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Family Rhyacophilidae

The insects of this group have a broader distribution and more of them extend to the south than any of the preceding families. The flies are small or of medium size. The larvae are found in rapid streams and, instead of making free cases with which to move about, their cases are formed of small stones fastened with silk to the rocks--a wise provision, considering the rapidly-running character of the streams which they inhabit. These cases are very slight and loosely formed--merely a few pebbles fastened to the lower surface of a large stone by a few threads of silk, but, living on the under surface of rocks as it does, the insect is less liable to be attacked by fish or predatory insects, and does not need so elaborate a case as do the free swimmers or crawlers. When it transforms to pupa a special protection is formed within the case in the shape of a brown cocoon, which is thin but of tough texture. We have a few widely-distributed species in this family, which belong to four genera.