Key to the genera of Hydrometridae occurring in French Polynesia

1. Head extremely elongate, length subequal to that of thorax and abdomen combined (Figs. 30–32); Marquesas Islands (Ua Huka).............................................................................. Dolichocephalometra

- Head not as elongate as above, length two-thirds or less the length of the thorax and abdomen combined (Figs. 1, 2, 10, 11, 12, 37)................................................................................................. 2

2. Legs bearing numerous long, erect setae, the lengths of these setae exceeding the widths of the segments on which they occur (Figs. 35, 36); head relatively short, less than half the length of the thorax and abdomen combined (Fig. 37); Marquesas Islands (Hiva Oa).................................................................................. Chaetometra

- Legs lacking numerous long, erect setae, although such setae may be present on the body ( Prohydrometra englundi n. sp.); length of head over half the length of the thorax and abdomen combined (Fig. 1); Society or Marquesas Islands .......... 3

3. Antennomere III very long, over 5 times the length of antennomere II (Fig. 11); fore-to-middle coxal spacing distinctly less than middle-to-hind coxal spacing, ratio 1.00/1.45 (Fig. 9); male abdominal sternum VII modified, bearing paired processes or patches of bristle-like setae (Figs. 19–24); Society Islands (Moorea, Raiatea, Tahiti)............... Prohydrometra n. gen.

- Antennomere III much shorter than above, slightly over 3 times as long as antennomere II (Fig. 1); fore-to-middle versus middle-to-hind coxal spacing subequal, ratio 1.00/1.20 (Fig. 3); male abdominal sternum VII unmodified, lacking paired processes or patches of bristle-like setae (Fig. 4); Marquesas Islands (Hiva Oa).................. Marquesametra n. gen.