The fly in the picture above is a typical member of the Sphaeroceridae, a family of over 1300 species of small flies that include some of the most ubiquitous of all insects. Despite their abundance, however, they rarely attract much attention from humans due to their small size, usually only about a couple of millimetres in length. Sphaerocerids are distinguished from other flies by the structure of the tarsus (the 'foot') on the hind leg. In most other flies, all the legs have the tarsi divided into elongate segments, but in sphaerocerids the first segment of the last tarsus is noticeably short and broad.
Sphaerocerids are sometimes referred to as 'lesser dung flies', referring of course to their small size and the diet of their larvae. However, lesser dung flies are not only associated with dung, but also feed on almost any form of decaying organic matter. They may be found on carrion or decaying vegetation or fungi. Some species may feed on a wide range of foodstuffs, but others are much pickier. One species, seemingly as yet unidentified, has been found in the excretory glands of a land crab. The coast-dwelling genus Thoracochaeta feeds on seaweed (Marshall & Buck 2010).
Though diverse, the majority of sphaerocerids are fairly conservative in appearance, including the Copromyza at the top of this post. There are, of course, notable exceptions. The individual just above is a representative of the Homalomitrinae, a rarely-collected subfamily of six known species in three genera from tropical Central and South America. The homalomitrines have a number of unusual features: their heads lack ocelli, have reduced bristles, and are more or less elongate; their thoraxes are relatively small; and the tarsi of all legs are short and broad. Two of the three genera, Sphaeromitra and Podiomitra, have markedly reduced venation in the wings. They do not have the appearance of strong fliers (though specimens have been collected in Malaise traps, indicating that they fly at least on occasion), but their extreme rarity (so far as is known) means that their lifestyles are almost unknown. Two species of Homalomitra, H. ecitonis and H. albuquerquei have been collected in association with army ants (Roháček & Marshall 1998), and it has been suggested that homalomitrines might develop in army ant middens, travelling phoretically by clinging to the ants with their modified tarsi (Marshall & Roháček 2003). However, it should be noted that this is largely speculative: the holotype of H. ecitonis, when collected, was reported to be walking amongst the supposed host ants, but otherwise little has been recorded of the nature of the association. It should also be noted that Roháček & Marshall (1998) suggested that H. ecitonis and H. albuquerquei form a clade to the exclusion of the other homalomitrine species, which further weakens confidence in extending the habits of these species to others.
Sphaerocerids have been suspected as potential disease vectors due to their association with waste and decay, but actual evidence of negative impact on humans is rare. There appears to be at least one recorded case of sphaerocerid-caused intestinal myiasis (Marshall & Richards 1987), and sphaerocerids may become numerous enough to be a nuisance in places with a concentration of potential food, such as abattoirs or mushroom farms. Whatever their negative impacts may be, they are almost certainly outweighed by the positive: the ubiquitous sphaerocerids are probably major players in the process of breaking down waste materials and returning nutrients to the environment. They're the little flies that save you from being knee-deep in shit.
Marshall, S. A., & M. Buck. 2010. Sphaeroceridae (small dung flies). In: Brown, B. V., A. Borkent, J. M. Cumming, D. M. Wood, N. E. Woodley & M. A. Zumbado (eds) Manual of Central American Diptera, vol. 2, pp. 1165-1187. NRC Research Press: Ottawa.
Marshall, S. A., & O. W. Richards. 1987. Sphaeroceridae. In: McAlpine, J. F. (ed.) Manual of Nearctic Diptera, vol. 2, pp. 993-1006. Biosystematics Research Centre: Ottawa.
Marshall, S. A., & J. Roháček. 2003. Podiomitra, a new genus of Homalomitrinae (Diptera: Sphaeroceridae) from Costa Rica. Proceedings of the Entomological Society of Washington 105 (3): 708-714.
Roháček, J. 2012. Wing polymorphism in European species of Sphaeroceridae (Diptera). Acta Entomologica Musei Nationalis Pragae 52 (2): 535-558.
Roháček, J., & S. A. Marshall. 1998. Revision of Homalomitrinae subfam. n. (Diptera: Sphaeroceridae), with the description of a new genus and three new species. European Journal of Entomology 95: 455-491.