Megadytes Sharp, 1882

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

Megadytes Sharp, 1882

Figs 6 , 12 , 13 , 21-24 , 39-47 , 54 , 55 , 60 , 68

Megadytes Sharp, 1882: 701.

Type species.

Megadytes latus Fabricius, 1801: 260.

Classification.

The concept of the genus presented here differs significantly from previous ones (e.g., Trémouilles and Bachmann 1980; Miller et al. 2007; Miller and Bergsten 2016). The type species of Megadytes is M. latus Fabricius, 1801, a species which differs in important phylogenetic characters from most other species historically Megadytes assigned to Megadytes . Here the species M. latus is placed together with Megadytes parvus comb. nov. in Megadytes , whereas the other species historically in Megadytes are placed in other genera based on the phylogenetic hypothesis developed here (Figs 75 , 76 ). Given the character distribution in this group and the diagnosis of the type species, M. latus , this new classification is unavoidably disruptive of the historical concept of the genus (which included all species here placed in the genera Bifurcitus stat. nov., Paramegadytes new status, and Trifurcitus new status, see below) and a new genus described below. The unusual characteristics of M. parvus were recognized by Trémouilles (1984) who placed the species in Cybister (Meganectes) Brinck, 1945.

Diagnosis.

Within Cybistrinae Megadytes are similar to Cybister (Figs 51 , 52 ) in having the medial margin of the lobes of the male abdominal sternum IX emarginate (Figs 54 , 55 ), but differ from Cybister (Fig. 5 ) in lacking a series of setae along the posterodorsal apical angle of the mesotarsomeres of males and pro- and mesotarsomeres of females (as in Fig. 4 ) This places them in an intermediate phylogenetic position between other Cybistrinae and Cybister (Figs 75 , 76 , see below). Males and females both have two metatarsal claws with the posterior claw strongly reduced (Figs 21-24 ). Third instar larvae (based on M. latus ) have (1) the median lobe of the frontoclypeus truncate apically with a tuft of setae, (2) the median and lateral lobes of the frontoclypeus separated by a wide emargination (Fig. 68 ), (3) the lateral lobes of the frontoclypeus apically simple (Fig. 68 ), (4) the lateral lobes of the frontoclypeus acutely angulate (Fig. 68 ), and (5) the cephalic capsule relatively long (head length / head width> 1.25).

Distribution.

Megadytes are found in the Neotropical region. Megadytes parvus is known only from the type locality in Bahia State, Brazil, and M. latus is known from Brazil, Uruguay, Argentina, French Guiana, and Venezuela.

Phylogenetic relationships.

Megadytes is sister to Cybister (Figs 75 , 76 ) based on both genera with distinctly emarginate medial margins of abdominal sternite IX in males (Figs 51-55 ). However, Megadytes lack a series of setae at the apicodorsal angle of the posterior surface of mesotarsomeres I-IV (and of protarsomeres I-IV of females). This feature is remarkably consistent across the numerous Cybister species in the world ( Miller et al. 2007).

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 9 - 14. Neotropical Cybistrinae species, habitus 9 Cybister festae 10 Cybister puncticollis 11 Nilssondytes diversus 12 Megadytes parvus 13 Megadytes latus 14 species " IR 57 " (Ribera et al. 2008). Scale bar: 10.0 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 30 - 50. Neotropical Cybistrinae species, male genitalia 30 - 32 Cybister festae: 30 median lobe, right lateral aspect 31 median lobe, ventral aspect 32 right lateral lobe, right lateral aspect 33 - 35 Cybister puncticollis: 33 median lobe, right lateral aspect 34 median lobe, ventral aspect 35 right lateral lobe, right lateral aspect 36 - 38 Nilssondytes diversus: 36 median lobe, right lateral aspect 37 median lobe, ventral aspect 38 right lateral lobe, right lateral aspect 39 - 43 Megadytes parvus: 39 median lobe, right lateral aspect 40 median lobe, ventral aspect 41 right lateral lobe, right lateral aspect 42 apex of median lobe, right lateral aspect 43 apex of median lobe, ventral aspect 44 - 47 Megadytes latus: 44 median lobe, right lateral aspect 45 median lobe, ventral aspect 46 right lateral lobe, right lateral aspect 47 apex of median lobe, ventral aspect 48 - 50 species " IR 57 " (Ribera et al. 2008): 48 median lobe, right lateral aspect 49 median lobe, ventral aspect 50 right lateral lobe, right lateral aspect.

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 58 - 61. Neotropical Cybistrinae species, female genitalia including left gonocoxosternite, ventral aspect, except Fig. 60 without internal genitalia 58 Cybister festae 59 Cybister puncticollis 60 Megadytes latus 61 Nilssondytes diversus. 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.