To Dung and Beyond

When most people think of a fly, odds are that they imagine one of the group of flies known as calyptrates. This is the clade that includes, among others, such animals as house flies, blow flies and flesh flies. Calyptrates are often reasonably large as flies go and they often have life styles (such as larvae feeding on decaying matter) that bring them close to humans and their homes. One of the most recognisable features of this clade, and the inspiration for its name, is enlargement of the lower calypter, a lobe at the base of the wing. This lower calypter can be moved semi-independently of the rest of the wing which is how calyptrate flies are able to fly acrobatically and avoid being swatted. Nevertheless, there is one significant subgroup of 'calyptrate' flies that has foregone the advantages of an enlarged calypter, commonly recognised as the family Scathophagidae.

Yellow dung fly Scathophaga stercoraria, copyright Derek Parker.

The Scathophagidae are a modestly diverse family of flies with about 250 known species, the great majority of which are found in the Holarctic region (Vockeroth 1987). Only a handful of species are found in more southerly regions, mostly at higher altitudes. They are medium-sized flies, ranging between three and eleven millimetres in length, fairly similar to a house fly in overall appearance but generally more slender and bristly. They are commonly referred to as 'dung flies', in reference to the larval diet of one of the most widespread and best known species, Scathophaga stercoraria (whose scientific name broadly and appropriately translates as 'shit-eater, thing of shit'). However, despite the unremarkable number of species, scathophagids are actually more diverse in their larval habits. As far as we know, adult scathophagids are all predators on other insects.

Scathophagids are divided between two subfamilies, distinguished by features of the male terminalia. In Scathophaginae, the sixth abdominal tergite (dorsal plate) of the male is hairy and usually separate from the following fused syntergosternite 7 + 8 (with dorsal and ventral plates of the segments fused to form a ring). In Delininae, the sixth tergite lacks hairs and is always fused to the following syntergosternite. The subfamilies also differ in life history. Larvae of Delininae are leaf-miners on monocots, hatching from eggs laid on the leaf surface. The Scathophaginae are more diverse. As already indicated, some are saprobes. As well as the dung-feeding S. stercoraria, the genus Scathophaga also includes species which specialise on rotting seaweed on the sea-shore (a milieu which, offhand, supports a range of fly species belonging to numerous families). Other species are, like the Delininae, miners in plant tissue though they are found in a wider range of hosts (both monocots and dicots) and their eggs are inserted by the female directly into the plant tissue. A handful are aquatic or semi-aquatic predators, feeding on small invertebrates along lake shores or in sewage, or on the eggs of caddisflies in fast-running streams.

Cordilura pubera, a plant-feeding scathophagid, copyright Aleksandrs Balodis.

Considering the more derived character of the male terminalia in the Delininae, and the more disparate life habits of the Scathophaginae, some authors have suggested that the latter may be paraphyletic to the former. It has also been presumed that the Scathophagidae as a whole is ancestrally saprobic, considering that saprobic habits are also the norm in related fly families such as the Muscidae (house flies). However, a molecular phylogenetic analysis of the Scathophagidae by Kutty et al. (2007) supported monophyly of both subfamilies. Their results indicated that the original scathophagids were plant-feeders with saprobic lineages arising within the family on two separate occasions. Predatory larvae also evolved twice, once as a further development from saprobes and once direct from plant-feeding ancestors. The diet of this family started out fresh but, somewhere along the line, some species decided they'd rather eat muck.

REFERENCES

Kutty, S. N., M. V. Bernasconi, F. Šifner & R. Meier. 2007. Sensitivity analysis, molecular systematics and natural history evolution of Scathophagidae (Diptera: Cyclorrhapha: Calyptratae). Cladistics 23: 64–83.

Vockeroth, J. R. 1987. Scatophagidae. In: McAlpine, J. F. (ed.) Manual of Nearctic Diptera vol. 2 pp. 1085–1097. Biosystematics Research Centre: Ottawa.

Flies on Stilts

Flies deserve a much better rep than they're usually given. They are animals of grace and poise that step lightly through the world. And perhaps few flies have an appearance that conveys that grace better than the stilt-legged flies of the Micropezidae. For today's post, I wanted to look at one particular subfamily of micropezids, the Taeniapterinae.

Scipopus sp., copyright Gail Hampshire.

Stilt-legged flies are found in most parts of the world but are particularly diverse in tropical regions. As their name indicates, they are light-bodied flies with notably long legs, the middle and hind legs being much longer than the fore legs. This legginess perhaps reaches its peak in the Madagascan genus Stiltissima, males of which have the hind femora alone at least 2.5 times the length of their thorax (Barraclough 1991). The adults are predators of small insects but are also attracted to decaying fruit or dung. Larvae of the family are little known but indications are that they feed on the aforementioned ordure or other rotting vegetation. Many of them are mimics of wasps such as ichneumons or ants with their slender figure resembling the narrow-waisted appearance of a wasp. Because micropezids belong to the brachyceran lineage of flies, in which the antennae are few-segmented and usually short, the front pair of legs is instead held out in front to imitate the wasp's antennae.

Habitus of Stiltissima violacea, from Barraclough (1991).

