Metaxydytes, Miller & Michat & Ferreira Jr, 2024

Miller, Kelly B., Michat, Mariano C. & Ferreira Jr, Nelson, 2024, Reclassification of Cybistrinae Sharp, 1880 in the Neotropical Region (Coleoptera, Adephaga, Dytiscidae), with description of new taxa, ZooKeys 1188, pp. 125-168 : 125

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.

Figures

Figure 75. One of seven cladograms derived from parsimony analysis of 57 morphological characters from Cybistrinae and other Dytiscidae (len = 105, CI = 68, RI = 92) with characters mapped using ' fast' optimization in WinClada. Black hash marks indicate unambiguous changes, white hash marks indicate homoplasious changes or reversals. Numbers above hash marks are character numbers, those below hash marks are derived state numbers.

Figure 76. Consensus cladogram of seven equally parsimonious cladograms from parsimony analysis of 57 morphological characters from Cybistrinae and other Dytiscidae morphology.

Figures 1 - 8. Cybistrinae morphological features 1 Bifurcitus lherminieri, right metatibia, posterior surface 2 Bifurcitus lherminieri, left metaleg, anterior surface 3 Trifurcitus robustus, left metaleg, anterior surface 4 Metaxydytes fraternus, left mesotarsomeres III-V, posterior surfaces 5 Cybister tripunctatus, left mesotarsomeres III-V, posterior surfaces 6 Megadytes latus, left half of metaventrite and left metacoxa (WV = metaventrite wing width, WC = metacoxal width) 7 Cybister tripunctatus, right metathoracic wing 8 Trifurcitus robustus, right metatrochanter, ventral surface. Scale bars: 1.0 mm.

Figures 15 - 29. Neotropical Cybistrinae species, metatarsal claws and tarsomere VI of males and females 15, 16 Cybister festae 17, 18 Cybister puncticollis 19, 20 Nilssondytes diversus 21, 22 Megadytes latus 23, 24 Megadytes parvus 25 species " IR 57 " (Ribera et al. 2008) 26, 27 Paramegadytes glaucus 28, 29 Metaxydytes fraternus. Scale bars: 1.0 mm.

Figures 51 - 57. Neotropical Cybistrinae species, male sternite IX, ventral aspect 51 Cybister festae 52 Cybister puncticollis 53 Nilssondytes diversus 54 Megadytes latus 55 Megadytes parvus 56 Paramegadytes glaucus 57 Metaxydytes fraternus. Scale bars: 1.0 mm.

Figures 62 - 66. Neotropical Cybistrinae species, female genitalia including left gonocoxosternite, ventral aspect 62 Metaxydytes fraternus 63 Paramegadytes glaucus 64 Bifurcitus lherminieri 65 Trifurcitus robustus 66 Cybister fimbriolatus. Scale bars: 1.0 mm.

Figures 67 - 71. Third instar larva, anterior clypeal margin 67 Bifurcitus magnus 68 Megadytes latus 69 Metaxydytes carcharias 70 Paramegadytes glaucus 71 Trifurcitus fallax.