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Ephemeridae (mayflies; subclass Pterygota, order Ephemeroptera) Family of insects whose nymphs occur in ponds, lakes, and large rivers, and have a burrowing habit. The head has a frontal process and long, tusk-like mandibles for loosening the silt, which is pushed sideways by the front legs and out of the burrow by the hind legs. The feathery gills are in continuous motion during digging, and the nymph feeds on detritus excavated from the burrow. The family contains some of the largest mayflies. Many have clear wings, although a few have spotted wings. Members of a particular species often emerge in large numbers.