Motonerus Hansen, 1989

Ek, Martin Fiká Č & Short, Andrew Edward Z., 2006, A revision of the Neotropical genus Motonerus Hansen (Coleoptera: Hydrophilidae: Sphaeridiinae), Zootaxa 1268, pp. 1-38 : 5

Motonerus Hansen, 1989

Motonerus Hansen, 1989: 256

Diagnosis. Very small to small beetles (1.5–2.8 mm); antennal grooves absent; median part of prosternum carinate, weakly divided from lateral parts, with shallow to very deep anteromedial excision ( Figs. 42–51 ); elytral margins with more or less distinct denticulation at least at elytral base ( Figs. 54–59 ); elytral intervals moderately convex to subcostate; each elytron with 9 elytral series; preepisternal elevation in the form of an elevate bulge bearing a longitudinal carina or plate (1.6–6.0 longer than wide), sometimes bearing a deep concave median area ( Figs. 32–41 ); postcoxal ridges on metaventrite not connecting but bent posteriad medially and forming a longitudinal ridge in anteromedian part of metaventrite; anterolateral ridge on metaventrite missing; femoral lines absent or only weakly developed; median part of male sternite 9 crescent­like ( Figs. 52, 53 ).

Differential diagnosis. Motonerus is easily distinguishable from all other New World megasternine genera by the combination of the following three characters: (1) prosternum without antennal grooves; (2) metaventrite without anterolateral ridge, and (3) preepisternal elevation laminar, narrow or widely elongate with deep median concave area. Additionally, the presence of the anteromedian excision on pronotum is unique for Motonerus and not known from any other genus in the Megasternini worldwide. However, while this character is very helpful for identification, it is often difficult to observe (see above for details). Smaller species can be easily confused with Oosternum Sharp, 1882 when the ventral characters are not examined.

The genus is also generally recognizable using the generic key presented by Hansen (1991), but with an adaptation of couplet 26:

26. Anterior margin of prosternum with shallow to very distinct anteromedian excision; antennal grooves missing; lateral margins of elytra with very distinct to fine denticulation at least basally; mesosternal elevation lamellar to widely prolonged (up to 1.6× longer than wide), when <3.0× longer than wide, then with deep median concave area; metasternum with or without median impression; elytral intervals moderately convex to subcostate ................................................................................................... Motonerus

– Anterior margin of prosternum without such excision; antennal grooves present or missing; elytral margin without denticulation or with fine serration; mesosternal elevation lamellar to variably shaped, never with deep median concave area; metasternum never with median impression; elytral intervals variably shaped ............................... 27

Many of the characters used by Hansen (1989, 1991) for delimiting the genus vary much more than previously known. Consequently, the modified generic diagnosis and key couplet are necessary to reliably distinguish the genus. However, as many species are still likely undescribed, we feel a full generic description is still premature.

Figures

FIGURES 41 – 53. Details of of Motonerus. 41, 49: M. sp. B; 42, 52: M. andersoni sp. n.; 43, 53: M. apterus sp. n.; 44: M. depressus sp. n.; 45: M. hanseni sp. n.; 46: M. oosternoides sp. n.; 47: M. nublado sp. n.; 48: M. obscurus Hansen; 50: M. problematicus sp. n.; 51: M. sp. A. 41 — preepisternal elevation of mesothorax and median part of metaventrite (scale bar = 0.2 mm); 42 – 51 — median part of prosternum; 52 – 53 — sternite 9 of male.

FIGURES 54 – 64. Details of Motonerus. 54, 60, 63: M. apterus sp. n.; 55, 64: M. oosternoides sp. n.; 56: M. depressus sp. n.; 57, 62: M. nublado sp. n.; 59: M. problematicus sp. n.; 61: M. obscurus Hansen. 54 – 59 — epipleuron and lateral part of elytron; 60 – 62 — abdominal ventrites; 63 – 64 — head, lateral view. Scale bar = 0.5 mm (54 – 59), 0.25 mm (60 – 62); 0.1 mm (63 – 64).

FIGURES 32 – 40. Preepisternal elevation of mesothorax and median part of metaventrite of Motonerus. 32: M. apterus sp. n.; 33: M. oosternoides sp. n.; 34: M. depressus sp. n.; 35: M. andersoni sp. n.; 36: M. hanseni sp. n.; 37: M. nublado sp. n.; 38: M. obscurus Hansen; 39: M. problematicus sp. n.; 40: M. sp. A. Scale bar = 0.2 mm.