Key to the known males of Caenis Stephens, 1835 from South India

(Male adult characters of Caenis sp. 2 is unknown)

1 Base of the antennal flagellum well dilated (Fig. 101)...................................... C. arunachalami sp. n.

- Base of the antennal flagellum slightly or not dilated (Fig. 23).................................................. 2

2 Forceps apically with few tuft of short spines (Srinivasan et al. 2023; Fig. 46)........................... C. americani

- Forceps apically with a rigid spine (Figs. 27, 67, 106, 147).................................................... 3

3 Foretarsus segments 2–3 each with a lateral and a median projections and segment 4 with an apico-median projection (Fig. 146)....................................................................................... Caenis sp. 1

- Foretarsus segments 2–4 each with an apico-median projection................................................. 4

4 Prosternal sclerite bell-shaped (Fig. 24)....................................................... C. kaegies sp. n.

- Prosternal sclerite differently shaped (Figs. 62, 103).......................................................... 5

5 Forceps apically with a short spine (Fig. 10).................................................... C. maduraiensis

- Forceps apically with a long spine (Fig. 66)................................................................ 6

6 Hind margin of penis with a well-developed medial incision (Fig. 67)......................... C. venkataramani sp. n.

- Hind margin of penis with slight or no medial incision (Srinivasan et al. 2023; Fig. 10)............................. 6

7 Forceps more or less parallel-sided (Srinivasan et al. 2023; Fig. 11)........................................ C. limai

- Basal half of forceps very broad, apical part abruptly narrowed (Malzacher 2015; Fig. 5b)................... C. maratha