Ephemera serica

It was identified by Hsu (1936-37) who examined specimen collected from Hong Kong in British Museum.
According to the paper of Zhou et. al. (2003), the description of Ephemera serica is given as follow.

General appearance
Body size medium up to 13mm long; cerci length 7mm
Frontal process concave anteriorly to bifurcate & its length slightly longer than width
Mandibular tusk symmetrical or right tusk slightly shorter than left tusk; its exposed length about its head length
Pronotum (dorsal part of prothroax) yellow without marking
Tergum I without marking; tergum II with a black spot on each side; terga IV-IX with a pair of pale black stripes, extra pair on terga VII-VIII (in 2 pairs of lines)

Distribution:
Hong Kong, China (Guangdong, Guizhou, Fujian, Jiangxi, Jiangsu, Shanghai, Zhejiang, Eastern & Southern China), Vietnam and Japan

Reference:
Zhou Changfa, Zhou Kaiya and Gui Hong (2003) The Description of Five Ephemera Species Larvae (Ephemeroptera: Ephemeridae) Journal of Nanjing Normal University (Natural Science) 26 (1): 69-73.