The forewings have at least a single longitudinal nervure - often two - reaching from base to tip of the wing.
Wings usually well developed, net-veined; the forewings of firmer texture than the hind-wings, whose anal area folds.
Forewings similar in texture to hind-wings, whose anal area folds fanwise.
Forewings similar in shape and texture to hind-wings, which do not fold.
The order must therefore be ancient, and as no evidence is forthcoming as to the mode of reduction of the hind-wings, nor as to the stages by which the suctorial mouth-organs became specialized, it is difficult to trace the exact relationship of the group, but the presence of cerci and a degree of correspondence in the nervuration of the forewings suggest the Mecaptera as possible allies.