Cybistrinae Sharp, 1880, 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

Cybistrinae Sharp, 1880

Cybistrini Sharp, 1880, as group of ' Dytisci complicati '.

Type genus.

Cybister Curtis, 1827.

Diagnosis and classification.

These are large to very large Dytiscidae (length 13.0-47.0 mm). The subfamily is demonstrably monophyletic and is characterized by the following synapomorphies (among others): in adults ( Miller et al. 2007; Miller and Bergsten 2014, 2016), (1) the apicoventral elytral setal patch small, composed of a field of short, coarse setae; (2) a large cluster of apically bifid setae present on the posteroapical surface of the metatibia (Fig. 1 ); (3) the anteroapical metatibial spur acuminate and broader than the posteroapical spur (Figs 1-3 ); and (4) the oblongum cell sub-triangular (Fig. 7 ); in larvae ( Ferreira Jr 2000; Michat et al. 2017), (5) the anterior margin of the frontoclypeus trilobed (Figs 67-71 ), (6) antennomeres II and III each subdivided into three articles (in three instars), (7) the premaxillary lobes well developed and projected anteriorly, (8) maxillary palpomere III subdivided into three articles in instars II and III, (9) labial palpomeres I and II each subdivided into two articles in all three instars, (10) a dense row of short spiniform setae in the third basal of the ventral margin of the protarsus (although also characteristic of a number of other diving beetle taxa), (11) protarsus with a ventral row of spines (spinulae, not setae), and meso- and metatarsus each with a row of setae (other dytiscids have spinulae on all tarsi), (12) tergal sclerites reduced to small rectangular plates in abdominal segments I to VI, (13) a subapically located anus, and (14) strongly reduced urogomphi. Male cybistrines have the synapomorphy of protarsomeres I-III broadly laterally expanded into a “palette” that is broader than its medial dimension with a large field of adhesive setae ventrally. Most cybistrines are dark greenish to black, often with a lateral yellow margin along the pronotum and/or the elytron, depending on the species, genus or subgenus. These features with several additional synapomorphies in the female genitalia, larvae, other morphological systems, and DNA sequence data make this group among the most characteristic in Dytiscidae ( Nilsson 1988; Ferreira Jr 2000; Miller 2000, 2001; Miller et al. 2007; Miller and Bergsten 2014, 2016; Michat et al. 2017).

Cybistrinae prior to this study included seven genera, several with single or few species and Megadytes and Cybister , each of which are species rich and include multiple subgenera. The most recent phylogenetic classification of the group was developed by Miller et al. (2007).

The subfamily Cybistrinae has long been associated with Dytiscinae as a tribe of that subfamily and sister to the rest of the clade (e.g., Miller 2000, 2001), but was somewhat reluctantly elevated to subfamily rank by Miller and Bergsten (2014) after they found cybistrines not resolved together with dytiscines. The two clades share an exceptional number of adult and larval features in common, however, and new data and additional taxon sampling may change an understanding of their relationships. Morphological characters supporting monophyly of Dytiscinae and cybistrines are numerous, adults have: (1) the anterior margins of the eyes rounded, not emarginate; (2) the median lobe of the male aedeagus bilaterally symmetrical with a distinct, elongate ventral sclerite; (3) females with a single genital opening in the female reproductive tract for both reception of sperm and oviposition (secondarily within Adephaga ); and (4) the female gonocoxae fused together along their dorsal margins, evidently plesiomorphically to facilitate endophytic oviposition although apomorphically this is lost ( Miller and Bergsten 2014, 2016). Also, larvae have (among other less clear features): (1) abdominal segments VII-VIII with distinct lateral fringes of natatory setae (present also on abdominal segment VIII in instars II and III of Coptotominae ), and (2) the larval antennomeres and maxillary and labial palpomeres subdivided into articles ( Ferreira Jr 2000; Michat et al. 2017). Although generally regarded as characteristic of Dytiscinae + Cybistrinae , the subdivision of larval antennomeres and palpomeres in the included taxa is quite variable and likely involves multiple independent characters requiring further investigation to determine homologies within this general condition (see discussion above and character coding scheme below). Subdivision of antennae and palps also occurs (probably homoplasiously) in other diving beetle taxa in different ways.

Immature semaphoronts.

