Rhyacobates Esaki, 1923

Leng, Zhaoqi, Tran, Anh Duc & Ye, Zhen, 2023, Taxonomic review of Rhyacobates Esaki, 1923, with descriptions of three new species (Hemiptera: Heteroptera: Gerridae), European Journal of Taxonomy 893 (1), pp. 1-73 : 3-10

publication ID	https://doi.org/ 10.5852/ejt.2023.893.2285
publication LSID	lsid:zoobank.org:pub:D9DA4879-BC1D-4947-96E8-7D574DE334BA
DOI	https://doi.org/10.5281/zenodo.8385636
persistent identifier	https://treatment.plazi.org/id/045E878D-FFDE-2930-3A44-FBFE720F0E73
treatment provided by	Plazi
scientific name	Rhyacobates Esaki, 1923
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Treatment

Genus Rhyacobates Esaki, 1923 View in CoL

Rhyacobates Esaki, 1923: 387 View in CoL .

Esakobates Lundblad, 1934: 22 View in CoL (syn. Hungerford & Matsuda 1959: 69).

Type species

Rhyacobates takahashii Esaki, 1923 View in CoL .

Diagnosis

Medium-sized ( Figs 1–2 View Fig View Fig ), females 6.79–12.21, males 5.42–8.41, females distinctly larger than males in the same species ( Figs 3–5 View Fig View Fig View Fig ). Dorsum mainly blackish with silvery pubescence, pronotum with a median yellowish marking ( Figs 3–4 View Fig View Fig ). Antennal tubercles pronounced, angularly produced in dorsal view; antennal segment I longer than other three segments combined, segment II shorter than segment III, segment IV curved, with whitish groove at distal two-fifths ( Figs 1–2 View Fig View Fig ). Fore femur slender, subapically without tooth on ventral surface. Middle femur with black spines along proximal three quarters of ventral margin, but usually not in distinct row. Middle coxa without apical spine, not elongate. Middle and hind tarsi without claws ( Fig. 1 View Fig ). Female: posterior abdominal segments curved dorsad or nearly straight ( Fig. 5 View Fig ); segment VII usually modified, length of sternum VII about twice the length of sternum VI. Female gonocoxa directed caudad, usually completely withdrawn into sternum VII. Male genitalia: pygophore simple, without lateral process; proctiger laterally produced into rounded or angular lobes ( Figs 6–7 View Fig View Fig ); paramere long and curved dorsad, without long setae ( Fig. 8 View Fig ).

Comparative notes

The differences between Rhyacobates and its closely related genera (i.e., Heterobates and Pleciobates ) were summarized by Andersen & Chen (1995). Three genera described after the study of Andersen & Chen (1995), Andersenius , Pleciogonus and Celerobates are also closely related to Rhyacobates . However, Rhyacobates can be distinguished from Andersenius by the hind coxa, which is shorter than wide in the former. In Andersenius , the hind coxa is distinctly longer than wide, i.e., 3.5–4.0 times as long as wide in the female and 1.2–2.0 times as long as wide in the male. Rhyacobates can also be distinguished from Pleciogonus by connexival segment VI of the female, which is simple, without a long caudal process ( Figs 1–2 View Fig View Fig ). Rhyacobates can be distinguished from Celerobates by the absence of distinct claws in the middle and hind tarsi ( Fig. 1 View Fig ).

Distribution

The genus Rhyacobates is distributed from eastern Asia (the Korean Peninsula, mainland China, and Taiwan Island) to Indochina ( Myanmar, Thailand, and Vietnam) ( Fig. 9 View Fig ).

Biology and ecology

Habitats

Species of Rhyacobates inhabit foot-hill and mountainous streams, rivers and sometimes pools. Most species are found only in running water with relatively cool temperatures ( Fig. 10A–C View Fig ). Elevations of habitats have been recorded from 22 to 2041 m, but mostly between 500 and 1000 m. Streams with emergent rocks in the current are typical habitats of Rhyacobates , as the rocks offer necessary resting areas for these skaters ( Esaki 1923; Tran & Yang 2006; Tran & Nguyen 2016).

Living forms

Usually, most adults found in the populations are the apterous form ( Fig. 11A–D View Fig ). However, in some rare situations, a population may completely consist of macropterous and dealated form, e.g., of R. chinensis ( Fig. 12B View Fig ). Tran & Nguyen (2016) reported one macropterous specimen (with dealated wings) of R. zetteli inhabiting an unshaded, tiny water flow ca 500 m away from the population in the main stream, indicating that the macropterous form might have good flying ability.

Perching behavior

Although most species of Rhyacobates can stride on torrent or fast-running water, they spend plenty of time resting on waterside rocks ( Figs 11A–B, E View Fig , 12A–C View Fig , 13 View Fig , 14A–D View Fig ), which was first observed by Esaki (1923). They are very alert and when potential enemies (predators or larger animals) are detected nearby, they will jump into the water and stride irregularly at an extremely fast speed ( Fig. 11D View Fig ). Esaki (1923) hypothesized that this swift gliding may cause disturbance of the water and make the insects unrecognizable to the predators.

Predatory behavior

Apparently, individuals of Rhyacobates access the water surface and stride against the torrent when they are searching for food. We have observed that they can rapidly locate living insects floating nearby. They sometimes jump onto waterside rocks soon after they catch their prey, where they can feed on it undisturbed ( Fig. 13 View Fig ).

Mating behavior During mating, the male jumps onto the dorsum of a female, ‘hugging’ the female with its fore legs ( Figs 12A–D View Fig , 13 View Fig , 14B–C View Fig ). The entire mating process may happen while skating on the water surface ( Fig. 12D View Fig ) or on waterside rocks ( Figs 12A–C View Fig , 13 View Fig , 14B–C View Fig ). The adults of Rhyacobates are often found as copulating pairs; they tend to remain connected even after being captured, seemingly too unwilling to separate from their mates. Abdominal segment VII of the female is elongate and highly modified, and the genital segments are often withdrawn into it ( Figs 3 View Fig , 5 View Fig ). We hypothesize that this structure might help females reject an unwanted mating.