Cybister giganteus Laporte, 1835: 99 by original designation (= Dytiscus lherminieri Guérin-Méneville, 1829).

Within Cybistrinae Bifurcitus have (1) the lateral margins of the pronotum and elytra margined with yellow, (2) males and females each with two equal-length metatarsal claws, and (3) the posterior metatibial spur bifid (Figs 1, 2). These are the largest of all diving beetles with adult specimens 36–47 mm in total length (Hendrich et al. 2019). Third instar larvae have (1) the median lobe of the frontoclypeus truncate apically with a tuft of setae (Fig. 67), (2) the median and lateral lobes of the frontoclypeus separated by a wide emargination (Fig. 67), (3) the lateral lobes of the frontoclypeus apically simple (Fig. 67), (4) the lateral lobes of the frontoclypeus obtusely angulate (Fig. 67), and (5) the cephalic capsule relatively short (head length / head width < 1.20).

Bifurcitus is sister group to the similar Trifurcitus (Figs 75, 76; Miller et al. 2007). Both have males and females with equal-length metatarsal claws and the anterior metatibial spur either bifid or trifid (although these two conditions may not be homologous) (Figs 1–3).

Although previous evidence suggested that the several subgenera of “Megadytes” are monophyletic, monophyly of this group is not supported here based in part on the discovery of undescribed cybistrine species with unique combinations of character states (Figs 75, 76). Given the situation, it seems appropriate to recognize these subgenera at the genus rank including Bifurcitus.

There are three currently valid species in Bifurcitus which were differentiated and characterized recently by Hendrich et al. (2019).

Dytiscus lateralis Fabricius, 1798.

Within Cybistrinae Cybister is characterized by the following: (1) a series of setae present along the posteroventral apical margin of the mesotarsomeres of males and pro- and mesotarsomeres of females (Fig. 5); (2) males with a single metatarsal claw, females with one or two, and if two, then the posterior claw small (some species with females dimorphic, some with a small posterior claw, others with only a single claw) (Figs 15–18); and (3) the medial margin of the lobes of the male abdominal sternite IX emarginate (Figs 51, 52). Larvae of Cybister (Neocybister) are unknown.

Cybister are found in all major biogeographic regions but are most diverse in the Afrotropical and Oriental regions, mainly in low latitudes. The group is not diverse in the Neotropical region where it is largely replaced in numbers of species and individuals by species previously in Megadytes (Megadytes) (most of these in a newly described genus, see below).

Cybister is the sister group to Megadytes as newly constituted (Figs 54, 76, see below). The Neotropical species of Cybister are in the subgenus Cybister (Neocybister) Miller, Bergsten & Whiting, 2007, which is restricted to the New World (Miller et al. 2007). This subgenus was resolved as sister group to all other Cybister in the analysis by Miller et al. (2007). It is resolved nested within Cybister here based on morphological data (Figs 75, 76), although previously examined molecular data (Miller et al. 2007) are not analyzed here. More investigation is needed. The two South American species are different from other Cybister in having (1) females always with a second, rudimentary posterior claw (Figs 15–18), (2) the medial margin of the gonocoxa distinctly emarginate (Figs 58, 59), and (3) the apex of the ventral sclerite of the male median lobe distinctly bifid (Figs 31, 34).

Panama, Darién, Matusagrati Lake (Laguna della Pita).

The syntype specimens are in Museo Regionale di Scienze Naturali di Torino (Museum of Turin, MRSN) (Fig. 72). Images of the specimens were examined (Fig. 72, courtesy of F. Giachino, MRSN). Two specimens are included in the type series, a male and a female (Fig. 72; Griffini 1895). Neither are dissected. The male specimen is here designated as the lectotype to stabilize the nomenclature of the species (see Material examined below; Fig. 72). Although the male is not dissected and the genitalia were not examined, the specimens agree well with the others examined for this study in size, shape, coloration, distribution, and other features.

This species differs from the other Neotropical Cybister species, C. puncticollis, in smaller size (TL = 20.3–21.7 mm in C. festae vs. 26.6–27.6 mm in C. puncticollis) and the shape of the male genitalia. The male median lobe in C. puncticollis is apically broadly expanded (Fig. 34) whereas in C. festae the median lobe is apically less strongly expanded with lateral margins that are somewhat more parallel-sided (Fig. 31). The specimens examined match the description of C. festae, but it is possible that the species illustrated and described here is not the same as C. festae since the lectotype (in MRSN) was not dissected.

