bats often goes under-appreciated. With over 1200 known species around the world (with new ones continuing to be named on a regular basis), they significantly outnumber any of the other traditionally recognised orders of mammals except the rodents. The basal relationships among the bats have been subject to some disagreement in recent years but it is generally agreed that living bats can be divided between three main lineages, one of which is the Pteropodidae, the fruit bats of the Old World. Pteropodids differ in a number of ways from all other bats, the most notable of which being that they mostly do not use echolocation (some do, but not in the same way as other bats: whereas non-pteropodids use sounds produced by their vocal chords for sonar, echolocating pteropodids click with their tongues or clap their wings). Instead, pteropodids use their over-sized eyes to see their way in the dark. The most familiar pteropodids may be the large flying foxes of the genus Pteropus, but they are not the only members of the family.

Not all pteropodids are as large as the flying foxes, either. Megaloglossus is a genus of one or two species of fruit bat found in lowland rain forests in tropical Africa. They are the smallest of Africa's pteropodids (forearm length is about four centimetres, which I'm guessing translates into a wingspan of about a foot?) and differ from other African pteropodids in their slender faces and long, protrusible tongue (the latter feature, of course, explaining the genus name). For a long time, Megaloglossus was classified with other long-tongued pteropodids in a subfamily Kiodotinae, of which it would have been the only African component. However, Bergmans (1997) reclassified it as part of a uniquely African tribe, the Myonycterini, on the basis of features such as partially webbed toes and a ventral collar of thick hair. The 'kiodotines' are now recognised as a polyphyletic assemblage that have evolved their protrusible tongues convergently, presumably to feed on flower nectar.

Woermann's fruit bat Megaloglossus woermanni being handled on a stick by an interfering human, copyright Natalie Weber.


Until recently, only a single species of Megaloglossus was recognised: Woermann's fruit bat M. woermanni, originally named for a specimen from Gabon. A separate subspecies, M. woermanni prigoginei, has been suggested for larger individuals in the eastern part of the larger country now called Congo but Bergmans (1997) noted that the type specimen of the species fell within the size range for 'prigoginei'. A recent molecular study of Myonycterini by Nesi et al. (2013) identified a genetic divide between specimens from Cameroon, Gabon and the Congos on one hand and Liberia and the Côte d’Ivoire on the other, and the authors proposed recognising the latter as a separate species Megaloglossus azagnyi for which they proposed the somewhat awkward vernacular name of 'western Woermann's fruit bat'. However, M. azagnyi is recognised solely on the basis of genetic distance; no morphological distinction has yet been identified between the populations. Nesi et al. (2013) claimed that specimens of M. azagnyi were generally smaller than M. woermanni, but their reported measurement ranges indicate a broad overlap between the two. Bergmans (1997), who examined a greater number of specimens than Nesi et al., suggested a broad cline of increasing size from the west to the east of Megaloglossus' range but no definite gap. Nesi et al.'s division also leaves the status uncertain of populations in intervening regions such as Nigeria. The distribution of Megaloglossus is divided into two by the 'Dahomey gap', a region of dry coastal savannah in Benin, Togo and Ghana that splits the wetter rainforests on either side. However, as noted by Nesi et al. themselves, palaeontological records indicate that the Dahomey gap has not been consistenly present in the past, and it should not be assumed that it corresponds to the division between M. woermanni and M. azagnyi.

REFERENCES

Bergmans, W. 1997. Taxonomy and biogeography of African fruit bats (Mammalia, Megachiroptera). 5. The genera Lissonycteris Andersen, 1912, Myonycteris Matschie, 1899 and Megaloglossus Pagenstecher, 1885; general remarks and conclusions; annex: key to all species. Beaufortia 47 (2): 11–90.

Nesi, N., B. Kadjo, X. Pourrut, E. Leroy, C. P. Shongo, C. Cruaud & A. Hassanin. 2013. Molecular systematics and phylogeography of the tribe Myonycterini (Mammalia, Pteropodidae) inferred from mitochondrial and nuclear markers. Molecular Phylogenetics and Evolution 66: 126–137.

