ALDER-FLY, the name given to neuropterous insects of the family Sialidae with long filamentous antennae and four large wings, of which the hind pair has the anal area folded fan-wise when at rest. They are sluggish insects found only near fresh water. The eggs are often very numerous and are laid in clusters on plants, etc., close to the water. The larvae are aquatic, are provided with biting jaws and breathe by seven or eight pairs of filamentous abdominal gills. The pupae are found in soil, moss, etc., and there is no cocoon. Sialis lutaria is the common alder-fly of Britain. In North America the genus Corydalis is remarkable for the great size of the jaws in the male and the insect has a wing-expanse of about I 20MM. : its larvae are called dobsons or bellgrammites by anglers who use them for bait.