Measurements. TL = 20.3–21.7 mm, GW = 11.7–13. mm, PW = 8.8–9.2 mm, HW = 5.1–5.5 mm, EW = 2.9–3.0 mm, TL/GW = 1.7–1.8, HW/EW = 1.7–1.8, WC/WV = 4.3–4.4. Body shape oval, widest slightly posteriad of middle; lateral margins broadly curved, continuously curved between pronotum and elytron. Depressed and somewhat flattened in lateral aspect.

Coloration. Head dark green-black, clypeus and labrum pale yellow. Pronotum dark green-black, with broad lateral yellow marginal band, medial margin of band distinctly delimited, band separated from posterolateral margin by narrow green strip (Fig. 9). Elytron dark green-black with broad lateral yellow marginal band, medial margin of band distinctly delimited, lateral margin of band slightly remote from lateral elytral margin for medial portion of length, extending laterally to margin of elytron anteriorly and posteriorly, apex of yellow band diffusing into coloration of elytron (Fig. 9). Ventral surfaces mostly black except antennae and palpi orange, pro- and mesothoracic legs with basal segments (trochanter and femur) pale orange and apical segments (tibia and tarsus) testaceous, metathoracic legs with mix of testaceous and orange, propleuron and elytral epipleuron pale orange to testaceous, and with small orange maculae laterally on abdominal ventrites IV–VI.

Sculpture and structure. Head broad, anteriorly produced, eyes prominent; dorsal surface evenly covered with exceptionally fine micropunctures but appearing smooth and shiny. Pronotum with lateral margins evenly and shallowly curved; surface of pronotum similar to surface of head in micropunctation; pronotum dorsally evenly curved. Elytron with margins very broadly curved, more strongly curved posteriorly; surface similar to surface of head in micropunctation and surface appearance. Prosternal process anteriorly rounded, surface nearly flat throughout and moderately broad, apex broadly elongate and sharply pointed. Metaventral wing narrow (WC/WV = 4.3–4.4); surface smooth, with extremely fine microsculpture, shiny. Lateral portion of metacoxa large, broad, surface smooth and shiny, with extremely fine microsculpture; metacoxal lines short and strongly curved, well-incised, extending anteriorly less than half distance across metacoxa. Abdominal ventrites smooth, unsculptured.

Male genitalia. Male median lobe in lateral aspect slender throughout length, apically slender, straight, and apically pointed, dorsal sclerite slender, evenly curved basally, apically straight, and slender, (Fig. 30); in ventral aspect broad basally, laterally somewhat constricted medially, apically broadly truncate (Fig. 31). Lateral lobe slender broad basally, apically strongly narrowed and slender, with long dorsal series of setae (Fig. 32).

Female genitalia (Fig. 58). Gonocoxosternite moderately broad, basal portion broadly ovate; gonocoxae together knifelike, evenly convergent to apex; rami smooth, short; vagina elongate; spermatheca elongate, ~ 2/3 length of vagina.

Sexual dimorphism. Males have a broad protarsal palette with ventral adhesive setae with apical structures in the form of elongate flattened spatulate surfaces; males also have mesotarsomeres with posteroventral fields of setae which are absent in females. Females lack natatory setae along the ventral margins of the metatarsomeres, but these are present in males. Males have a single metatarsal claw (the anterior, Fig. 15), whereas females have a second posterior claw that is ~ 2/3 length of the anterior, curved, and apically sharp (Fig. 16).

Variation. Two specimens were examined, a male and a female. The female has the extent and intensity of greenish-rufous coloration somewhat greater than the male and is larger, but otherwise the two specimens are similar.

This species is known from the type locality in the Darién in Panama (Griffini 1895; Brinck 1945) and Venezuela, Apure State (new country record).

The lectotype (here designated) in MRSN is labeled, “Laguna d. Pita (Darien) [handwritten]/ Cybister festae ♂ Griffini tipo./ Cybister festae (tipo) Griffini Darien [green label, black line border, horizontal black lines).” Other than the type specimens (not directly examined), two specimens were examined from Apure State, between La Ye and Bruzual, Venezuela from a roadside lake (7.6443333 -69.3000667) (SEMC, accession numbers: SEMC0846768, SEMC0846766).

Cybister puncticollis: Bolivia, San Miguel. Cybister kemneri: Brazil, La Plata, Amazonas, Rio Autaz.

Cybister puncticollis, in MNHN (not examined). Cybister kemneri, in MZLU (not examined).

This species is larger (Fig. 10) than the other Neotropical species in the genus, C. festae (Fig. 9) and the male genitalia are distinctly different (Figs 33–35). See above under that species for details about this and additional diagnostic differences between these two species of Cybister.

