Eggs of representative genera of all families of Ephemeroptera except the Palingeniidae were studied. Their external morphology is reviewed, and hypotheses have been proposed to explain the evolution of several chorionic features and types of attachment structure. The archetypical Ephemeroptera egg is considered to have been round, with a smooth chorion, non‐fibrous adhesive layer, funnelform micropyle, and a suprachorionic sperm guide. Subsequent evolution resulted in several different micropyles and many different chorionic sculpturings and attachment structures. Data obtained from studying the egg stage are utilized as aids to understanding the phytogeny of the Ephemeroptera. The proposed classification divides the order into 21 families arranged into 6 superfamilies.

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