Key to males of the I. crassicornis group

1. Gonarcus (fused gonocoxites 11) with a posteriorly produced process or pair of processes (Fig. 31) ........................ 2

- Gonarcus without any posterior process....................................................................................................................... 4

2. Posterior process of gonarcus bifurcate (Fig. 32) ......................................................................................................... 3

- Posterior process of gonarcus paired, slender and digitiform (Fig. 26) ...................................... I. digitiformis sp. nov.

3. Ninth gonocoxite with a feebly prominent stylus on inner side (Fig. 30) .............................................. I. fujiana Yang

- Ninth gonocoxite with a long and acutely produced stylus on inner side (U. Aspöck et al. 2009: fig. 5)..................... .............................................................................................................................. I. shinohara Aspöck, Liu & Aspöck

4. Endophallus proximally with a pair of processes dorsolaterally (Fig. 6) ..................................................................... 5

- Endophallus proximally without any process (Fig. 15)................................................................................................6

5. Endophallus distally with a pair of tufts of bristles ....................................................................... I. japonica Okamoto

- Endophallus distally with one pair of bristles or none (Fig. 6)................................................... I. biprocessus sp. nov.

6. Ectoproct in lateral view produced posterodorsad; gonarcus obtusely prominent ventrolaterally; endophallus with a pair of bristle tufts. ........................................................................................................... I. taiwana Aspöck & Aspöck

- Ectoproct in lateral view rounded posterodorsad (Fig. 14); gonarcus not prominent ventrolaterally (Fig. 15); endophallus with two pairs of bristle tufts (Fig. 15) ........................................................... I. crassicornis (Schummel)