Measurements. TL = 26.6–27.6 mm, GW = 15.0–15.9 mm, PW = 10.8–11.6 mm, HW = 6.5–6.8 mm, EW = 3.7–3.8 mm, TL/GW = 1.7–1.8, HW/EW = 1.7–1.8, WC/WV = 2.9–3.2. Body shape oval, widest slightly posterad of middle; lateral margins broadly curved, continuously curved between pronotum and elytron. Depressed and somewhat flattened in lateral aspect.

Coloration. Head dark green, clypeus and labrum pale yellow. Pronotum dark green, with broad lateral yellow marginal band (Fig. 10), medial margin of band distinctly delimited, band separated from posterolateral margin by narrow green strip (Fig. 10). Elytron dark green with broad lateral yellow marginal band, medial margin of band distinctly delimited, lateral margin of band slightly remote from lateral elytral margin for medial portion of length, extending laterally to margin of elytron anteriorly and posteriorly, apex of yellow band diffusing into coloration of elytron, less distinct (Fig. 10). Ventral surfaces mostly black except antennae and palpi orange, pro- and mesothoracic legs mostly orange, tarsi dark orange, metathoracic legs testaceous, anterodorsal surface of tibia orange, propleuron and elytral epipleuron pale orange to testaceous, and with small orange maculae laterally on abdominal ventrites IV–VI.

Sculpture and structure. Head broad, anteriorly produced, eyes prominent; dorsal surface evenly covered with exceptionally fine micropunctures but smooth and shiny. Pronotum with lateral margins evenly and shallowly curved; surface of pronotum similar to surface of head in micropunctation; pronotum dorsally evenly curved. Elytron with margins very broadly curved, more strongly curved posteriorly; surface similar to surface of head in micropunctation and surface appearance. Prosternal process anteriorly distinctly emarginate medially, remaining surface nearly flat throughout and moderately broad, apex broadly elongate and sharply pointed. Metaventral wing narrow (WC/WV = 2.9–3.2); surface smooth, with extremely fine microsculpture, shiny. Lateral portion of metacoxa large, broad, surface smooth and shiny, with extremely fine microsculpture; metacoxal lines short and curved, extending anteriorly less than half distance across metacoxa. Abdominal ventrites smooth, relatively unsculptured.

Male genitalia. Male median lobe in lateral aspect moderately slender throughout, evenly curved, apically abruptly expanded with a narrowly rounded apex, ventral sclerite slender and evenly curved basally, apically straight and apically pointed (Fig. 33); in dorsal aspect median lobe moderately broad basally, lateral margins broadly sinuate, apex broadly expanded, with apex broadly sub-truncate, medially broadly pointed (Fig. 34). Lateral lobe moderately broad throughout length, evenly curved, apex narrowly rounded, with long series of setae along dorsal margin (Fig. 35).

Female genitalia. Vagina extremely elongate, slender; with enlarged area at base of common oviduct and spermatheca, with enlarged lateral sacs on this enlarged area; spermatheca extremely slender and elongate (Fig. 59); gonocoxae together slender and apically pointed (Fig. 59); gonocoxosternite broad, anterolateral lobe broad, medial margin distinctly, narrowly emarginate, basomedially with field of setae (Fig. 59).

Sexual dimorphism. Males have a broad protarsal palette with ventral adhesive setae with apical structures in the form of elongate flattened spatulate surfaces; males also have mesotarsomeres with posteroventral fields of setae which are absent in females. Females lack natatory setae along the ventral margins of the metatarsomeres, but these are present in males. Males have a single metatarsal claw (the anterior claw, Fig. 17), whereas females have a second, posterior claw that is ~ 1/2 the length of the anterior, is abruptly curved apically and sharply pointed (Fig. 18).

Variation. Specimens vary somewhat in size and coloration, with some specimens more greenish and others darker, less greenish.

This species is known from Argentina, Bolivia, Brazil, French Guyana, and Peru. There is also a previously unpublished record from Paraguay.

Specimens were examined from Argentina, Bolivia, Peru, and Paraguay.

Megadytes latus Fabricius, 1801: 260.

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 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.

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).

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.

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).

South America.

Syntypes in Zoological Museum der Universität Kiel, Germany, not examined.

This species differs from M. parvus in its larger size (TL = 19.5–23.2 mm in M. latus, Fig. 13 vs. TL = 13.0–14.6 mm in M. parvus, Fig. 12), broader shape (TL/GW = 1.7–1.8 in M. latus, Fig. 13, vs. TL/GW = 1.9–2.1 in M. parvus, Fig. 12) as well as features of the male genitalia. In M. latus the male median lobe is relatively simple and evenly curved in lateral aspect with the apex narrowly rounded to somewhat pointed (Fig. 44). In M. parvus the median lobe is very broad in lateral aspect with the dorsal sclerite strongly expanded ventrally and curved with the apex strongly recurved (Fig. 39). The lateral lobe in M. latus is very slender apically (Fig. 44), but in M. parvus it is relatively broad throughout (Fig. 41).

