Caccothryptus arakawae

Caccothryptus arakawae is a species of minute marsh-loving beetle in the subfamily Limnichinae. The species was described alongside five other Caccothryptus species by Natural History Museum entomologist Keita Matsumoto in 2021, using specimens gathered by Martin J. D. Brendell at Kathmandu, Nepal in 1983. Six of Brendell's specimens were distinguished from its initial classification of Caccothryptus testudo due to distinct genital structures. The species was named for Hiromu Arakawa, the creator of manga series Fullmetal Alchemist.

Taxonomy
The Limnichidae (minute marsh-loving beetle) genus Caccothryptus was first described by David Sharp in 1902. In 2014, the genus was divided into five species groups by Carles Hernando and Ignacio Ribera.

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 Caccothryptus arakawae specimens identified by Matsumoto have elytral lengths of 3.39–3.68 mm, and elytral widths of 2.32–2.52 mm. Their prothoraces are 0.90–1 mm in length and 2.03–2.11 mm in width. C. arakawae has an elongated oval body, brown in colour except for reddish tibia and long, white setae, which are distributed across both the upper and lower sides of the beetle. Its tarsi 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 the male and female specimen, beyond a longer fifth abdominal ventrite on the female.

The beetle's genitals display a number of differences from the original classification of C. testudo, which enabled its classification as a separate species. C. arakawae 's median lobe narrows to a slightly pointed tip. It has a U-shaped depression between the V-shaped notches on the tip of the genitalia. The genitals' dorsum are wavy, forming two concave curves. Females have a long ovipositor.