Gyrinus (Neogyrinus) gibbus Aubé, 1838

Gustafson, Grey T. & Short, Andrew E. Z., 2017, Review of the whirligig beetle genus Gyrinus of Venezuela (Coleoptera: Gyrinidae), Acta Entomologica Musei Nationalis Pragae 57 (2), pp. 479-520 : 484-490

Gyrinus (Neogyrinus) gibbus Aubé, 1838

( Figs 2, 5 , 10–19 , 51–52 , 70–71 , 75 )

Gyrinus gibbus Aubé, 1838: 709 (original description). Gyrinus apicalis Sharp, 1877: 117 (original description); synonymy by RÉGIMBART (1883): 186. Gyrinus (Neogyrinus) gibbus: OCHS (1935a) : 126 (new status).

Type localities. Gyrinus gibbus : ‘Brazil’. Gyrinus apicalis : ‘Santa Cruz’, Brazil’ [likely Santa Cruz do Sol, Brazil]. Material examined. VENEZUELA: AMAZONAS: Stream along Rio Sipapo, 4°55.849’N, 67°44.645’W, 16.i.2009, leg. Short /Miller, VZ09011602 (1 spec. KBMC); swift trib. to Rio Siapa, 5.ii.1989, leg. D. A.Polhemus, CL8007 (5 spec. USNM); 40 km S Puerto Ayacucho, at Tobogan, 19.ii.1986, leg. P.J.Spangler, colln#1, USNM ENT 00717233– USNM ENT 00717236 (4 spec. USNM); nr Iboruwa, “Tobogancito”, 5°48.414’N, 67°26.313’W, 13.i.2009, leg. Short /Miller, VZ09011302 (2 spec. KBMC); riv nr Orinoco /Sipapo confl., 5°03.707›N, 67°46.768›W, 15.i.2009, leg. Short /Miller, VZ09011501 (4 spec. KBMC). BOLÍVAR: ca. 25 km E. El Burro, 6°13’4.6”N, 67°14’26.4”W, 60 m, 7.viii.2008, leg. A.Short, M.García, L.Joly, rocky morichal, AS-08-077, SM0827797 (1 spec. SEMC); Los Pijiguaos, 6°35.617’N, 66°49.238’W, 12.i.2009, leg. Short /Miller, morichal, VZ09011201 (4 spec. KBMC). CAPITAL DISTRICT: Libertador, El Valle, 22.vi.–22, leg. L.R.Reynolds, F. Psota Col.(40 spec. FMNH); Caracas Valley, 6.v.–22, leg. L.R.Reynolds, F. Psota Col. (1 spec. FMNH); Las Trincheras, 9.vi.–22, leg. L.R.Reynolds, F.Psota Col. (1 spec. FMNH). Non-Venezuelan material examined. BOLIVIA: BENI: Chacobo Indigenous Village, Río Benicito, ‘60W-12:20S’, 26.vii–4.viii.1960, leg. B.Malkin (6 spec. FSCA). BRAZIL: RIO DE JANEIRO: Itatiaia, 17.iv.1960, leg. Borvs Malkin, temporary “muddy” puddle (46 spec. FSCA); SÃO PAULO: São Paulo leg. J. Metz, Karl Brancsik Coll. (1 spec. FMNH). MEXICO: (2 spec. FMNH). PERU: MARISCAL RAMÓN CASTILLA: Estiron, Rio Ampiácu, Loreto, 15–25.xi.1961, leg. B.Malkin, forest stream (66 spec. FSCA).

Diagnosis. Body form ( Figs 2 and 5 ) broadly oval, strongly convex in lateral view; pronotal and elytral margins broad, darkly colored except elytral margin apically, often red or lightly colored; elytral disc with non-uniform reticulation, medially appearing polished, laterally bronzy-metallic in appearance; striae VI–XI evident ( Fig. 10 ) with distinct, widely spaced punctures ( Fig. 19 ), striae VIII–IX at most weakly sulcate ( Fig. 10 ); elytral intervals all similarly convex; elytral apex rounded ( Fig. 11 ), border complete ( Fig. 11 ), epipleural angle indistinct; metanepisternal ostiole absent; aedeagus ( Fig. 13 ) with median lobe just shorter than parameres, narrow, evenly attenuated, weakly laterally expanded in apical 1/5 at swollen apex; gonocoxae ( Fig. 51 ) elongate with obliquely truncate apices.

