Key to species of the genus Hylomus species in Vietnam

1. Paraterga wing-shaped ........................................... H. spectabilis

- Paraterga antler- or spine-shaped (Figs. 3, 7, 9, 11) ................... 2

2. Paraterga antler-shaped (Figs. 3, 7, 11) ...................................... 3

- Paraterga spine-shaped (Fig. 9) ................................................ 11

3. Epiproct unmodified, without conspicuous setiferous knobs near tip (Fig. 18 A−B) ......................................................................... 4

- Epiproct modified, with several evident setiferous knobs near tip (Figs. 7F, 28B)............................................................................ 7

4. Metaterga smooth, more or less shining. Male femora unmodified ......................................................................................... H. asper

- Metaterga rough, dull, granular. Male femora modified, inflated. ..................................................................................................... 5

5. Metaterga with more than 4 + 4 posterior setiferous spines on midbody segments (Fig. 17A). Body pale or weakly pigmented. Lamina lateralis of gonopod solenophore with densely setose region (Figs. 19−20) ................................................... H. songoku

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- Metaterga with less than 4 + 4 posterior setiferous spines on midbody segments. Body pigmented. Lamina lateralis of gonopos solenophore without densely setose region ................................ 9

6. Metaterga with 3 + 3 posterior setiferous spines. Male femora 6, 7 and 9 humped ventrally. Fifth sternum with a pair of tubercles between coxae 4 ....................................................... H. cervarius

- Metaterga with 2 + 2 posterior setiferous spines. Male femora 6 and 7, sometime femur 5 modified, each with a large hump on ventral side. Fifth sternum with either rectangular process or bifid trapeziform lamina between coxae 4.......................................... 7

7. Body light or darkish brown. Fifth sternum with a bifid trapeziform lamina between coxae 4.......................................... 8

- Body reddish or pinkish. Fifth sternum with four round tubercles on a prominent, elevated, rectangular lamina between male coxae 4 .................................................................................. H. enghoffi

8. Male femora 6, 7 humped ventrally (Fig. 28D). Tip of solenophore broadly rounded lobuliform (Figs. 29−30) ............... ........................................................................................ H. namek

- Male femora 6, 7, sometimes 5, humped ventrally (Fig. 23B). Tip of solenophore acute (Figs. 24−25) ............................. H. saiyans

9. At least 3 + 3 posterior metatergal spines on metaterga........... 10 - Only 1 + 1 posterior poor setae on metaterga (Fig. 3B, D) ........... ............................................................................... H. cattienensis

10. Colour pink to red. Metaterga with numerous microsetae, and 4 + 4 posterior spines (Fig. 4A, B, D, E) ............................ H. pilosus

- Colour dark to castaneous brown. Metaterga with microgranulation, and 3 + 3 posterior spines ............ H. proximus

11. A row of 1 + 1 posterior spines on metaterga. Gonopod subfalcate, femorite slightly curved; solenomere long .................. .................................................................................... H. specialis

- A row of 2 + 2 posterior tubercles/spines on metaterga. Gonopod falcate; solenophore short, pointed terminally ......................... 12

12. Fifth sternum with two independent setiferous tubercles between coxae 4. Antenna long and slender.............................. H. grandis

- Fifth sternum with a bifid setiferous trapeziform lamina between coxae 4. Antenna short and stout (Fig. 9B) ................... H. holstii