Unexpected discovery of a new Podonychus species in Kyushu, Japan (Coleoptera, Elmidae, Elminae, Macronychini)

Abstract Podonychus gyobusp. nov., a second species of the genus Podonychus Jäch & Kodada, 1997, hitherto known only from Indonesia, is described from Kyushu, Japan. This new species is similar to P. sagittarius Jäch & Kodada, 1997, but differs from it in the straight penis, arcuate 2nd labial palpomere, and in the 3rd antennomere being longer than wide. The endophallic structures and the larva of P. gyobusp. nov. are described. A character matrix of the Macronychini genera and a key to the Japanese genera are provided.


Introduction
The riffle beetle fauna of Japan is well studied and 17 genera with 57 species are reported so far (Kamite et al. 2018). Some undescribed species and taxonomic problems still remain (Kamite et al. 2018;Nakajima et al. 2020). The larval stages of the Japanese species are well known and except for a few taxa, most larvae were described recently (Yoshitomi and Satô 2005;Hayashi 2009Hayashi , 2013Hayashi and Sota 2010, 2015Yoshitomi 2014, 2015;Kamite 2015;Hayashi et al. 2016. The genus Podonychus Jäch & Kodada, 1997 (Elminae, Macronychini) was hitherto known only from Siberut Island, Indonesia (Jäch et al. 2016). It has been regarded as monotypic so far. This genus is peculiar in having 6-segmented antennae, which is the smallest number of antennomeres within the family Elmidae (Jäch and Kodada 1997;Kodada and Jäch 2005). Unexpectedly in 2018, some specimens of this genus were collected in Kyushu, Japan. After repeated field investigation and closer examination, it was clear that the specimens represent a new species, closely related to P. sagittarius Jäch & Kodada, 1997. In the present paper, we describe this new species including endophallic structures and the larva.
The holotype and some paratypes will be deposited in the Ehime University Museum, Matsuyama, Japan (EUMJ), and the remaining paratypes in the Naturhistorisches Museum Wien, Austria (NMW), the Kitakyushu Museum of Natural History and Human History, Kitakyushu, Japan (KMNH), and the Hoshizaki Institute for Wildlife Protection, Izumo, Japan (HOWP).
Morphological terms used to describe the genitalia and the larva follow Kodada and Jäch (2005) and .
General observations and dissections were made under a stereoscopic microscope (Leica MZ95). Microstructures of the dissected parts were mounted on hollow slides with pure glycerine and observed under a microscope (Olympus BH-2). After observation, the dissected parts were mounted on slides with Canada Balsam. SEM photographs were taken using a scanning electron microscope (JCM-6000 Neoscope; JEOL Ltd., Tokyo, Japan).
Morphological abbreviations used in this study are as follows: EL length of elytra PW width of pronotum EW width of elytra PL length of pronotum TL total length (EL + PL) The average measurement is given in parentheses after the range. For male genitalia, we made the following measurements after Hayashi and Yoshitomi (2015, fig. 1 Jäch & Kodada, 1997 Diagnosis. The genus is very distinctive in having the following characteristics of the adults (see also Table 1): 1) antennae 6-segmented; 2) pronotal median groove very thin and shallow; 3) pronotal sublateral carinae absent; 4) lateral declivity of the pronotum with oblique impressions; 5) each elytron with one sublateral carina on interval VIII. The larva is also distinctive in having Y-shaped projections (Fig. 7A