The Neomiodontids: Brackish-Water Bivalves of the Mesozoic

Cast from internal mould of Myrene tetoriensis, from here.


During the early part of the Cretaceous, an area corresponding to the modern Sea of Japan was occupied by a massive brackish-water lake that has been called Lake Tetori, after the geological Tetori Formation that it left behind it. My impression is that Lake Tetori was not an overly hospitable place: warm, shallow, and probably low in oxygen, it was home to a fairly depauperate fauna dominated by only two species of clam, known as Tetoria yokoyamai and Myrene tetoriensis (Kondo et al. 2006). Tetoria I shall leave for another time; Myrene (and its ilk) is the one I want to look at today.

Myrene tetoriensis belonged to a now-extinct family of bivalves known as the Neomiodontidae that lived during the Jurassic and Cretaceous periods. Neomiodontids were most diverse in the northern continents, though species have also been assigned to this family from India and Australia (Moore 1969). Not dissimilar in appearance to a modern pipi, though generally smaller in size, neomiodontids are primarily known from brackish-water or freshwater deposits. The late Triassic/early Jurassic saw something of a flush of bivalve lineages in low salinity environments: the separation of the ex-Pangaean continents resulted in an increase in continental margins, while high carbon dioxide concentrations in the atmosphere stymied the growth of calcium-heavy marine forms (Kondo & Sano 2009).

A rock full of fossils of Neomiodon, from here.


Most neomiodontids were found in sandy habitats, though Myrene tetoriensis lived in mud. They were shallow burrowers, living buried in the sand with the tip of the shell at surface level. Deep-burrowing bivalves possess elongate tubular siphons through which they breath and feed; the mantle boundary inside the shell of such species has a cavity called the pallial sinus into which the siphons can be retracted. In neomiodontids, the pallial sinus is undeveloped, indicating a proportional lack of development of any siphons. Neomiodontids would have mostly been suspension feeders, capturing food particles floating in the water; because of its muddier habitat, Myrene may have been a deposit feeder (Nishida et al. 2013). The narrow, relatively slim shape of neomiodontid shells suggests that they could probably burrow into their substrate rapidly if they became exposed by water action or potential predators.

When the Neomiodontidae was first established as a distinct family, Casey (1955) suggested that it could include the ancestors of the Sphaeriidae, a living group of small freshwater clams known as the pea clams. If this was the case, then neomiodontids did not truly go extinct during the Cretaceous but live on in their descendants. However, more recent authors do not seem to support this relationship. Kondo et al. (2006) note that the decline of brackish-water shallow burrowers such as neomiodontids correlated with the diversification of deeper-burrowing families and suggest a causal connection between the two, but it is worth noting that Tetoria, mentioned above as peaceful cohabitant of Myrene, was a deep-burrower. Nishida et al. (2013) see the extinction of neomiodontids somewhat differently. Citing the often transient nature of the habitats preferred by neomiodontids and other non-marine bivalves, they suggest that the neomiodontids were not a single lineage but represented numerous independent colonisations of non-marine habitats by members of the related marine family Arcticidae, with the 'neomiodontid' habitus the result of convergent evolution.In this view, the reasons for 'neomiodontid' extinction should be sought not with the neomiodontids themselves, but with the extinction of their arcticid progenitors.

REFERENCES

Casey, R. 1955. The Neomiodontidae, a new family of the Arcticacea (Pelecypoda). Proceedings of the Malacological Society 31: 208–222.

Kondo, Y., T. Kozai, N. Kikuchi & K. Sugawara. 2006. Ecologic and taxonomic diversification in the Mesozoic brackish-water bivalve faunas in Japan, with emphasis on infaunalization of heterodonts. Gondwana Research 10: 316–327.

Kondo, Y., & S. Sano. 2009. Origination of extant heteroconch families: ecological and environmental patterns in post-Paleozoic bivalve diversification. Paleontological Research 13 (1): 39–44.

Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology pt N. Mollusca 6. Bivalvia vol. 2. The Geological Society of America, Inc., and The University of Kansas.

Nishida, N., A. Shirai, K. Koarai, K. Nakada & M. Matsukawa. 2013. Paleoecology and evolution of Jurassic–Cretaceous corbiculoids from Japan. Palaeogeography, Palaeoclimatology, Palaeoecology 369: 239–252.

Juniper Gall Midges

Sometimes, the evidence of an insect's presence may be much more visible than the insect itself. Imagine passing by a common juniper tree Juniperus communis and seeing a structure like the one in the photo above (copyright Jean-Yves Baugnée). You might think it was some sort of reproductive structure. You would be right, though it is not the tree that is reproducing. This is the gall of a juniper gall midge Oligotrophus juniperinus, and were you to cut the gall in half you would possibly find a single gall midge larva lurking within. Many insects (and other animals) cause the development of galls on their host plants, thus providing themselves with both shelter and food in one convenient location.

Male Oligotrophus betheli, from Simova-Tošić et al. (2010).


The adult gall midge is a minute, very delicate fly, unlikely to be spotted by the casual observer. Gall midges are classified in the family Cecidomyiidae, an extremely diverse group of which not all members cause galls as larvae (some feed on plants without causing galls, others feed on fungi, a few are even predators or parasitoids). Cecidomyiids are divided between a number of subfamilies and tribes, with Oligotrophus belonging to the tribe Oligotrophini. In the past, this tribe has been used to cover a heterogeneous mix of relatively unspecialised cecidomyiids, but the most recent classification of the tribe strips it down to two genera, Oligotrophus and Walshomyia, found in the Holarctic region (Harris et al. 2006). Adults of these genera have legs with simple tarsal claws and long empodia (the soft pads between the claws), and as larvae they all live in galls on trees of the cypress family Cupressaceae. The exact form of the gall produced may differ between species, and it is often (though not always) possible to determine the species responsible for a gall by its form. For instance, three species that cause galls on Juniperus communis in Europe are Oligotrophus juniperinus, O. panteli and O. gemmarum. The first two species have galls formed from whorls of leaves pressed into a vase shape, but whereas in galls of O. juniperinus the leaves splay outwards towards the tip, in galls of O. panteli they remain parallel. The third species, O. gemmarum, has much smaller galls formed from only slightly modified buds; though very different from mature galls of the other two species, they may be confused with young undeveloped galls (Harris et al. 2006).

Despite their diversity, and the fact that some species are economically significant to humans, cecidomyiids are not a widely studied group. Part of the reason for this is that their small size and build makes them difficult to handle; diagnostic work on adults often requires slide-mounting them. I have made one not-very-successful attempt at slide-mounting cecidomyiids, and I can confirm that it is a fiddly process. Because the different body parts often have to be examined from different angles, slide-mounting first requires dissection of the animal into sections (so, for instance, the head can be placed on the slide face-on, the body side-on, and the terminalia top-up). In my experience, instructions for slide-mounting animals requiring such dissections will always tell you to arrange the various bits appropriately on the cover-slip before placing the slide (or the other way around, if you prefer). And if you know how to attach slide to cover-slip without having all your carefully arranged body parts immediately zooming off to a completely different spot on the slide from where you put them, then you're a far more skillful slide preparer than I am.

REFERENCES

Harris, K. M., S. Sato, N. Uechi & J. Yukawa. 2006. Redefinition of Oligotrophus (Diptera: Cecidomyiidae) based on morphological and molecular attributes of species from galls on Juniperus (Cupressaceae) in Britain and Japan. Entomological Science 9: 411–421.

Simova-Tošić, D., D. Graora, R. Spasić & D. Smiljanić. 2010. Oligotrophus betheli Felt (Diptera: Cecidomyiidae), a new species in the fauna of Europe. Arch. Biol. Sci. 62 (4): 1219–1221.

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.