Rhagophthalmus Motschulsky, 1854 is the most speciose genus in Rhagophthalmidae, distributed in the region encompassing South, East, and Southeast Asia. Here, we describe R. nanus sp. nov. from the Houaphanh Province of eastern Laos, which represents the smallest known species in Rhagophthalmus and one of the smallest in Rhagophthalmidae. We compare it with the morphologically similar and geographically close congeners and provide a preliminary identification key to adult males of Rhagophthalmus species from mainland Southeast Asia. Additionally, we discuss the morphology and variability of male genitalia within Rhagophthalmus.

Bioluminescent beetles, distribution, diversity, Elateroidea, identification key, neoteny, Oriental Realm, Southeast Asia, taxonomy

The family Rhagophthalmidae (Coleoptera, Elateroidea) includes 66 species classified in 12 genera distributed in South, East, and Southeast Asia (Kundrata et al. 2022). These soft-bodied beetles are characterized by a significant sexual dimorphism. While males are fully winged, all known females are larviform and, similar to predaceous larvae, occur in soil and leaf litter (Wittmer and Ohba 1994; Li et al. 2008; Kawashima et al. 2010). Such modifications in female morphology in various elateroid groups are related to paedomorphosis (e.g., Cicero 1988; Ferreira and Ivie 2022; Hoffmannova and Kundrata 2022). Another interesting phenomenon present in Rhagophthalmidae is the ability to emit light (bioluminescence) which they share with their closest relatives, i.e., Phengodidae, Lampyridae (fireflies), and Sinopyrophoridae (Kusy et al. 2021). No fossils of Rhagophthalmidae are known to date; however, the first fossil of their sister-group family Phengodidae was recently described from the mid-Cretaceous Burmese amber (Roza et al. 2023).

Rhagophthalmus Motschulsky, 1854, is the most species-rich genus in the family Rhagophthalmidae. To date, 34 species have been described mainly from East Asia. The highest diversity of this genus lies in China, however several species are distributed in the region ranging from India and Sri Lanka through the Himalayas, to Japan and Indonesia (Kundrata et al. 2022). Rhagophthalmus is in need of a complete taxonomic revision. Its generic limits were challenged by some authors, especially considering its relationships with the genera Ochotyra Pascoe, 1862 and Menghuoius Kawashima, 2000. Although the synonymy of Ochotyra with Rhagophthalmus (Wittmer and Ohba 1994) is generally accepted, the status of Menghuoius differs among authors (Li et al. 2008; Kawashima et al. 2010; Kundrata and Bocak 2011; Liu et al. 2020). Li et al. (2008) synonymized Menghuoius with Rhagophthalmus based on the similar morphology of their highly paedomorphic larviform females, which, however, bear significantly fewer diagnostic characters than adult males. Kundrata et al. (2022) considered Menghuoius a separate genus until a detailed revision of this group is carried out.

During the examination of elateroid material in the collection of the National Museum in Prague, Czech Republic, we identified a yet undescribed Rhagophthalmus species from Laos which attracted our attention by its unusually small body size. In this paper, we describe this species as R. nanus sp. nov., compare it with its congeners and provide an identification key to adult males of Rhagophthalmus species from the mainland Southeast Asia.

The genitalia were dissected after a short treatment in hot 10% KOH. Main diagnostic characters were photographed using a digital camera attached to a stereoscopic microscope. The measurements were taken with a scale bar in an eyepiece. Body length was measured from the fore margin of head to the apex of elytra (since abdomen is highly flexible in Rhagophthalmus), body/elytra width at humeri, head width including eyes, minimum interocular distance in the frontal part of cranium, maximum eye diameter in the lateral view, pronotal length at midline, scutellar shield length at midline, and scutellar shield at the widest part. We follow the definition of Rhagophthalmus, i.e., including Ochotyra but excluding Menghuoius, by Kundrata et al. (2022). Label data are cited verbatim. The type material is deposited in the National Museum, Prague, Czech Republic (NMPC). To compare the new species from Laos with its congeners from adjacent regions, we examined the type and non-type material of Rhagophthalmus from the following collections: Muséum national d’Histoire naturelle, Paris, France (MNHN), Naturkundemuseum Erfurt, Germany (NKME), Natural History Museum, London, United Kingdom (BMNH), and the Naturhistorisches Museum, Basel, Switzerland (NHMB).

