Rhyacobates Esaki 1923
Creators: name: Leng, Zhaoqi
name: Tran, Anh Duc
name: Ye, Zhen
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Insecta
Hemiptera
Gerridae
Rhyacobates

description: Genus Rhyacobates Esaki, 1923
Rhyacobates Esaki, 1923: 387.
Esakobates Lundblad, 1934: 22 (syn. Hungerford & Matsuda 1959: 69).
Type species
Rhyacobates takahashii Esaki, 1923.
Diagnosis
Medium-sized (Figs 1–2), females 6.79–12.21, males 5.42–8.41, females distinctly larger than males in the same species (Figs 3–5). Dorsum mainly blackish with silvery pubescence, pronotum with a median yellowish marking (Figs 3–4). 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). 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). Female: posterior abdominal segments curved dorsad or nearly straight (Fig. 5); 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 Female gonocoxa directed caudad, usually completely withdrawn into sternum VII.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); paramere long and curved dorsad, without long setae (Fig. 8).
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). Rhyacobates can be distinguished from Celerobates by the absence of distinct claws in the middle and hind tarsi (Fig. 1).
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).
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). 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). However, in some rare situations, a population may completely consist of macropterous and dealated form, e.g., of R. chinensis (Fig. 12B). 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, 12A–C, 13, 14A–D), 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). 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).
Mating behavior During mating, the male jumps onto the dorsum of a female, ‘hugging’ the female with its fore legs (Figs 12A–D, 13, 14B–C). The entire mating process may happen while skating on the water surface (Fig. 12D) or on waterside rocks (Figs 12A–C, 13, 14B–C). 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, 5). We hypothesize that this structure might help females reject an unwanted mating.
description: Published as part of Leng, Zhaoqi, Tran, Anh Duc & Ye, Zhen, 2023, Taxonomic review of Rhyacobates Esaki, 1923, with descriptions of three new species (Hemiptera: Heteroptera: Gerridae), pp. 1-73 in European Journal of Taxonomy 893 (1) on pages 3-10, DOI: 10.5852/ejt.2023.893.2285, http://zenodo.org/record/8385630