Diagnosis.
The new species is morphologically similar to P. sagittarius, and differs from it in the following characteristics: 1) penis almost straight from base to near apex in lateral view (slightly curved ventrally in P. sagittarius); 2) lateral margins of the 2 nd labial palpomeres strongly arcuate (weakly arcuate in P. sagittarius); 3) 3 rd antennomeres longer than wide (as long as wide in P. sagittarius). The endophallic structures of P. sagittarius were not described in detail, but judging from the aedeagus illustrations (Jäch and Kodada 1997, figs 78-80) they are similar to those of P. gyobu sp. nov.
Head visible dorsally, well exposed from prothorax, trapezoidal, widest at apical 1/3, densely covered with short spines, bearing short setae. Eyes (Fig. 6C, E) lacking lens of stemmata. Antennae (Figs 5D, 6F) relatively long; antennomere I as long as wide; antennomere II long, with long and slender sensorial appendage; antennomere III shorter than sensorial appendage on antennomere II, with short sensorial appendage at apex. Labrum (Fig. 5E) transverse, with a row of pectinate setae in anterior part of dorsal surface, with a pair of long apodemes projecting from postero-lateral corners. Mandibles (Fig. 5F) subtriangular, with long and pectinate setae on lateral parts; two apical teeth short and blunt; basal setose processes long.   Maxillae (Figs 5G, 6D) with relatively long palpi. Labium (Figs 5G, 6D) with a pair of long setae in middle. Thorax serrate in lateral parts, widest at metathorax. Prothorax with seven ventral sclerites; mesal one small, situated between procoxae; antero-lateral ones wide, bearing plumose setae on anterior margin; postero-lateral ones quadrate, bearing three long plumose setae. Mesothorax with five ventral sclerites; mesal one wide and transverse, sinuate at posterior margin, bearing short bipectinate setae near midcoxae; antero-lateral ones oblong, bearing long bipectinate setae. Metathorax with five ventral sclerites; mesal one wide and transverse, bearing short setae. Granules on dorsal surface of thorax and abdomen (Fig. 7D, E) distributed linearly and somewhat irregularly, bearing short setae in posterior end, with long and curved setae in basal part. Y-shaped projections (Fig. 7A, D, F) present on caudal margins of thoracic and abdominal segments. Spiracles thumb-shaped, on mesothorax and abdominal segments I-VIII, situated near lateral margin. Legs 5-segmented, short and stout; apical segment stout, with short inner seta. Abdomen ( Fig. 8C) with pleural sclerites on segments I-VII, gently tapering caudally. Abdominal segment IX (Fig. 8G) long, as long as abdominal segments VI-VIII combined, flat ventrally, serrate at apex (Fig. 8E), with line of granules in midline of dorsal surface, bearing four long lateral setae and one ventral seta, densely covered with short spines. Ventral operculum (Fig. 8G, H) oblong, situated in caudal 1/3 of abdominal segment IX.
Biological notes. This species lives in small rivers at low elevation. Larvae and adults were collected from submerged roots of reeds using a net. They were collected with Elmomorphus brevicornis Sharp, 1888 andStenelmis nipponica Nomura, 1958. Distribution. So far, this species is known only in the Iroha and Yakkan river systems, in the eastern part of Kyushu, Japan.
Etymology. "Gyobu" is an NPO (nonprofit organization) in Kitakyushu, Fukuoka. This new species was discovered during a survey of water beetle fauna by Mr D. Inoue, president of "NPO Kitakyushu Gyobu". The epithet is a noun in apposition.
Key to Japanese genera of Macronychini (adult)

Discussion
Podonychus is currently known only from Indonesia and Kyushu, Japan, representing a disjunct distribution across the equator. We think this genus is probably distributed more widely in the Oriental Region but has not been found in other areas because of the small body size and unusual microhabitat. All specimens of the new species were collected from the submerged roots of reeds, and we think this microhabitat is important for the genus. In the future, additional species of this genus are expected to be discovered from east and southeast Asia, e.g., China, Taiwan, Vietnam, and the Philippines.
Podonychus sagittarius was collected from a small stream densely shaded by forest (Jäch and Kodada 1997), while the new species was collected from a small river running in open land (Fig. 1F). Judging from the long legs, both species live on submerged roots or rotten wood.
The endophallic structures of Podonychus, with a well sclerotized long apical tube, are unique not only in the tribe Macronychini, but also in the family Elmidae. The endophallic structures within the family are usually membranous as is probably the case in many Coleoptera (Dam 2014;Hayashi and Yoshitomi 2015). To obtain new relevant information about fine morphology, the endophallus must be fully everted for examination (Hayashi and Yoshitomi 2015).
The larval morphology of this genus is unique, particularly in having Y-shaped projections (Fig. 7A, E, D) on the thorax and abdomen. The characteristic of the larvae is basically similar to the other Macronychini genera (Hayashi and Sota 2010; * The larvae of Urumaelmis and Sinonychus are unknown. Hayashi and Yoshitomi 2015), but the shape of the granules on the surface is similar to that of the larvae of Pseudamophilus japonicus (Hayashi and Sota 2010), which live on the surface of rotten wood or submerged roots of reeds in rivers. It is thought that the similarity of the shape of the granules on the surface of the larvae of Podonychus and Pseudamophilus is due to the larval microhabitat.