Measurements. TL = 19.5–23.2 mm, GW = 11.4–13.4 mm, PW = 8.7–10.3 mm, HW = 5.3–6.2 mm, EW = 3.1–4.0 mm, TL/GW = 1.7–1.8, HW/EW = 1.6–1.7, WC/WV = 2.7–3.4. Body shape large and broadly oval, widest slightly posteriad of middle; lateral margins broadly curved, continuously curved between pronotum and elytron. Depressed and somewhat flattened in lateral aspect.

Coloration. Head dark green to dark brown, anteriorly somewhat green-rufous, clypeus pale rufous, labrum pale yellow. Pronotum dark green to dark brown, laterally broadly dark green-rufous. Elytron dark green with broad lateral green-rufous margins in some specimens. Ventral surfaces black, legs dark rufous.

Sculpture and structure. Head broad, anteriorly produced, eyes prominent; dorsal surface evenly covered with exceptionally fine microsculpture and dispersed micropunctures. Pronotum with lateral margins evenly and shallowly curved; surface of pronotum similar to surface of head in microsculpture and micropunctation; pronotum somewhat swollen anteriorly. Elytron with margins very broadly curved, more strongly curved posteriorly; surface similar to surface of head in microsculpture, but with extensive, very fine micropunctation over entire surface. Prosternal process anteriorly rounded, surface flat and moderately broad, apex broadly elongate and sharply pointed. Metaventral wing moderately broad, ~ 1/3 width of lateral portion of metacoxa; surface smooth, without sculpturing. Lateral portion of metacoxa large, broad, surface smooth, without sculpturing; metacoxal lines short and fine, extending anteriorly less than half distance across metacoxa. Abdominal ventrites smooth, unsculptured.

Male genitalia. Male median lobe in lateral aspect moderately slender throughout, evenly curved, apically narrowed, apex narrowly pointed (Fig. 44); in dorsal aspect moderately narrow, apically narrowed to narrowly rounded apex, ventral sclerite slender throughout length, apically abruptly expanded, apex rounded (Fig. 45). Lateral lobe slender throughout length, apically very slender, with long series of setae along dorsal margin (Fig. 46).

Female genitalia. The only female examined has the internal genitalia missing. Externally, the female gonocoxosternite is broad with the medial margin linear; the gonocoxae are together fused and knifelike, broad anteriorly, abruptly constricted subapically and apically linear to narrowly rounded apex (Fig. 60); rami short; other internal structures (vagina, spermatheca, etc.) not observed.

Sexual dimorphism. Males have a broad protarsal palette with ventral adhesive setae with apical structures in the form of elongate flattened structures; males also have mesotarsomeres with posteroventral fields of setae. Females lack natatory setae along the ventral margins of the metatarsomeres, but these are present in males. Males and females each have two metatarsal claws with the posterior shorter, but females have the posterior claw slightly longer than in the male (Figs 21, 22).

Variation. Specimens exhibit variation in size (TL = 19.5–23.2 mm) but are consistent in shape (TL/GW = 1.7–1.8), and male genitalic shape and other features are relatively consistent across the range of the species. There is some variation in coloration with most specimens dark green-black, but a single specimen from the Gran Sabana, Venezuela (MIZA) is dorsally strikingly green. This specimen is also smaller than most (TL = 19.5 mm) and may represent a distinctive regional population or separate species.

This species is known from Argentina, Brazil, French Guiana, Uruguay (Sharp 1882; Blackwelder 1944; Mouchamps 1957; Trémouilles and Bachmann 1980; Trémouilles 1989b), and Venezuela (MIZA, new country record).

Few specimens of this species exist in collections. Two specimens from Argentina, one from Bolivia (new country record), one from Brazil (KBMC), and a specimen from the Gran Sabana, Venezuela (MIZA) were examined for this study.

Brazil, Bahia State, Santa Rita.

Holotype and nine paratypes in Museu de Zoologia, Universidade de São Paulo, Brazil and two paratypes, one male and one female, in Museo de La Plata, La Plata, Argentina.

This species differs from the other species in the genus, M. latus, in smaller size (TL = 13.0–14.6 mm), narrower shape (TL/GW = 1.9–2.1) (Fig. 12) and features of the male genitalia (Figs 39–43). See above under M. latus for details about these differences between the two species.