Gyrinus gibbus is most similar to G. ovatus and in addition to the difference discussed under the diagnosis of G. ovatus , can be distinguishedby the much starker difference between the polished appearance of the reticulation of the medial elytral disc, and the bronzy metallic reticulation of the lateral regions ( Figs 2 and 5 ). The elytral apex of G. gibbus is also distinct among the Venezuelan Neogyrinus species in that it has a complete border to the rounded elytral apex ( Fig. 11 ).

Redescription. Size. Length = 4.5–6.0 mm, width = 2.0– 3.5 mm. Habitus. Body form broadly oval, weakly attenuated anteriorly and posteriorly, widest point just anterior to elytral midlength; in lateral view strongly dorsoventrally convex, greatest convexity posterior to scutellar region, weakly depressed anteriorly and posteriorly.

Coloration ( Figs 2 and 5 ). Dorsally, head, pronotum, elytra bronzy-green, lateralmargins of pronotum and elytral similarly colored as remainder of pronotum and elytra, elytral lateral margin lightly colored apically, often red or yellow; ventrally lightly colored, ventral surface of pedicel, hypomeron, and elytral epipleuron light yellow, abdomen slightly darker yellow to orangish-yellow in color, remainder of venter especially meso- and metaventrites, often more darkly colored orange.

Sculpture and structure. Pronotum with broad lateral margins. Elytra ( Fig. 10 ) with striae I–V present as reticulate stripes ( Fig. 18 ), Vwith very sparse weakly impressed punctures; VI–XI evident ( Fig. 10 ), composed of widely spaced distinct punctures; striae VIII–IX at most weakly sulcate, with largest and most well-impressed punctures ( Fig. 19 ); stria Xwith smaller, widely separate punctures; stria XI mostly marginal, weakly elevated in basal 1/3. Elytral intervals I–VI ( Fig. 18 ) with weakly impressed reticulation composed of meshes with small sculpticells, producingapolished appearance; intervals VII withmorestrongly impressed reticulation basally, meshes composed of larger sculpticells producing a metallic appearance; intervals VIII–IX entirely with more strongly impressed metallic reticulation ( Fig. 19 ); all intervals evenly convex. Elytra without medial pre-apical plica ( Fig. 10 ); apices round to subtruncate; border complete ( Fig. 11 ); epipleural angle indistinct, never with denticle. Metanepisternal ostiole absent. Ultimateabdominal tergite often with strong medial acumination.

Male genitalia ( Figs 13 and 14 ). Aedeagus with median lobeshorter than parameres, gradually narrowing apically, weakly constricted afterbasal 2/3 becomingmore weakly attenuated, laterally expanded in apical 1/5 forming small round process, apex swollen, rounded, weakly bifid medially; parameres with apex obliquely truncate, often weakly emarginate. Female genitalia ( Figs 51 and 52 ). Gonocoxae elongate, apices obliquely truncate, left gonocoxa more-so than right.

Variability. This species is most variable in the development of the elytral reticulation, dorsoventral convexity, and body size. The material examined includeda very unique population of G. gibbus from the Caracas Valley in northern Venezuela ( Fig. 75 ). These specimens are much larger in size and more robust in general appearance than others examined ( Fig. 5 ). These specimens also have acoarser metallic reticulation of the elytra, occupying a larger area than exhibited in other specimens, being present on most of elytral interval V andthe entirety of VI–IX. Together this gives these specimens a much more bronzy appearance and creates a starker contrast between the lateral and medial region of the elytra disc where the weakly impressed reticulation composed of smallersculpticells gives a stronger polished appearance ( Fig. 5 ). The elytral apices are more evenly rounded ( Fig. 5 ) and the Caracas specimens have the ultimate abdominal tergite without a strong medial acumination.

