This group is estimated to include up to 2,000 North American species. It is the most widespread and often most abundant aquatic insect family. The larvae and pupae can be found in almost any aquatic habitat. They live on the bottom, sometimes within slender tubes of silk and detritus. The larvae must be slide-mounted in order to be identified to genus. Different species vary greatly in pollution tolerance. Shed pupal skins can be collected from surface water and identified, sometimes more easily than for larvae or adults. Larval feeding strategies vary greatly, ranging from collector-gatherers, to predators. Larvae are often abundant and an important link of the food web, as most aquatic predatory species use them as a primary food resource at some point during their life cycle. The larvae pupate on the bottom, and then the pupae swim to the surface to emerge as adults, which usually survive for only a few days to mate and lay eggs.
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 horizontal or oblique plane. Head complete and fully exposed. Hook-bearing prothoracic and anal prolegs paired, though division may be slight and only at apex. Spiracles absent (apneustic). Body segments usually without conspicuous dorsaltubercles and setae as in some Ceratopogonidae.
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