The Taeniapterinae are the most diverse of three subfamilies recognised within the Micropezidae. Distinctive features of this subfamily include ocelli sitting relatively forward on the top of the head, a dense vertical fan of bristles on the sternopleuron (the sclerite on the side of the thorax just between the base of the fore and middle legs) and a vestigial subscutellum (Jackson et al. 2015). Though cosmopolitan in distribution, and the only micropezid subfamily known from sub-Saharan Africa (Barraclough 1991; the only non-taeniapterines known from the Afrotropical region are restricted to the Mascarene islands), taeniapterines are most diverse in the Neotropical region.

Mesoconius dianthus contrasted with its ichneumon model Cryptopteryx, from Marshall (2015).

The Taeniapterinae have been divided into two tribes based on the length of the cup cell near the base of the fore wing, the short-celled Rainieriini and the long-celled Taeniapterini (Jackson et al. 2015). All taeniapterines found outside the Neotropical region belong to the Rainieriini, as well as a number of Neotropical genera. The Taeniapterini are restricted to the New World. Genera of Taeniapterinae are often poorly distinguished with the relationships between species obscured by the evolution of features related to mimicking their wasp models. A phylogenetic analysis of selected Taeniapterinae by Jackson et al. (2015) indicated many recognised genera were non-monophyletic. It also cast doubt on the tribal classification with the Taeniapterini rendering the Rainieriini paraphyletic.

REFERENCES

Barraclough, D. A. 1991. Review of the Madagascan Taeniapterinae (Diptera: Micropezidae), with the description of a remarkably elongate-legged new genus and first record of Rainieria Rondani from the subregion. Annals of the Natal Museum 32: 1–11.

Jackson, M. D., S. A. Marshall & J. H. Skevington. 2015. Molecular phylogeny of the Taeniapterini (Diptera: Micropezidae) using nuclear and mitochondrial DNA, with a reclassification of the genus Taeniaptera Macquart. Insect Systematics and Evolution 46: 411–430.

The Asteiids: Overlooked Flies

Flies are incredibly diverse, but they may be one of the least appreciated of the major insect groups. There are many significant fly lineages whose presence goes all but unnoticed by a small number of afficionados.

Asteia amoena, copyright Mick E. Talbot.

One of the largest clades of flies is the Schizophora, including many such familiar animals as house flies, blowflies, and fruit flies (of both varieties). The most distinctive feature marking this lineage is the ptilinal fold, a groove that runs around the face of the flies along the inner margin of the eyes and across above the antennal insertions. This groove marks the position of a large fold of soft cuticle, the ptilinum, that is used by the fly when it emerges from the hardened case, the puparium, in which it metamorphoses from a larva. The ptilinum expands like a bellows as the fly pumps its head full of liquid until the pressure causes the cap of the puparium to pop open. After this, the excess cuticle is folded away inside the head, never to emerge again, but the mark of its presence remains.

House fly Musca domestica emerging from its puparium, showing the inflated ptilinum. Copyright Alex Wild.

Schizophorans have commonly been divided between two main groups, referred to as the calyptrates and acalyptrates. The basis of this division is the presence (calyptrates) or absence (acalyptrates) of the calypters, lobes at the base of the wings that help in controlling flight. This is not an entirely phylogenetic system: the calyptrates are a single clade but the acalyptrates are not. The most familiar flies belong to the calyptrates (which include house flies and blowflies) despite the fact that acalyptrate flies are considerably more diverse. This is in part because many acalyptrates are very small flies, easily overlooked by the casual observer.

The Asteiidae are one such group of overlooked flies. They are found pretty much world-wide and can be very abundant in some habitats. Nevertheless, they are apparently not common in collections: their soft-bodiedness makes them tricky to preserve, and Grimaldi (2009) noted that tropical species living on rolled leaves of herbs such as bananas and gingers were reluctant fliers and so unlikely to be collected by passive intercept traps. Noteworthy features of asteiids compared to similar flies include a reduced wing venation, and an antennal arista bearing alternating rays (Friedberg 2009). In two genera, Polyarista and Anarista, the arista is reduced or lacking, replaced by a collection of long setae arising from the first flagellomere (Papp 2013).

Diagnostic features of Asteia amoena, from Walker's Insecta Britannica Diptera.

Because of their low collection rates, the natural history of asteiids is poorly known. As already noted, a number of species are found in association with vegetation; others have been raised from fungi. Grimaldi (2009) described Asteia species running "over the surface of a leaf in all directions with uniform effort, including backwards and sideways, which gives them an appearance of floating over the surface". Some species have mating rituals involving trophallaxis, in which a male attempts to entice a female by offering her a regurgitated droplet. If his offering meets her standards, they will collaborate to produce a new generation that will carry on in the same obscurity as the last.

REFERENCES

Freidberg, A. 2009. Asteiidae (asteiid flies). In: Brown, B. V., A. Borkent, J. M. Cumming, D. M. Wood, N. E. Woodley & M. A. Zumbado. Manual of Central American Diptera pp. 1093–1096. NRC Research Press: Ottawa.

Grimaldi, D. A. 2009. The Asteioinea of Fiji (Insecta: Diptera: Periscelididae, Asteiidae, Xenasteiidae). American Museum Novitates 3671: 59 pp.

Papp, L. 2013. A new genus of Asteiidae with a key to the Old World genera (Diptera). Annales Historico-Naturales Musei Nationalis Hungarici 105: 199–205.