Specimens from Amazonian Venezuela differed strongly in appearance to those from the Caracas Valley, being much less convex, and less rounded in appearance ( Fig. 2 ). They also differed considerably inthe appearance of the medial elytral reticulation ( Fig. 2 ). In the Amazonian Venezuelan populations, the reticulation of the medial elytra discs was more strongly impressed, with slightly larger sculpticells, resulting in less contrast in comparison to that of the metallic reticulation found laterally. However, the distinctly more polished appearance of the medial reticulation is still evident. The elytral apices of the Amazonian specimens are less strongly rounded, and the ultimate abdominal tergite has a strong medial acumination ( Fig. 2 ). However, the aedeagus between the Caracas Valley specimens ( Fig. 13 ) and the more southern Venezuelan populationsdoes not differconsiderably ( Fig. 14 ) compared to theaedeagi of other species (cf. Figs 29–32 ) and both shared all the diagnostic features of the aedeagus.

The variation in the G. gibbus population from Caracas Valley is similar to those of specimens examined from outside the Amazon, in Rio de Janeiro and São Paulo, Brazil and Mexico, whichalso have an evenlyrounded appearance, are strongly convex, and have astark contrast between the lateral and medial elytral disc reticulation. These populations also have the elytral apices more rounded in appearance, but they don’t all lack the medial acumination of the ultimate abdominal tergite.

Specimens examinedfrom further southin the Amazon in Peru and Bolivia were similar to those from Amazonian Venezuelain external appearance ( Fig. 2 ). However, all specimens from across the species entire rangeexhibited the same diagnostic externalfeatures providedabove, as well as having very similar aedeagi ( Figs 12–17 ). The aedeagi exhibit all the diagnostic features of the median lobe, with variation evident in the apex being similar among adjacent populations (i.e. Amazonian Venezuela and Peru: Figs 14 and 15 ) and consistent differences between Amazonian ( Figs 14–16 ) vs. non-Amazonian ( Figs 12, 13 , and 17) specimens primarily in the appearance of the parameres. Amazonian specimens have the parameres apically with a greater acute medial angle to the apex, which in combination with the emargination gives them amore obliquely truncate appearance. Specimens fromoutside the Amazon region have the parameres broader apically with the medial angle of the apex less acute.

Habitat. In Venezuela this species has been primarily found in association with streams and river habitats ( Figs 70–71 ).

Distribution. This is a very widespread Neotropical species found from Argentina through Central America ( OCHS 1948), potentially as far as Mexico based on historic specimens examined here from the FMNH. Within Venezuela, this species has a disjunct distribution with most records coming from the Amazonian region and historic specimens from the Caracas Valley in the north ( Fig. 75 ). See discussion below.

Discussion. The considerable amount of variation among populations of G. gibbus has long been recognized ( RÉGIMBART 1883). There isa general pattern inthe variation exhibited based on distribution, whether the specimens comefrom withinor outside of the Amazon (discussed above in the variability section). In Venezuela, this is exemplified by the specimens from the Caracas Valley ( Fig. 5 ), compared to those from Amazonas ( Fig. 2 ). RÉGIMBART (1889) identified the specimens examined by him from Caracas as G. gibbus var. apicalis Sharp, 1877 . Gyrinus apicalis was originally described as a distinct species from southern Brazil by SHARP (1877), distinguished from G. gibbus by the different elytral reticulation and larger size. However, RÉGIMBART (1883: 186) considered this mere variation and synonymized G. apicalis with the typical G. gibbus from Brazil (without further locality specification).

The specimens from within the Amazonian region of Venezuela ( Fig. 2 ) also showed consistent external differenceswith those fromother parts of the Amazon (discussed above under the variability section). The male genitalia ( Figs 12–17 ), the indicator of species boundaries in this study, show subtle, yetconsistent, differences between the Amazonianpopulations and non-Amazonian populations (primarily in association with the parameres) across G. gibbus ’ range. The gonocoxae are also very similar between the populations ( Figs 51 and 52 ), especially compared to those of other species. In Venezuela, the disjunct distribution exhibited by G. gibbus is quite odd, especially compared to other Venezuelan Neogyrinus species ( Figs 72–75 ). The extensive sampling done by the Venezuelan aquatic insect survey precludes the explanation of this being a sampling artifact. Thus, the current definition of G. gibbus may include two taxa, one from the Amazon region and the other from outside of it. Based solely on morphology, the variation between these two ‘taxa’ would at most warrant subspecific status, when compared to the interspecific variation in morphology of the other Neogyrinus species (cf. Figs 1–6 and 29–32 ). Molecular phylogenetic studies of these populations will ultimately be needed to clarify the relationships of these populations and determine their relative taxonomic status. As our focus was a morphological taxonomic study of the Venezuela Gyrinus species , we will leave the question of the status of G. gibbus and its Amazonian and non-Amazonian populations up to future work.

