New 1.37-mm round fungus beetle from Brunei distinguished by arrow-shaped genital tip

A single adult male collected in Brunei’s Ulu Temburong rainforest has been formally described as a new species of round fungus beetle, Dermatohomoeus sagittarius. The 1.37-millimeter specimen, found in leaf litter near the Kuala Belalong Field Studies Centre, is the first recorded species of Dermatohomoeus from the country.

The new beetle was recovered during a taxon expedition in primary lowland dipterocarp rainforest along the Ashton Trail, adjacent to the Kuala Belalong Field Studies Centre in Ulu Temburong National Park, Temburong District. The collection site was at 4.545° north, 115.157° east and an elevation of 150 meters. On the afternoon of 8 October 2025, between approximately 16:00 and 17:00, researchers manually collected and sieved 10 liters of leaf litter.

The material was sifted on site with leaf-litter sieves having 1-square-centimeter mesh. The flow-through was placed in fabric bags and suspended in two Winkler collectors at the field studies centre. Collection containers filled with 70% ethanol were examined daily for seven days. The resulting male specimen was designated as the holotype, cataloged as UBDM.3.07895 and deposited in the Zoology Museum of Universiti Brunei Darussalam.

Molecular analysis was conducted in a portable laboratory at the field centre. DNA was extracted from abdominal soft tissue with a DNeasy Blood & Tissue Kit using a modified protocol. Samples were incubated in ATL buffer with proteinase K at 56°C for at least three hours and then overnight at room temperature. DNA was eluted twice in 50 microliters of TE buffer, while integrity was assessed by electrophoresis on a 0.8% agarose gel and concentration was estimated against mass ladders of known DNA quantities.

The researchers amplified the mitochondrial cytochrome c oxidase subunit I region with the Folmer primers, modified with 5′ Oxford Nanopore adapter sequences. Products of approximately 710 base pairs were checked on agarose gels, uniquely indexed, pooled at equimolar concentrations, purified with AMPure XP magnetic beads and quantified with a Qubit 4 fluorometer. A library prepared from 1 microgram of pooled amplicons was sequenced on a MinION with an R10.4.1 flow cell, using the specified ligation kit while omitting fragmentation and end-preparation.

After basecalling, demultiplexing and consensus generation, the researchers retained sequences with quality scores of at least 20 and lengths from 705 to 715 base pairs, then clustered them at 90% sequence identity. The analysis produced 3,346 high-quality reads. The largest cluster contained 3,047 reads, or 91% of the total, which the authors interpreted as indicating minimal sample contamination. Searches against a locally downloaded NCBI nucleotide database returned Leiodidae matches with 100% query coverage but less than 89% sequence identity. The holotype sequence was deposited in the Barcode of Life Database as TXEX082-26.

External morphology was examined with a Nikon SMZ445 stereomicroscope at magnifications up to 35 times. The beetle has an ovoid, amber-colored body, a pronotum only slightly wider than the elytra at the shoulders, well-developed subglobose eyes, clavate antennae, and dense, strong puncturation on the head and pronotum. Measurements were made in ImageJ and calibrated against a ruler photographed at the same magnification. Reproductive structures were dissected and embedded on a slide in polyvinylpyrrolidone, while images of the aedeagus were traced and interpreted following Daffner’s method.

The diagnostic features are concentrated in the male genitalia. The aedeagus is strongly curved near its base, while the tip of the median lobe extends into a narrow, arrow-shaped point. Lateral branches of the operculum protrude on either side as spatulae, and the endophallus contains a symmetrical pair of comma-shaped basal sclerites. The authors state that this combination distinguishes D. sagittarius from all known Asian species. A backward-oriented tooth on each side of the tegmen before its apex is shared with D. rufus from India. The epithet sagittarius, meaning “archer,” refers to the arrow-shaped median-lobe tip.

This taxonomic description is based on one male holotype from a single locality in Brunei, with no female specimen or additional population reported in the study. Dermatohomoeus previously contained 56 species distributed across Oriental Asia and Oceania, and only four had been recorded from Borneo before this addition. The authors note that the genus is numerous and diverse in Southeast Asian rainforest but remains understudied because the beetles are small, cryptic, externally uniform and often identifiable only through genital examination. They expect additional undescribed species to await description, and argue that expert-guided work by non-specialists can make productive contributions to taxonomy while increasing appreciation for biodiversity.

Subject of Research: Taxonomic description of a new round fungus beetle species from Brunei based on one male specimen, external morphology, genital anatomy and mitochondrial barcoding.

Article Title: A new Dermatohomoeus from the Ulu Temburong forest, Brunei ( Coleoptera , Leiodidae )

Article References: Lasco, I. J., Rossen, N., Focaia, R., Rossi, N., van Rijn, B., Koopmans, H., Koomen, P., Alford, D., Beuger, B., Diaconu, D., Fiorentino, F., Zulkarnaen, R., Holbrook, L., Gómez-Zapata, P., Kahar, R. S., Delledonne, M., Cicuzza, D., Njunjić, I., & Schilthuizen, M. (2026). A new Dermatohomoeus from the Ulu Temburong forest, Brunei (Coleoptera, Leiodidae). Biodiversity Data Journal, 14, Article e202669. https://doi.org/10.3897/bdj.14.e202669

Image Credits: AI Generated

DOI: 10.3897/BDJ.14.e202669

Keywords: Dermatohomoeus sagittarius, new species, round fungus beetles, Leiodidae, Brunei, Ulu Temburong, taxonomic description, leaf litter, male genitalia, DNA barcoding, Dermatohomoeus, Temburong