Key to six Ephemera species upon eggs

1. The egg size is 250 µm or more, and the surface of the chorion is smooth without or with a weakly expressed very fine reticulation (Figs 27–28).................................................................... E. (S.) japonica

- The egg size is less than 250 µm; surface of the chorion with reticulation (Figs 6, 12, 23, 36, 45)...................... 2

2. Sperm guide well expressed (Figs 2, 9, 16, 27, 39)........................................................... 3

- Sperm guide absent (Fig. 31–32).............................................................. E. (E.) shengmi

3. Sperm guide relatively deep (Figs 2, 9, 16)................................................................. 4

- Sperm guide is shallow and almost flat (Figs 38–39).............................................. E. (E.) strigata

4. Sperm guide is relatively oval and short; the micropylar canal is 12–18 µm (Figs 3–4)................... E. (E.) orientalis

- Sperm guide an elongated, micropylar canal less than 10 µm (Figs 10–11, 18–19).................................. 5

5. Sperm guide is almost rectangular; the distal margin is weakly defined and almost open; the surface of the chorion is intermittently fractured (Figs 10, 11, 13)............................................................... E. (E.) sachalinensis

- Sperm guide oval, with pointed, close distal margin; surface of the chorion with small-mesh reticulation (Figs 18, 19, 23)...................................................................................... E. (E.) transbajkalica