Taxonomy
The Limnichidae (minute marsh-loving beetle) genus Caccothryptus was first described by David Sharp in 1902. Native to southern and eastern Asia, Caccothryptus beetles generally live in forests, inside piles of water-logged dead wood adjacent to small streams. In 2014, the genus was divided into five species groups by Carles Hernando and Ignacio Ribera. A group named for C. testudo, its type species, is distinguished by its separated paramere (part of the external genitalia of males), and—unlike the C. compactus group—the lack of a long longitudinal furrow along the median lobe of the aedeagus (male reproductive organ), alongside a lack of differentiation between the anterior claws on males.
A group of Caccothryptus specimens were collected in 1983 in Kathmandu, Nepal by Martin J. D. Brendell, curator of Coleoptera Department of Entomology at the Natural History Museum, London. These were labeled as C. ripicola and C. testudo, first described by forester Harry George Champion in 1923. These were stored at the Natural History Museum. In 2021, museum entomologist Keita Matsumoto identified a number of distinct species from these specimens. A male specimen from Kathmandu was labeled as the holotype for Caccothryptus arakawae. Five specimens (comprising three males and two females) were labeled paratypes. C. arakawae was placed in the species group C. testudo and named for Japanese manga artist Hiromu Arakawa, the creator of Fullmetal Alchemist.
Description
The C. arakawae specimens identified by Matsumoto have elytra ranging from of 3.39–3.68 mm in length, and 2.32–2.52 mm in width. Their prothoraces (the frontmost thorax segments) are 0.90–1 mm in length and 2.03–2.11 mm in width. C. arakawae has an elongated oval body, brown in color except for reddish tibia, which are distributed across both the upper and lower sides of the body. The setae on the upper side are long and white, and densely concentrated into wavy patterns. The setae on the underside of the body has somewhat shorter, but still relatively long.
Its tarsi (the last portion of the leg) are divided into five segments, of increasing length, with the fifth segment as long as the others combined. It has long, narrow, light brown tarsal claws. The beetle's head, smooth and punctured, is slightly retracted into its pronotum, and features eleven-segment antennae. Little sexual dimorphism exists between males and females, beyond a longer fifth abdominal ventrite on the female.
The beetle's male genitals display a number of differences from the original classification of C. testudo, which enabled its classification as a separate species. The aedeagus is enlongated and highly sclerotised (hardened by sclerotin), with a median lobe about two-fifths of the length of the genitalia, longer than the parameres. This lobe narrows to a slightly pointed tip. A U-shaped depression lies between the V-shaped notches on the tip of the parameres, while a deep concave, rounded indentation lies between the parameres. The upper side of the parameres is wavy, forming two concave curves. Females have a long ovipositor and pointed gonocoxites (structures covering the underside of the genitals).
