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.