Soft-bodies
with short setaceous (bristle-like) antennae and vestigial mouthparts; wings
held vertically at rest, hind pair much reduced; intercalary veins and many
crossveins present; abdomen with long cerci, and with or without a medial
caudal filament; larvae (nymphs) aquatic, campodeiform (elongated and flattened)
with tracheal gills of varied form; true adult preceded by a subimago (winged
instar).
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