These flies are known as buffalo gnats, black gnats, and black flies (blackflies). They are 2-5 mm long, with a short body and variable in color from gray to dark yellow (Fig. 7.7e). The thorax is strongly developed and has a humped appearance. Larvae develop in relatively nonpolluted, fast-flowing streams. Both sexes possess elongated mouthparts, but the mandibles of the females are broad, and blade-like, and capable of piercing animal skin. The mouthparts of the male are weak and not adapted for piercing skin. Females of many species are blood-feeders, but some (Simulium) suck the blood of insects. Adults occur in large numbers during late spring and early summer, and they are some of the most persistent human-biting insect pests. These flies are cosmopolitan and abundantin all zoogeographic regions from arctic to equatorial latitudes. Urban habitats for these flies are limited, butpopu-lations often occur in rivers and streams in cities, especially those with reduced pollution. Artificial habitats for these flies in urban areas are provided by ornamental fountains and landscape ponds, which have flowing water and sufficient organic material.

Pest status is based on large numbers of these flies annoying people outdoors. Black flies are active and bite during the day and usually when there is little surface wind. They do not bite at night outdoors, or in the daytime indoors, and they enter buildings only accidentally. The bite from a simuliid results in a reddened, swollen area; the site may itch and remain irritated for several days. The only known disease transmitted to humans by Simuliidae is onchocerciasis, river blindness, which is caused by the filarial worm Onchocera volvulus. However, around the world simuliids are best known for their biting and nuisance swarming behavior. They are sometimes called buffalo gnats in the USA, because when viewed from the side they resemble a buffalo, 'potu' fly (Simulium indicum) in the Himalayas, and 'no-no' (S. buissoni) in theMarquesas Islands.

Not all black flies suck blood, even among those with fully formed biting mouthparts. Human-biting behavior is generally restricted to three genera: Prosimulium, Austrosimulium, and Simulium. The identification of species involved in pest outbreaks is sometimes difficult. The taxonomy of simuliids includes the use of a species complex for an assemblage of sibling species, which can only be distinguished using non-morpohological (chromosomal) criteria. Chromosome data often provide the only reliable means of identifying individual members of a complex.

Eggs are deposited in large numbers directly into streams or rivers, or on rocks or other objects projecting out of the water. Females lay 200-500 eggs singly in the water or in masses on vegetation or other objects near the water. Hatching occurs in 3-7 days, but the eggs laid in the fall may not hatch until the following spring. Eggs ofdisease vectors and pest species will not develop unless the female has had a blood meal before ovipositing. Larvae are 10-15 mm long and brown to gray, cylindrical and 12-segmented. They move on rocks and debris under water using a looping motion and use an anterior proleg that is modified into a prehensile, toothed disk, and a posterior sucker. Larvae cover their place of attachment under water with silken threads produced by the salivary glands. They feed on diatoms, algal filaments, animal plankton, and other organic matter that is collected on brush-like structures at their mouth opening. There are six or seven instars, and development takes about 10 days. Cocoons and pupae are attached to rocks and debris under water. Adults emerge under water, but they are carried to the surface in air bubbles; they fly to vegetation and mate soon after. Adults live about 3 weeks. Most Simulium species are capable of dispersing 15-35 km from their breeding site, and some species make wind-assisted migrations of 150225 km. S. arcticum has occurred in pest numbers at a distance of 225 km from its breeding site.

Buffalo gnat, Cnephia pecuarum This is the simuliid originally called the buffalo gnat. It is a serious pest to humans and livestock throughout the region. The common name, buffalo gnat, may be linked to the general shape of this fly, or to its association with livestock. This species is distributed in southern USA, primarily in the Mississippi river basin.

Western black fly, Prosimulium exigens Adults are 2-4.5 mm long and dull black to grayish black. They crawl over the face, neck, and hands, and often bite. This species occurs along streams in May and June in the Pacific northwest of the USA.

Golubatz fly, Simulium colombaschense Adults are about 2.5 mm long and grayish black. Larval developmentis 20 days at 20-25 °C; the pupal stage is about 5 days. This species occurs in middle and southern Europe, in the Danube river basin, and is a periodic pest of livestock and humans. In 1923 two large swarms of this species appeared in southern Romania and caused the death of 16474 domestic animals, including cattle, horses, and pigs.

Turkey gnat, Simulium meridionale Adults are about 2 mm long and grayish to brownish black. It is distributed from eastern to southeastern USA, and in spring it is a common pest of humans, domestic animals, and poultry. Adults bite the combs and wattles of poultry, and cause symptoms similar to the disease cholera. This species is also known as the cholera gnat. These flies can occur in large numbers and may be a serious pest.

White-stockinged black fly, Simulium venustum complex

Adults are distinguished from other common species by having silvery-white tibiae. Adults occur in large numbers during June and July and persist throughout the summer. In the Adirondack mountains of New York, it appears later in the year than Prosimulium hirtipes, and is not considered a serious pest of humans. People in the region say that, when the black flies put on their white stockings in June, the trouble with black flies is about over. This means that the most common species then is S. venustum, which is not a serious biting pest. This species is distributed in Canada and eastern USA.

Common black fly, Simulium vittatum complex Adults are 2-3 mm long and dark gray to velvety black. It does not ordinarily bite humans, but attacks livestock and other domesticated animals. However, this species will fly around the heads of people in agricultural areas or working around large animals. This species is widely distributed in North America and Iceland.

Other Simuliidae Simulium quadrivittatum is an important biting species in various habitats in Belize and Panama. S. arakawae usually has a localized distribution, but is often a serious biting pest in Japan. S. tuberosum (species complex) occurs in Scotland and northern Europe and is a pest throughout this region. In the UK, S. bredign may become established in landscape ponds and ornamental fountains and waterfalls. The Blanford fly S. posticatum occurs primarily in southern UK, and can occur in large numbers as a local pest. In southeastern Lithuania, the mostcommon blood-sucking species is Byssodon maculatus.This species overwinters in the egg stage, there is one generation per year, and the adults are active until the end of June. Cnephia pecuarum is the southern buffalo gnat, and it is a severe pest of people and livestock in the Mississippi valley and other regions in southern USA. Other species that attack humans include S. erythrocephalum in central Europe, and S. aokii in Japan.

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