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any insect of the megalopteran family Sialidae, characterized by long, filamentous antennae and two pairs of large wings (anterior wing length 20 to 50 mm [ 34 inch to 2 inches]), membranous and well-developed, with part of the hind wing folding like a fan. The adult alderfly is dark-coloured, 15 to 30 mm (35 inch to 1 15 inches) long, sluggish, and a weak flier. It usually inhabits shore vegetation, especially alder trees.