Cybistrinae larvae are very characteristic within Dytiscidae (see diagnostic features above). They are often prominent and abundant large predators in systems where they occur. Knowledge of larvae in the group is increasing, but lags behind knowledge of adults, and even lags well behind knowledge of larvae of other diving beetle groups, despite their conspicuousness, although they have been investigated within the context of the phylogeny and taxonomy of Dytiscinae and Cybistrinae ( Larson et al. 2000; Alarie et al. 2011; Michat et al. 2017). Table 2 details the state of descriptive knowledge of the morphology of the three instars of each genus group and a key is presented to the known taxa (see below). The pupa of Megadytes (Paramegadytes) glaucus Brullé was described by Crespo (1982). Eggs are unknown for Cybistrinae in general.

Distribution.

Cybistrinae are found throughout the world, mainly at low latitudes. Most members of the group are tropical, although some occur north to southern Canada and northern Europe and south through temperate South America and Australia and throughout southern Africa.

Phylogeny.

Parsimony analysis of the matrix resulted in seven equally parsimonious cladograms (length 102, CI = 68, RI = 93) one of which is shown in Fig. 75 with characters and states optimized on branches. Disagreement among trees is primarily within Cybister , but also in relative placement of Nilssondytes and Paramegadytes . In some solutions, Paramegadytes is resolved as sister to a clade of species previously in Megadytes (in a new genus described below), and in other solutions is sister to the clade Megadytes + Cybister . Similarly, Nilssondytes is either resolved as sister to a large clade containing a new genus (previously in Megadytes ), Paramegadytes , Megadytes , and Cybister or is sister to Megadytes + Cybister . This conflict resulted in a consensus cladogram (Fig. 76 ) with Nilssondytes and Paramegadytes in unresolved positions with respect to a new genus (previously in Megadytes ) and Megadytes + Cybister .

Cybistrinae and Dytiscinae have historically been regarded as individually monophyletic and together monophyletic (with cybistrines as a tribe within Dytiscinae ) based on a large number of adult and larval morphological characters (e.g., Miller 2001, and see above). The most extensive phylogenetic analysis of the family to date by Miller and Bergsten (2014), however, resulted in Cybistrinae and Dytiscinae not together monophyletic, with each of the groups individually monophyletic as historically constituted. In the analysis presented here which is admittedly more limited only to morphological features and fewer taxa, Cybistrinae and Dytiscinae are each monophyletic, and they are together monophyletic (Figs 75 , 76 ).

Taxonomic implications of phylogenetic analyses.

An analysis of Cybistrinae and reclassification was presented by Miller et al. (2007) (as Cybistrini in Dytiscinae ). Based on that work, Cybistrinae include certain genera characterized by apomorphic features, but also genera characterized by plesiomorphies that do not include any Neotropical species. These are primarily Australian in distribution including Spencerhydrus Sharp, 1882, Austrodytes Watts, 1978, Onychohydrus Schaum & White, 1847, and Sternhydrus Brinck, 1945, but also the one Afrotropical species in the genus Regimbartina Chatanay, 1911 ( Miller et al. 2007). The remaining two genera, Megadytes Sharp, 1882 (as historically defined), and its several subgenera, and Cybister Curtis, 1827 (also with several subgenera) were found to be together monophyletic based especially (unambiguously) on the presence of an oblique groove across the posterior surface of the metatrochanter (Fig. 8 ), as well as DNA sequence data, with strong support ( Miller et al. 2007). These genera occur in the Neotropical region. Megadytes (as then defined) was found to be monophyletic as was Cybister ( Miller et al. 2007). All South American species of Cybistrinae are evidently part of this clade since they have an oblique, ventral metatrochanteric groove ( Miller et al. 2007, and see below).

The analysis presented here is somewhat limited as regards taxon sampling overall, but it expands the Cybistrinae taxa available with morphological data and results largely support previous analyses including; 1) monophyly of Cybistrinae , 2) monophyly of the Australian genera ( Regimbartina not included here) 2) monophyly of Cybister (except Cybister parvus ), and 3) monophyly of taxa previously included in Megadytes together with Cybister (Figs 75 , 76 ; Miller et al. 2007; Miller and Bergsten 2014). However, the addition of newly discovered taxa and poorly known historical taxa with unique new combinations of morphological features resulted in some new phylogenetic relationships. Specifically, Megadytes , as historically constituted, is not monophyletic (Figs 75 , 76 ). Previously recognized subgenera of Megadytes (several elevated to genus rank, see below) and some of those species historically in Megadytes (Megadytes) (here placed in a new genus, see below) are not monophyletic (Figs 75 , 76 ). An undescribed species from northern South America is ambiguously resolved near these two groups based on a unique combination of features requiring a new genus (Figs 75 , 76 , see below). In addition, two species (previously Megadytes latus Fabricius and Cybister parvus ) with an intermediate character combination between Megadytes and Cybister are resolved in a monophyletic group between these other two groups requiring generic reclassification, as well (see below).

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