Measurements. TL = 13.0–14.6 mm, GW = 6.2–7.7 mm, PW = 5.4–5.9 mm, HW = 3.3–3.4 mm, EW = 2.0–2.1 mm, TL/GW = 1.9–2.1, HW/EW = 1.6–1.7, WC/WV = 4.1–4.8. Body shape elongate oval, widest slightly posteriad of middle; lateral margins broadly curved, continuously curved between pronotum and elytron (Fig. 12). Depressed and somewhat flattened in lateral aspect.

Coloration. Head dark green to green-rufous throughout. Pronotum dark green with broad lateral green-rufous margins. Elytron dark green with broad lateral green-rufous margins. Ventral surfaces dark rufous.

Sculpture and structure. Head broad, anteriorly produced, eyes prominent; dorsal surface evenly covered with extremely fine microsculpture and micropunctures. Pronotum with lateral margins evenly and broadly curved; surface similar to surface of head in microsculpture and micropunctation; pronotum somewhat swollen anteriorly. Elytron with margins very broadly curved; surface similar to surface of head in microsculpture, but with extensive, very fine micropunctation. Prosternal process anteriorly rounded, surface flat and broad, apex elongate and sharply pointed. Metaventral wing moderately broad, ~ 1/3 width of lateral portion of metacoxa; surface smooth, without sculpturing. Lateral portion of metacoxa large, broad, surface smooth, without sculpturing; metacoxal lines short, extending anteriorly less than half distance across metacoxa. Abdominal ventrites smooth, unsculptured.

Male genitalia. Male median lobe in lateral aspect broad throughout, subapically somewhat expanded, apically narrowed, apex with multiple small processes (Fig. 39); in dorsal aspect moderately narrow, evenly narrowed apically, apex narrowly lobed, ventral sclerite apically broadly lobed, extending to near apex (Fig. 40). Lateral lobe broad throughout length, evenly curved, apex rounded, with long series of setae along ventral margin (Fig. 41).

Female genitalia. The single female specimen available for examination lacks female genitalia which apparently have been dissected and lost.

Sexual dimorphism. Males have a broad protarsal palette with ventral adhesive setae with apical structures in the form of elongate flattened structures; males also have mesotarsomeres with posteroventral fields of setae. Females lack natatory setae along the ventral margins of the metatarsomeres, but these are present in males. Males and females each have unequal length metatarsal claws with the posterior shorter, but in males the posterior claw is relatively longer than in females, and distinctly, but only somewhat, shorter than the anterior (~ 4/5 of length).

Variation. Two paratype specimens were examined, a male and a female. The female has the extent and intensity of greenish-rufous coloration somewhat greater than the male, but otherwise the two specimens are similar.

This species is known only from Santa Rita, Bahia State, Brazil. Indication by Trémouilles (1984) of the locality “Santa Rita” to Goyas State, Brazil is erroneous. In Brazil, there are several locations called Santa Rita, but in all labels of the type materials indicate “Santa Rita BA” in clear reference to Bahia State, Brazil (BA = abbreviation of Bahia) and not to Goiás State (= GO). “Goyaz” is an old spelling of Goiás State.

Nothing is known of the habitat of this species.

Twelve specimens in MZSP – holotype male and nine paratypes, six males and three females, each specimen labeled, “Santa Rita BA – Brasil IV.1958 E. Dente col. [label with black line border]/ Cybister (Cybister) parvus E. Tremouilles [handwritten] 1990 det. E. R. Tremouilles [label with black line border]/ Cybister (Meganectes) parvus. 1984 Tremouilles [red label, black line border, handwritten]”. Holotype, one paratype female and the other paratypes respectively labeled, “Holotypus [red label, black line border], Holotypus [red label, black line border], Paratypes [label with black line border]”; two paratypes in MLP – one male and one female, each specimen labeled, “Santa Rita BA – Brasil IV.1958 E. Dente col. [label with black line border]/ Cybister (Cybister) parvus E. Tremouilles [handwritten] 1990 det. E. R. Tremouilles [label with black line border]/ PARATYPUS/ MUSEO DE LA PLATA PARATIPO Cybister (Meganectes) parvus. 1984 Tremouilles [red label, black line border, handwritten].”

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

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).

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.

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).

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.

Nilssondytes diversus sp. nov., by current designation.

