Permian mayfly nymphs

Although adult mayflies are not infrequently found in the fossil state, nymphs have been very rarely collected.The few fossil nymphs which are known have been placed in six families, four of them extant (Siphlonuridae, Leptophlebiidae, Ephemerellidae and Baeti- dae), and two extinct (Hexagenitidae and Misthodotidae) .Three of the existing families are known in the geological record only from these nymphs.The Leptophlebiidae are known from the Pliocene of Australia (genus Atalophlebia doubtfully determined) and from the Jurassic of the Soviet Union ( Mesobaetis ) and the Oligocene of Colorado (also Mesobaetis) .The Baetidae are known from the Pliocene of Australia ( Cloeon ) and the Ephemerellidae from the Jurassic of China {Turfinella) and the Jurassic of the Soviet Union {Mesoneta) .Since, as noted above, these three families are known as fossils only by the nymphs, the family determinations are probably not very reliable.One existing family, Siphlonuridae, is known from the Jurassic of the Soviet Union and is represented by wing fragments and nymphs ( Stackelbergisca , Tshernova, 1967).The extinct family Hexagenitidae, known only from the Jurassic of Siberia, is represented by adults as well as nymphs (Ephemeropsis) .The adults are well-known and have been fully described (Tsher- nova, 1961) but the nymphs are less satisfactorily known.The Misthodotidae, known from Lower Permian deposits in North Amer- ica and the Soviet Union, is represented by adults and a single, fragmentary nymph (Tshernova, 1965).Finally, mention should be made of two extinct genera, Dyadentomum and Phthartus , both based on nymphs from the Permian of the Soviet Union, but so little-known that they have not been placed in any family (familiae incertae) P The present paper deals with the first well-known Permian rep-