Key to Groups of New World Hydraena

1 Posterior ridge of genae present, at least laterally (e.g., Figs. 2 c, 2d); abdominal intercoxal ventrite various; scintilla absent ... 2

1' Posterior ridge of genae absent (e.g., Figs. 44, 126) or nearly imperceptible; abdominal intercoxal ventrite flat, with posterior margin straight or weakly arcuate; scintilla (Fig. 2 b) present or absent........................................... 5

2 Elytra and labrum with spines (Figs. 21, 22); elytron with five rows of punctures between suture and humerous; two species from South America ...................................................................... Multispina Group

2' Elytra and labrum without spines; elytron with more than five rows, or with many random punctures................... 3

3 Abdominal intercoxal ventrite concave, with posterior margin markedly arcuate; elytral punctures serial or, in few adults, with few random punctures on disc; spermatheca C-shaped; female gonocoxite not divided; Canada and the United States ............................................................................................... Circulata Group

3' Abdominal intercoxal ventrite flat, with posterior margin straight or weakly arcuate; elytral punctures random, at least near base; spermatheca disc-shaped (e.g. Figs. 151–154) or (in H. paeminosa) trilobed (Fig. 155); female gonocoxite divided (Figs. 148–150); South America .............................................................................. 4

4. Elytral punctures coarse and very random (e.g., Fig. 3); pronotum more elongate; mesoventral intercoxal process (P2) very narrow, tapering to sharp point that is contiguous with midlongitudinal carina of metaventrite (e.g., Fig. 2 c).................................................................................................... Paeminosa Group

4'. Elytral punctures serial except in small area at base (Figs. 16–18); pronotum less elongate, sides more sinuate; mesoventral intercoxal process (P2) wide, apex blunt (e.g., Fig. 2 d)............................................ Curvosa Group

5 Pronotal scintilla (Fig. 2 b) present or absent; pronotum always with posteromedial foveae (PF2; Fig. 2 a)...... Leechi Group

5' Pronotum never with scintilla; pronotum without posteromedial foveae, or if present very shallow and pronotum coarsely punctate (e.g., Figs. 110, 118)........................................................... Marginicollis Group