There are over 255 North American species in this family. The genera Simulium and Prosimulium include most of these. Larvae spin a pad of silk and glue it it to the substrate, then use hooks on the posterior end to grasp the silk pad and suspend themselves in the current, where they feed. If a larva is disturbed by a predator it uses a strand of silk from their mouth that is tethered to the substrate to climb back to their rock once the predator has left. They use comb-like fanning structures on their head to filter food from the water. Generally, this group is considered tolerant to pollution, however, they can be found only in clean water as well, depending on the species. Adult black flies are terrestrial, and the females are biting pests. The bites can cause allergic reactions in some people, and in cases where the numbers are high can cause blood loss and become dangerous for some mammal species. In the tropics, flies in this family can transmit diseases such as river blindness. Fly fishermen often refer to members of this group as smuts, and sometimes use larvae or pupae in their fly patterns.
Wings and wing pads absent. Eye spots sometimes visible, but compound eyes absent. Segmented legs absent, but sometimes fleshy prolegs present. Sometimes with distinct head, often without head or with head drawn deeply into thorax. Body flattened, cylindrical, or maggot-like.
Mandibles move against each other along a horizontal or oblique plane. Head complete, fully exposed, distinct from thorax, and usually without lengthwise incisions. Head capsule usually with pair of conspicuous fans. Prothorax with single prolegventrally. Thoracic segments usually individually distinguishable. Posterior abdominal segments 5–8 swollen,with ballooned or bowling-pin appearance. Last abdominal segment ringed with several radiating rows of small hooks. Mature larvae usually 3–8 mm long, or greater.
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