They can be found under stones or buried into the sediment, feeding on tiny plant particles and bacteria. Some make small silk-like tubes that they live in. As a group, midge larvae are considered pollution tolerant organisms. They are widespread throughout the River and are one of the few macroinvertebrates that are able to survive within the River downstream of Old Forge. Red midges are especially pollution tolerant and are found within the lower reach.
Some midges are more pollution sensitive and are found within upper reaches.
Most larvae are grey or brownish in color. Female adult black flies are among the most important insect pests of humans and animals in many parts of the world, because of the toxic and allergic properties of their bite. Black fly larvae are pollution tolerant organisms. They are not very common in the River, however, they have been found throughout its reach. Large populations of black fly larvae usually are not allowed to develop, because of the potential for health problems from the adults.