From other Cybistrinae this genus differs in having: (1) the metatibial spurs apically simple, (2) metacoxal lines clearly present, (3) the pronotum and elytron with broad, distinct lateral yellow bands along margins (Fig. 11), (4) males and females each with two metatarsal claws, the posterior much reduced in both sexes (Figs 19, 20), (5) the prosternum and prosternal process relatively shallowly but distinctly sulcate, (6) the medial margins of the male sternite IX straight, not emarginate (Fig. 53), (7) no cluster or line of setae at the apicodorsal angle of the posterior surface of the mesotarsomeres, and (8) the ventral surface of the metatrochanter with an oblique, transverse groove. The single species in this genus (described below) is somewhat similar in size, shape and coloration to Metaxydytes laevigatus (Olivier) and may be present among series of that species in collections. Nilssondytes differ from M. laevigatus in several features (see above) including the presence of yellow lateral elytral margins (Fig. 11) which are absent in M. laevigatus. Larvae are unknown.

This genus is named Nilssondytes from the Latin dytes meaning “diver,” and Nilsson, after the great diving beetle worker and excellent friend, Anders Nilsson, in honor of his inestimable contribution to the science of diving beetle biology.

The single species of Nilssondytes is part of the clade that includes species with an oblique metatrochanteric groove, but it has an unresolved position with respect to other genera (Figs 75, 76). The presence of a reduced posterior metatarsal claw in both males and females (Figs 19, 20) with the straight medial margins of the male abdominal sternite IX (Fig. 53) is a unique combination of features within Cybistrinae. Unique among this larger clade is also the sulcate prosternum and prosternal process which is somewhat similar to the Australian genera Spencerhydrus and Sternhydrus.

Venezuela, Amazonas State, roadside pond ca. 7 km S Samariapo 5°10.900'N, 67°46.078'W, 95 m elev.

This is the only species in the genus and is characterized by its diagnostic combination (see above). Typically, species-level features include the shape of the male median lobe which is unique. In ventral aspect the apex is abruptly constricted with the apex narrowly truncate with laterally pointed processes (Fig. 37). In lateral aspect, the median lobe is moderately evenly curved on the dorsal margin, lobe apically abruptly narrowed with the apex elongate and slender, apically narrowly rounded (Fig. 36).

Measurements. TL = 16.7–19.4 mm, GW = 9.6–10.7 mm, PW = 7.0–8.1 mm, HW = 4.2–4.7 mm, EW = 2.7–2.9 mm, TL/GW = 1.7–1.8, HW/EW = 1.6–1.7, WC/WV = 3.1–3.2. Body shape suboval, slightly expanded posteriorly, widest at ~ 3/5 of length; lateral margins evenly, continuously curved between pronotum and elytron. Depressed and somewhat flattened in lateral aspect (Figs 11, 73).

Coloration (Figs 11, 73). Head dark green, anterior clypeal margin yellow, more so laterally, testaceous near eyes. Pronotum dark green with broad lateral yellow margin, posteriorly interrupted and green in one of the four examined specimens, in other specimen yellow extending to posterior angle. Elytron dark green with broad lateral yellow band, separated narrowly from lateral margin, slightly expanded near apex. Ventral surfaces largely black, testaceous on head, basal leg segments and elytral epipleuron.

Sculpture and structure. Head broad, frontoclypeal lines elongate, straight, strongly oblique; anterior clypeal margin broadly, shallowly, and evenly concave; dorsal surface evenly covered with fine microsculpture and micropunctures. Pronotum with lateral margins evenly and broadly curved; surface similar to surface of head in microsculpture and micropunctation. Elytral lateral margin evenly and slightly curved for most of length, apically broadly curved; surface of elytron similar to surface of head in microsculpture and micropunctation. Prosternal process apically rounded, ventral surface distinctly sulcate, apex robust, acutely pointed. Metaventral wing broad, slightly less than 1/3 width of lateral portion of metacoxa (WC/WV = 3.1–3.2); surface smooth, without sculpturing. Lateral portion of metacoxa large, broad, surface smooth, without sculpturing; metacoxal lines short, extending less than half distance across metacoxa. Abdominal ventrites smooth, unsculptured.

Male genitalia. Male median lobe in lateral aspect shallowly curved, apically abruptly narrowed, apex narrowed, slightly curved, apically narrowly rounded, broad medially (Fig. 36). In dorsal aspect broad throughout most of length, apically abruptly narrowed, apex laterally produced, submedially with broad, elongate lobes on each side, ventral sclerite short, apically sharp, acuminate, extending to 3/5 length of median lobe, apex sharply pointed (Fig. 37). Lateral lobe broad in basal half, apically distinctly narrowed, apex narrowly rounded, with series of elongate setae along more than apical half of dorsal margin of lateral lobe (Fig. 38).

Female genitalia. With a single genital opening, vagina elongate, slender, with enlarged, bulbous region at base of common oviduct; spermatheca short, curved, at apex of enlarged region, with soft tissue region on each side of enlarged region (Fig. 61); gonocoxae together broad, apically broadly pointed (Fig. 61); gonocoxosternite broad, with elongate anterolateral lobe, with medial margin sublinear, without conspicuous setae (Fig. 61).

