The unique locomotor apparatus of whirligig beetles of the tribe Orectochilini (Gyrinidae, Coleoptera)

Abstract

Whirligig beetles, which are known for their rapid gliding on the water surface, have evolved a unique locomotor apparatus. External and internal thoracic structures of Orectochilus villosus (Orectochilini) are described in detail and documented with microcomputed tomography, computer‐based 3D reconstructions, and scanning electronic microscopy (SEM). The results are compared with conditions found in other genera of Gyrinidae and other groups of Coleoptera. The focus is on structures linked with locomotion, especially on the unusual flight apparatus, which differs strongly from that of other beetles. As in the other Orectochilini, the prothorax of Orectochilus displays characters typical for Gyrinidae, with triangular procoxae and forelegs transformed into elongated, sexually dimorphic grasping devices. The musculature of this segment is similar to the pattern found in other Coleoptera. Similar to all other extant Gyrinidae, the mesothorax is characterized by an extensive and flat mesoventrite, suitable for gliding on the water surface. As in Heterogyrinae and the other Gyrininae, the pterothoracic legs are transformed into paddle‐like structures, enabling the beetles to move with high speed on the surface film. The musculature of the mesothorax is reduced compared to other Coleoptera, but similar to what is found in the other Gyrininae. The metathoracic skeleton and musculature are simplified in Orectochilini compared to other Gyrininae and other groups of Coleoptera. In O. villosus, only 10 metathoracic muscles are preserved. 36 are present in an archostematan beetle, a condition probably close to the coleopteran ground plan. The metathoracic dorsal longitudinal bundles are absent in Gyrininae, muscles that play a role as indirect flight muscles in most other neopteran insects. The rest of the posteromotoric flight apparatus is distinctly modified, with a limited number of skeletomuscular elements taking over more functions. The large muscle M84 (IIIdvm7) M. noto–trochanteralis, for instance, functions as dominant wing levator, but is also responsible for the powerful and rapid backstroke of the hind legs. The presence of this muscle is a synapomorphy of Heterogyrinae and Gyrininae. The narrow metafurca in the latter group is likely linked to its large size. The elytra likely contribute to the control of the flight of the beetle, whereas they shield and inhibit the flight apparatus during swimming.

On the thoracic anatomy of the MadagascanHeterogyrus millotiand the phylogeny of Gyrinidae (Coleoptera)

“…d; e.g. Larsén, , ; Beutel, ; Liu et al ., ), and the medially inflected metapostnotum, both very unusual features in Adephaga and other groups of beetles (Larsén, ; Beutel & Komarek, ; Friedrich & Beutel, ; Friedrich et al ., ). These modifications are likely linked with the complete loss of the dorsal longitudinal muscles (Mm60, 61) and of M. dorsoventralis primus (M64) (Figs ), which are normally crucial elements of a flight apparatus.…”

“…An evolutionary burst in terms of structural modification (e.g. Larsén, ; Beutel & Roughley, 1993; Liu et al ., ) was linked with the rise of orectochilines. The tribe comprises c .…”

“…The characters are coded and entered in a data matrix, combined with information from published studies on adults and immature stages (e.g. Hatch, ; Larsén, ; Beutel & Roughley, , ; Beutel, ,b, ; ; Arndt et al ., ; Miller & Bergsten, ; Michat et al ., ; Liu et al ., ). Another aim of this study is a phylogenetic placement of the genus and an assessment of the phylogenentic relationships of the family given new data.…”

The morphological evolution of the Adephaga (Coleoptera)

“…Orectochilini are arguably the most ‘derived group of Gyrininae', with a row of setae on the elongated fused gonocoxosterna as a steering organ (char. 99.1), and a strongly simplified pterothoracic muscle system (Liu et al ., ), despite a functional flight apparatus.…”

“…Beutel & Roughley, 1988;Baca et al, 2017a), have evolved highly specialized surface-swimming habits as adults, linked with numerous autapomorphic features (e.g. Larsén, 1966;Beutel, 1989aBeutel, , 1989bBeutel et al, 2017Beutel et al, , 2019aLiu et al, 2018).…”