Masculine.

Agraphydrus punctatellus Régimbart, 1903: 34; by monotypy.

Small beetles, body length 1.4–4.8 mm. Body shape elongate to broadly oval in dorsal view, weakly to moderately convex in lateral view, rarely strongly convex (Figs 18, 19). Surface of head and pronotum smooth, usually with shallow ground punctation. Body ranging from pale/yellowish to dark brown (Figs 18, 19), either uniform across body regions or with different regions colored differently (e.g., darker head, paler elytra and margins of pronotum; Fig. 18A, B). Eyes with anterior margin straight in lateral view (not emarginate), in dorsal view slightly projecting from outline of head. Clypeus moderately convex, with distinct systematic punctures, with anterior margin slightly to clearly emarginate. Labrum not concealed by clypeus. Mentum nearly 1.5 × wider than long, with variable surface, with wide and moderate median anterior depression limited by low transverse carina. Antennae with eight or nine antennomeres, with slightly asymmetric cupule, round in outline. Maxillary palps elongate, 0.7–1.5 × width of head, with inner margin of palpomere 2 usually straight and palpomere 4 nearly as long to slightly longer than palpomere 3 (Fig. 12G). Pronotum with ground punctation usually moderate. Elytra without sutural striae, not laterally explanate, with serial punctures usually absent; systematic punctures usually rather sparse and aligned in four rows along elytra. Prosternum slightly convex, not carinate medially. Posterior elevation of mesoventrite variable, from simply bulged, to bearing variously shaped elevations; anapleural sutures variable in shape and orientation. Metaventrite with posteromedian glabrous patch. Metafemora without distinct tibial grooves, either mostly pubescent (only glabrous at apex), or with pubescence reduced to small basal area (“Gymnhelochares”). Metatarsomere 1 shorter than 2; metatarsomere 2 slightly shorter than 5; metatarsomere 5 similar in length to metatarsomeres 3 and 4 combined. Fifth abdominal ventrite apically emarginate, sometimes very slightly, or rounded, with or without fringe of stout setae. Aedeagus trilobed in form (Fig. 20); basal piece shorter to longer than parameres; outline of apical region of parameres variable; median lobe triangular, with well-developed lateral basal apodemes, usually rounded at apex; gonopore well developed.

Agraphydrus can be considered highly variable both morphologically and ecologically. Given their usually small to very small size, in the regions where Agraphydrus is distributed, they may be confused with smaller species of Helochares, from which Agraphydrus can be distinguished by the presence of a posteromesal glabrous patch on the metaventrite (metaventrite uniformly and densely covered by hydrofuge pubescence in Helochares); their size allows to differentiate them from the much larger Colossochares and Peltochares. The lack of sutural stria in Agraphydrus allows to recognize the larger Agraphydrus from similarly sized Crephelochares. The maxillary palps tend to be shorter in Agraphydrus. Most Agraphydrus have moderately punctate head and pronotum and generally lack elytral serial punctures; although they may have very coarse systematic punctures somewhat aligned in rows, these rows are not quite uniform as in many Old World Helochares or Acidocerus. The outer margins of the elytra of Agraphydrus are only slightly flared, as opposed to laterally expanded which differentiates them from Batochares. The most similar genus to Agraphydrus would be the Neotropical genus Tobochares, but they do not co-occur; the body shape in Agraphydrus, in general, tends to be more elongated (1.1–1.4 × longer than wide), whereas in Tobochares it tends to be only slightly longer than wide (1.07–1.15 × longer than wide); in addition, the metafemora in Tobochares are always mostly glabrous, with scattered setae, and their serial punctures are well aligned longitudinally.

Afrotropical: Angola, Botswana, Cameroon, Democratic Republic of the Congo, Djibouti, Eritrea, Eswatini, Ethiopia (in doubt), Gabon, Ghana, Guinea, Ivory Coast, Kenya, Madagascar, Malawi, Mozambique, Namibia, Nigeria, Oman, Republic of South Africa, Saudi Arabia, Sudan, Tanzania, United Arab Emirates, Yemen, Zimbabwe. Australasian: Australia (New South Wales, Northern Territory, Queensland, Western Australia), Indonesia (Java, Papua), Papua New Guinea. Indo-Malayan: Bhutan, Brunei, China (Fujian, Guangdong, Guangxi, Guizhou, Hainan, Himachal, Hong Kong, Hunan, Jiangxi, Taiwan, Yunnan, Zhejiang), India (Arunachal Pradesh, Assam, Goa, Himachal Pradesh, Kerala, Karnataka, Madhya Pradesh, Maharashtra, Meghalaya, North Andaman Island, Sikkim, Tamil Nadu, Uttar Pradesh, Uttarakhand), Laos, Malaysia, Myanmar, Nepal, Philippines, Sri Lanka, Thailand, Vietnam. Palearctic: China (Anhui, Gansu, Hubei, Shaanxi, Shandong, Sichuan, Tibet), Iran, Japan, Korea, Pakistan, South Korea; Fig. 4.