Sexual dimorphism. Males have a characteristic broad protarsal palette with ventral adhesive setae. Males also have mesotarsomeres with clumps of posteroventral setae. Females lack pro- and mesotarsal expansions or adhesive setae. Both males and females have two metatarsal claws with the posterior shorter than the anterior, but females have the posterior somewhat more curved than in males (Figs 19, 20). Females have distinctive microsculpture on the surface of the elytron anteriorly in the form of a field of short striae which is absent in males.

Variation. Five specimens were examined. One specimen has the lateral pronotal yellow band extending to the posterior margin of the pronotum, the others have a narrow dark green separation from the posterior margin.

This species is known from few localities in Venezuela along the northwestern margins of the Guiana Shield craton (Fig. 74).

The only natural history information available from labels is “roadside pond,” “river margin,” and “rock outcropping.”

The species is named from the Latin diversus, meaning “different,” in recognition of the different lengths of the metatarsal claws in both males and females (Figs 19, 20).

Holotype , male labeled, “VENEZUELA: Amazonas State 5°10.900'N, 67°46.078'W, 95 m ca. 7 km S. Samariapo 15.i.2009; leg. Short, Miller, García, Camacho, Joly VZ09-0115-02X: roadside pond/ SM0846115 KUNHM-ENT [barcode label]/ HOLOTYPE: Nilssondytes diversus Miller, Michat and Ferreira-Jr., 2023 [red label with double black line border].” Paratypes, 1 male labeled, “Suapure VENEZ. Caura River 4.20.1900 [handwritten] E.A. Klages.”, 1 female labeled “VENEZUELA: Bolivar State 7°41'23.6"N, 64°1'56.0"W, 134 m ca. 14 km E Rio Aro; 5.viii.2008 leg. A. Short $ M. García AS-08-073; rock outcropping/ SM0829328 KUNMH-ENT [barcode label],” 1 female labeled “VENEZUELA: Guárico State 8°6.226'N, 66°26.228'W, 52 m UCV San Nicolasito Field Station: Rio Aguaro; 10.i.2009 leg. Short, Miller, Joly, García, Camacho; VZ09-0110-01A/ SEMC0852602 KUNHM-ENT,” 1 male labeled “VENEZUELA: Bolivar State 6.58694°N; 67.02912°W Rio Caripito 12.i.2009; leg. Short Miller VZ09-0112-02A: river margin/ SM0844405 KUNHM-ENT [barcode label].” All paratypes with, “…PARATYPE Nilssondytes diversus Miller, Michat and Ferreira-Jr., 2023 [blue label with black line border].”

Dyticus glaucus Brullé, 1837: 46 by original designation.

Like Metaxydytes these species have both metatibial spurs apically simple, the medial margins of male abdominal sternite IX straight, and both males and females with two metatarsal claws, males with equal-length claws and females with the posterior claw reduced (Figs 26, 27). Females also share the characteristic of the medial margins of the gonocoxosternite with a series of spinous setae (Fig. 63). The lateral pronotal margin has a diffuse, but distinctive lateral pale band. From Metaxydytes these specimens are larger with the metaventrite wings relatively broader (see above under Metaxydytes for details of diagnostic comparisons). Third instar larvae have (1) the median lobe of the frontoclypeus truncate apically with a tuft of setae (Fig. 70), (2) the median and lateral lobes of the frontoclypeus separated by a wide emargination (Fig. 70), and (3) the lateral lobes of the frontoclypeus bilobed (Fig. 70).

This may be the sister genus to Metaxydytes (Figs 75, 76; Miller et al. 2007) although here it is in an unresolved position relative to Nilssondytes, Metaxydytes and Megadytes + Cybister (Figs 75, 76). See under Metaxydytes for further discussion.

There are currently two valid species in this genus, P. australis (Germain) and P. glaucus (Brullé). Trémouilles and Bachmann (1980) characterized and differentiated them.

Cybister fallax Aubé, 1838b: 54, by original designation.

These are former Megadytes species with the anterior metatibial spur apically trifid (Fig. 3). Specimens are very large for diving beetle species (TL = 27–36 mm). They are somewhat similar to Bifurcitus specimens. See under that genus for diagnostic comparisons. Larvae are distinctive in having the median lobe of the frontoclypeus sharp apically without an apical tuft of setae.

Trifurcitus is sister group to Bifurcitus (Figs 75, 76). Both males and females have equal-length metatarsal claws and the anterior metatibial spur is either bifid (Figs 1, 2) or trifid (Fig. 3), although these two conditions may possibly not be homologous.

