Metaxydytes gen. nov.

Figs 4, 28, 29, 57, 62, 69

Megadytes sensu auctorum.

Type species.

Megadytes fraternus Sharp, 1882: 708, by current designation.

Diagnosis.

These species have males with equal-length metatarsal claws and females with two claws of unequal length with the posterior claw distinctly reduced (Figs 28, 29). The medial margins of male sternite IX are linear, not emarginate (Fig. 57). This group and Paramegadytes are similar in having females with the medial margins of the gonocoxosternite with a series of spinous setae (Fig. 62). From Paramegadytes these species differ in being smaller (≤ 24 mm in Metaxydytes, compared with ≥ 27 mm in Paramegadytes) and having the metasternal wings relatively narrow (WC/WV = 2.5-2.6 in Metaxydytes, compared with WC/WV = 1.8-1.9 in Paramegadytes). Third instar larvae have; (1) the median lobe of the frontoclypeus truncate apically with a tuft of setae (Fig. 69), (2) the median and lateral lobes of the frontoclypeus separated by a narrow emargination (Fig. 69), and (3) the lateral lobes of the frontoclypeus apically simple and acutely angulate (Fig. 69).

Etymology.

Metaxydytes is from the Greek metaxy, meaning “between,” and dytes, meaning “diver,” the root word for many genera of Dytiscidae including in this subfamily. The genus is named to signify its intermediate phylogenetic placement among other genera of Cybistrinae .

Phylogenetic relationships.

This genus may be sister group to Paramegadytes based especially on the presence of distinctive stiff, spinous setae along the medial margins of the female gonocoxosternite (Miller et al. 2007), although in the analyses presented here the group is ambiguously resolved near Nilssondytes, Paramegadytes and Megadytes + Cybister (Figs 75, 76).

Discussion.

These species were previous placed in Megadytes . The type species of Megadytes s. str. is M. latus which belongs to a different genus from all other known species previously placed in Megadytes (Figs 75, 76) requiring this new name for those species now in Metaxydytes . The species of Metaxydytes have never been completely revised, although Trémouilles (1989a, b) and Trémouilles and Bachmann (1980) addressed the species in southern South America. The genus is in need of a comprehensive revision.