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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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The higher classification of the extant Ephemeroptera of the world is reviewed. The suborders Sch.istonota (to include the Baetoidea, Leptophlebioidea, and Ephemeroidea) and Pannota (to mclude the Ephemerelloidea, Caenoidea, and Prosopistomatoidea) are introduced ~nd defined .on the ?asis of thoracic structure and other characters. The families Ephemerel-hda~ and Tncoryth1~ae ar~ remov~d fro~ ~he ~eptophlebioidea and placed as the Ephemer-ello~d.ea .. The phyletlc basis f<;>r this cl.ass1f1cat10n and the evolutionary history of the super-fam1l~es 1s presented. The S1phlonundae and Leptophlebiidae are paraphyletic groups as constituted. All extant mayfly lineages take their origin within the Siphlonuridae and their deri~a.tio~s are. discussed. The Leptophlebiid~e gave rise to the Ephemeroidea. Classificatory !11od1f1c~t10ns 1~.the makeup of th~ S1phlonun~ae, Siphlaenig~atidae, Baetidae, Ametropod-1dae, Ohgoneumdae, Ephemerelhdae, and Tncoryth1dae are discussed. On the basis of phyletic int~_rmediacy, the SiJ?hl~enigmatinae is_ recognized as a subfamily of Baetidae, and the Isonychnnae and Colobunscmae are recognized as subfamilies of Oligoneuriidae.