A new species of Thraulus from Nanjing (eastern China) with single first gill (Insecta: Ephemeroptera: Leptophlebiidae).

Thraulus femoratus sp. nov., a new species found in eastern China, is described from imagos, subimagos, nymphs, and eggs. The nymphs are unique in the genus as well as in the Thraulus group because of their single lanceolate gills 1. The imagos can be differentiated from those of related species by the following combination of characters: slightly robust hindwing with a bluntly rounded costal projection that is situated over half the distance from wing base to apex, darkly pigmented basal portion of hindwing, and subapical dark band on hind femur. Subimagos resemble the imagos in color pattern. Eggs have eight pairs of long threads on the polar portion and slight ridges on the surface. The reduced gills 1 in combination with entirely fringed gills 2-7 represent a new evolutionary lineage and ecological type in the Thraulus group, but provide limited information regarding phylogenetic relationships.