Agraphydrus can be found in an extremely broad range of habitats, from rivers, streams and forest pools, to hygropetric environments around waterfalls or seepages over rocks; a few species have been collected in terrestrial habitats by sifting moss and leaves from near water bodies, or in the gravel along the bank of a river; in many cases specimens have been found associated with floating vegetation, mosses and algae (Komarek 2018, 2019, 2020, Komarek and Freitag 2020, Komarek and Hebauer 2018).

Only the larvae of two species of Agraphydrus are currently known: A. narusei (Satô) (first and third instars; Minoshima and Hayashi 2011), and A. hanseni (Satô and Yoshitomi) (third instar; Minoshima et al. 2013). Minoshima (2016) offers a diagnosis for Agraphydrus larvae.

Originally described as a genus by Régimbart in 1903; downgraded to a subgenus of Enochrus by d’Orchymont (1919c: 155); transferred as a subgenus to Helochares by d’Orchymont (1927a: 250); generic status re-established by Satô (1965: 128). Hansen (1991: 148) placed Gymnhelochares as a subgenus of Agraphydrus; Komarek and Hebauer (2018: 17) placed Gymnhelochares as a synonym of Agraphydrus given that they could not identify any unique morphological traits that allowed the two genera to be differentiated. Minoshima et al. (2015: 7) synonymized Megagraphydrus with Agraphydrus also based on the lack of morphological traits in support of their separation. Short and Fikáček (2013) recovered Horelophopsis and Agraphydrus as sister taxa within the Acidocerinae (Horelophopsis had been described as, and was prior to Short and Fikáček (2013), its own subfamily of Hydrophilidae). These affinities between Agraphydrus and Horelophopsis were also recognized by Minoshima et al. (2013) based on larval characters. Finally, Short et al. (2021), based on their molecular phylogenetic analyses, synonymized Horelophopsis with Agraphydrus, as Horelophopsis was recovered as a lineage nested within Agraphydrus. The genus was redescribed by Komarek (2020). For more details on the taxonomic history of the genus and its synonyms see Minoshima et al. (2015).

With 201 described species, Agraphydrus is currently the largest genus of Acidocerinae, due to a series of recent revisions and monographs (Minoshima et al. 2015; Komarek 2018, 2019, 2020; Komarek and Hebauer 2018; Komarek and Freitag 2020), making it the fifth largest genus of Hydrophilidae (behind Berosus Leach, Laccobius Erichson, Cercyon Leach, and Enochrus Thomson). The condition of the maxillary palpomere 2 being straight (with inner margin straight) is not unique to Agraphydrus but shared with Tobochares and some Helochares. Minoshima et al. (2015) proposed the V-shaped male abdominal sternite 9 as a possible synapomorphy of the genus, but the condition is shared with some members of the Tobochares group.

The genus appears well supported as monophyletic as currently defined, despite its substantial morphological and ecological variation (Short et al. 2021). Although previous decisions to synonymize derived genera (e.g., Megagraphydrus, Pseudopelthydrus, Horelophopsis) were necessary to preserve the monophyly of the broader concept of Agraphydrus, it has rendered the genus unmanageably large and with no internal formal or informal classification system. The lineage would be well-served by further phylogenetic studies to define species groups or to partition into subgenera.

Hebauer (2002a) listed several species of Agraphydrus as “in press”, and some specimens in collections bear associated red and orange holotype or paratype labels bearing these names; however, those were never formally published. Many of these taxa appeared in Komarek and Hebauer (2018) or subsequent revisions by Komarek (2019, 2020), with names different from those proposed by Hebauer (2002a).

Agraphydrus anatinus Komarek, A. attenuatus (Hansen), A. coomani (d’Orchymont), A. decipiens Minoshima, Komarek & Ôhara*, A. hanseni (Satô & Yoshitomi), A. insidiator Minoshima, Komarek & Ôhara*, A. ishiharai (Matsui), A. kempi (d’Orchymont), A. luteilateralis (Minoshima & Fujiwara)*, A. malayanus (Hebauer)*, A. masatakai Minoshima, Komarek & Ôhara*, A. minutissimus (Kuwert), A. narusei (Satô), A. pauculus (Knisch), A. politus (Hansen), A. pygmaeus (Knisch), A. siamensis (Hansen), A. stagnalis (d’Orchymont), A. thaiensis Minoshima, Komarek & Ôhara, and numerous unidentified specimens. For species marked with an asterisk, paratype specimens were studied.

Minoshima et al. 2015: character discussion, taxonomic history, synonymization of Megagraphydrus, description of seven new species; Komarek and Hebauer 2018: 17: synonymized the subgenus Gymnhelochares with Agraphydrus, taxonomic revision for China and Taiwan describing 33 new species; Komarek 2018: taxonomic revision for India describing 36 new species; Komarek 2019: taxonomic revision for South East Asia (except Philippines) and Australasian Region, describing 60 new species; Komarek and Freitag 2020: revision of the species from the Philippines describing nine new species and providing barcodes for the species treated therein; Komarek 2020: revision of the African and Western Asian species, describing 25 new species and redescribing the genus; Short et al. 2021: synonymization of Horelophopsis with Agraphydrus, phylogenetic placement of Agraphydrus.