Eggs have a sub-triangular form of 0.15 to 0.30 mm in length. Larvae have a cylindrical shape of grey or brown colour; they measure from 4 to 12 mm in

length. They have a head capsule with well-developed, crushing mouth parts. The larva is fixed on its support by hooks present on its abdomen. The larva is mobile with characteristic movements. Pupae are immobile, enclosed in a silk cocoon which remains fixed on its support. Adults are bulky, dark and similar to a small fly of 1 to 6 mm in length (see Figure 1.10).

1.5.3 Biology

The major bio-ecological characteristic of the blackflies is the development of the immature stages in running freshwater habitats. The female lays about 150 to 500 eggs on a partially immersed support: stone, various plants or branches.1 The female generally lays four to five batches of eggs during its lifetime. The egg incubation is short (two to seven days), except in temperate climate (diapause of several months). The larval phase comprises six to eight stages which last from one to two weeks in tropical zones, but there can be hibernation in a cold period/zone. The larvae live in great numbers anchored to immersed inorganic (e.g. stones) or vegetational substrates (e.g. plants, branches), or even the bodies of other arthropods (e.g. freshwater crabs, shrimps, dragonfly larvae). The larvae are mobile as they use their circle of hooks and the prothoracic pseudopodes for moving. The larvae are especially prone to live in strong running water (0.30 to 1.50 ms-1), that is highly oxygenated, such as cascades, rapids, dams, etc. The pupa is motionless and does not feed (stage duration: 210 days). Emergence of the adult takes place at daytime in about one minute. Females bite (pool feeding) during the day, outdoors and they visually target their hosts. Blackflies are not exclusively anthropophilic but rather opportunistic. There are marked seasonal variations with important pullulations representing a considerable nuisance. Their longevity is about two to three weeks. Blackflies develop in zones of savannas, forests and mountainous areas, in the torrents at heights of up to 4520 m in Kenya and 4700 m in the Andes, but the anthropophilic blackflies do not seem to go above 1500 m. Blackflies are excellent fliers with an active flight of several kilometers (15 to 35 km, and up to 80 km).

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