The discovery of R. nanus sp. nov. from Laos contributes to our better understanding of this little known group of Asian bioluminescent beetles. The species currently classified in Rhagophthalmus are usually medium-sized, most of them being 8.0–11.0 mm long. The exception is R. longipennis Pic, 1925 from China, which is with 18.0 mm by far the largest species within this genus (Pic 1925). Until the discovery of here described R. nanus sp. nov., the smallest representatives of the genus Rhagophthalmus were R. filiformis Olivier, 1912 from Sri Lanka (5.0–6.0 mm; Olivier 1912) and R. minutus from Thailand (5.8 mm; Kawashima and Satô 2001). With the body length of 3.50 mm, R. nanus sp. nov. is much smaller than those species and represents not only the smallest known species in the genus Rhagophthalmus, but also one of the smallest representatives of the whole family Rhagophthalmidae. Among the formally described rhagophthalmids, the genus Falsophrixothrix Pic, 1937 contains small species, ranging from about 4.00 to 6.50 mm. However, this also includes the abdomen since elytra are shortened in this genus. Some undescribed rhagophthalmids are even smaller, having less than 3.5 mm (personal observations of authors).

Li et al. (2008) discussed the importance of male genitalia in distinguishing species of Rhagophthalmus. It should also be noted that the authors correctly mentioned that Wittmer and Ohba (1994), followed by some other authors, erroneously mixed up dorsal and ventral sides of the aedeagus. Based on the shape of the aedeagus (information available to authors for 30 out of 35 species), Rhagophthalmus can be divided into two separate groups. The first group (at least 19 species) is characterized by the phallobase, which is typically elongate, anteriorly medially emarginate, and covers the substantial portion of parameres in ventral view (e.g., Fig. 3D–F). The parameres are usually surpassing the apex of a median lobe [in for example, the type species R. scutellatus or R. obscurus (Pic, 1917)]; however, in rare cases the median lobe is slightly surpassing the parameres, as in for example, R. burmensis (Wittmer and Ohba 1994) and R. nanus sp. nov. The second group (at least 11 species) includes representatives with much smaller phallobase, which is of variable shape (usually wider than long) and never covers the significant portion of parameres in ventral view, and with a median lobe always distinctly surpassing apices of parameres [e.g., R. flavus, R. laosensis, R. minutus, and R. tonkineus; see Kawashima and Satô (2001)]. The genus Menghuoius, which is morphologically similar to Rhagophthalmus and considered its synonym by some authors (Li et al. 2008), has a somewhat intermediate aedeagus, with the more or less transverse phallobase covering only relatively small portions of parameres in ventral view, and a relatively short median lobe with its apex not, or only very slightly, surpassing apices of parameres (Kawashima 2000; Kawashima 2002; Li et al. 2008).

Taking into account above-mentioned morphological differences in male genitalia within Rhagophthalmus, it would be interesting to test the phylogenetic relationships among the species of this genus using independent molecular data. However, future phylogenetic studies should focus not only on the relationships within the genus, but also on testing the limits of Rhagophthalmus by including morphologically similar genera like Menghuoius and Dioptoma Pascoe, 1860.

We thank J. Hájek (NMPC) for the loan of the examined specimen, and M. Geiser, M. Barclay (both BMNH), A. Mantilleri, A. Taghavian (both MNHN), M. Hartmann (NKME), and I. Zürcher (NHMB) for access to the Rhagophthalmus material in their care.