Figures

Figs 1–6. Dorsal habitus of Neogyrinus species, scale bar = 1 mm. 1 – Gyrinus sabanensis sp. nov. 2 – G. gibbus Aubé, 1838 from Amazonian Venezuela. 3 – G. ovatus Aubé, 1838. 4 – G. guianus Ochs, 1935. 5 – G. gibbus ‘var. apicalis’ Sharp, 1877 from Caracas valley, Venezuela. 6 – G. rozei Ochs, 1953.

Figs 7–11. 7–9 – SEM images of Gyrinus ovatus Aubé, 1838 elytral morphology. 7 – elytron, box indicates region shown in Fig. 8, scale bar = 1 mm. 8 – elytral apex, box indicates region shown in Fig. 9. 9 – denticle of epipleural angle of elytral apex. 10–11 – SEM images of G. gibbus Aubé, 1838 elytral morphology. 10 – elytron, box indicates region shown in Fig. 11. 11 – elytral apex of G. gibbus, showing complete border.

Figs 12–17.Dorsal and lateral view of the aedeagus of G. gibbus Aubé, 1838 specimens from throughout its range, not to scale. 12 – historical specimen from Mexico (FMNH). 13 – Caracas valley, Venezuela ‘var. apicalis’ Sharp, 1877 (FMNH). 14 – Amazonian Venezuela:Amazonas, river near Orinoco /Sipapo confluence (KBMC). 15 – Amazonian Peru: Mariscal Ramón Castilla, Estiron, Rio Ampiácu (FSCA). 16 – Amazonian Bolivia: Beni, Chacobo indigenous village, Río Benicito (FSCA). 17 – São Paulo (FMNH).

Figs 18–21. SEM images of elytral interval reticulation and strial punctures. 18–19 – Gyrinus gibbus Aubé, 1838. 18 – intervals II and III, striae I–III. 19 – intervals IX and X, striae VIII–X. 20–21 – G. ovatus Aubé, 1838. 20 – intervals II and III, striae I–III. 21 – intervals IX and X, striae VIII–X.

Figs 51–59. Ventral view of gonocoxae of Venezuelan Gyrinus species, not to scale. 51–56 – Neogyrinus species. 51 – G. gibbus Aubé, 1838 from Caracas valley, Venezuela (FMNH). 52 – G. gibbus from Amazonian Venezuela: Amazonas stream along Rio Sipapo (KBMC). 53 – G. ovatus Aubé, 1838 from Venezuela: Falcón: W. Curimagua, Blackwater Lagoon.54 – G. guianus Ochs, 1935 from Amazonian Venezuela:Amazonas:river near Orinoco /Sipapo confluence (KBMC). 55 – G. rozei Ochs, 1953. 56 – G. sabanensis sp. nov. 57–59 – Oreogyrinus species: 57 – G. iridinus sp. nov. 58 – G. venezolensis Ochs, 1953. 59 – G. vinolentus sp. nov.

Figs 68–71. Habitat of Gyrinus species in the Guiana Shield region of Venezuela. 68 – Bolívar State, Gran Sabana, collecting event AS-08-063, habitat of G. rozei Ochs, 1953. 69 – Bolívar State, Gran Sabana, collecting event AS- 08-066, habitat of G. sabanensis sp. nov. 70 – Amazonas, tributary of the Rio Sipapo, collecting event VZ09-0116- 02X, habitat of G. guianus Ochs, 1935 and G. gibbus Aubé, 1838. 71 – Bolívar State, 25 km E. El Burro, collecting event AS-08-077, habitat of G. gibbus.

Figs 72–75. Distribution maps of Gyrinus species within Venezuela.

Figs 29–33. Dorsal and lateral view of the aedeagus of Neogyrinus species, not to scale. 29 – G. ovatus Aubé, 1838. 30 – G. sabanensis sp. nov. 31 – G. guianus Ochs, 1935. 32 – G. rozei Ochs, 1953.