Blackflies of the genus Simulium (including several hundred species - see Adler & Crosskey, 2008, for a listing if you're really keen) are found around the world and familiar to most people for their blood-sucking habits. Like other members of the fly clade Culicomorpha, such as mosquitoes and chironomid midges, the larvae of blackflies are aquatic filter-feeders while only the females feed on blood as adults. In fact, blackflies (like other culicomorphs) are primarily nectar- rather than blood-feeders. Both males and females feed on nectar as adults and the females may require only a single blood meal to complete development of their eggs (Cupp & Collins, 1979).
Which is not to say that the role of blackflies as parasites is negligible. Askew (1971) refers to a plague of Simulium columbaschense in 1923 causing the deaths of nearly 20,000 head of livestock along the banks of the Danube. The most notorious blackflies are members of the Simulium damnosum complex of Africa (with one species, S. rasyani, in Yemen) which carry the human-parasitic worm Onchocerca volvulus. As well as the debilitating skin disease infection with Onchocerca causes in the majority of cases, approximately 250,000 people are rendered blind by Onchocerca infections every year. The various species of the Simulium damnosum complex are largely indistinguishable externally and require examination of their chromosome arrangements to be identified; however, different species differ significantly in their potential as Onchocerca vectors. Some 55 'species' have been identified in the complex, making it one of the largest known assemblages of cryptic species (Post et al., 2007).
Adler, P. H., & R. W. Crosskey. 2008. World blackflies (Diptera: Simuliidae): a fully revised edition of the taxonomic and geographical inventory. http://blackflies.info/sites/blackflies.info/files/u13/blackflyinventory_2008_Adler___Crosskey_1.pdf. Accessed 23 March 2010.
Askew, R. R. 1971. Parasitic Insects. Heinemann Educational Books: London.
Cupp, E. W., & R. C. Collins. 1979. The gonotrophic cycle in Simulium ochraceum. American Journal of Tropical Medicine and Hygiene 28 (2): 422-426.
Post, R. J., M. Mustapha & A. Krueger. 2007. Taxonomy and inventory of the cytospecies and cytotypes of the Simulium damnosum complex (Diptera: Simuliidae) in relation to onchocerciasis. Tropical Medicine and International Health 12 (11): 1342-1353.