See above under Bifurcitus for more discussion of these two taxa. Six species are currently recognized. Although they have not been revised thoroughly, most of the species were described or illustrated by Trémouilles (1989a) and Trémouilles and Bachmann (1980).

This single male specimen of a cybistrine from Peru presents some problems. It appears to be an undescribed species based on the male genitalia (Figs 48–50). The specimen was DNA sequenced and analyzed for a project by Ribera et al. (2008) where it was found to be in a group with species then assigned to Megadytes (including species of M. (Bifurcitus), M. (Paramegadytes) and M. (Megadytes)). The male specimen currently includes a single metathoracic leg (the other is absent). On it, there are two unequal length metatarsal claws with the posterior short, much shorter than the anterior (Fig. 25), which places it outside the historical diagnosis of Megadytes which includes males with equal-length metatarsal claws (although see above). However, the specimen is missing important morphological structures for further interpreting its placement within Cybistrinae including the mesothoracic legs (which are important for examining the posterodorsal series of setae on the mesotarsomeres) and components of the genital capsule (which are important for examining the emargination of the medial margins of abdominal sternite IX). Because of this, the specimen cannot be placed within a known genus. Nor is it reasonable to place it in a new genus or expand the definition of an existing genus to include it given the lack of information about its features. Hopefully, additional specimens will be found to allow this species to be described and placed. The species is described here to the extent possible to allow for future identification and investigation.

Figures 9–14. 

Neotropical Cybistrinae species, habitus 9 Cybister festae 10 Cybister puncticollis 11 Nilssondytes diversus 12 Megadytes parvus 13 Megadytes latus 14 species “IR57” (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 “IR57” (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 “IR57” (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.

Peru, Atalapa, Rio Carbon at Rio Madre de Dios, in river, Apr 1999 (Ribera et al. 2008).

Measurements. TL = 17.5 mm, GW = 10.4 mm, PW = 12.8 mm, HW = 4.8 mm, EW = 2.4 mm, TL/GW = 1.7, HW/EW = 2.0, WC/WV = 4.2. Body shape (Fig. 14) broad, expanded posteriorly, widest at ~ 3/5 of length; lateral margins evenly, continuously curved between pronotum and elytron. Depressed and somewhat flattened in lateral aspect.

Coloration. All dorsal surfaces dark reddish brown, without yellow margins on pronotum or elytron. Ventral surfaces entirely dark reddish brown, somewhat more reddish on ventral surfaces of prothorax and pro- and mesothoracic legs.

Sculpture and structure. Head broad; anterior clypeal margin broadly, shallowly and evenly concave; eyes large (HW/EW = 2.0). Dorsal surface shiny and evenly covered with fine micropunctures on head and pronotum, very few sparse micropunctures on elytron. Pronotum with lateral margins evenly and broadly curved. Elytral lateral margin evenly and slightly curved for most of length, apically broadly curved. Prosternal process apically broadly, shallowly concave, ventral surface flat throughout, apex robust, acutely pointed. Metaventral wing narrow, ~ 1/4 width of lateral portion of metacoxa (WC/WV = 4.2); surface smooth, with extremely fine punctation. Lateral portion of metacoxa large, broad, surface smooth, with dispersed, very fine micropunctures; metacoxal lines short, extending only ~ 1/3 distance across metacoxa. Abdominal ventrites smooth, unsculptured.

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 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.

Figure 72. 

Cybister (Neocybister) festae, lectotype male (left) and paralectotype female (right). Photograph courtesy of F. Giachino, MRSN.

Figure 73. 

Nilssondytes diversus, holotype specimen. Scale bar: 5.0 mm.

Figure 74. 

Distribution of Nilssondytes diversus in northern South America.

Male genitalia. Male median lobe in lateral aspect slender, broader submedially and gradually narrowed apically to slightly curved apex, apex bluntly rounded (Fig. 48). In dorsal aspect slender, evenly and gradually narrowed to narrowly rounded apex; dorsal sclerite very slender and elongate (Fig. 49). Lateral lobe extremely slender throughout, broadly curved with long series of long setae along dorsal margin (Fig. 50).

Female genitalia. Females are not known.

Sexual dimorphism. Only a single male was examined. However, this male has a characteristic broad protarsal palette with ventral adhesive setae. Males also have mesotarsomeres with clumps of posteroventral setae. Other typical sexually dimorphic features cannot be compared.

Variation. Only a single specimen was examined.

A single male specimen examined labeled, “271297 PERU ZUNGARO COHA 16u:TO [handwritten, somewhat illegible].”