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Insects and other arthropods

Black flies


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Black flies are tiny, stocky flies from 1.4 to 6 mm long. Despite their name, their colour varies depending on the species. Their convex thorax gives them a characteristic humpbacked appearance. The six fairly short legs have white bands in certain species.

The females’ abdomens, which absorb blood meals, are particularly expandable. Their short, sharp mouthparts are adapted to cutting through skin and sucking up blood.

Life cycle

These insects’ life cycle varies greatly from one species to another. The egg, larva and pupa are all aquatic, while the adult is aerial. They require running water to develop in almost all cases.

Males and females mate on the ground shortly after emerging. In most species, the female must then seek a blood meal to produce her eggs. The laying process depends on the species.

The eggs are from 0.15 to 0.3 mm long and swollen on one side. The larva has a pair of labial brushes that open up like fans. Mature larvae are from 4 to 12 mm long. During their development they moult from six to nine and sometimes even up to eleven times. Most of the time they live attached to some kind of object in running water.

When the larva pupates, it first remains in its exuviate, and then weaves a silken cocoon of different shapes. The cocoon is attached to a submerged object. The pupa remains motionless for two to six days. When it emerges, the adult may stay still on the water surface or fly off immediately.

Depending on the species, these insects overwinter as eggs or larvae. In Quebec, there are two to four generations a year in the south and a single generation in the north.

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