The primitive and unique characters of extant mayflies

Extant Ephemeroptera retains a long series of primitive and unique characters, they contributed significantly to reconstruction of insect ground plan, investigation on origin of wing, evolution of venation and appendages and so on. These characters (prometabola, aquatic nymphal stage, copulation behavior, wing venation, corrugation and position at rest, more appendages and molt times plus mouthparts, reproductive system, etc.) are discussed collectively, and different views on origin and development of them are reviewed.