Caccothryptus yunnanensis is a species of minute marsh-loving beetle in the subfamily Limnichinae. The species was described in 2018 by enomologist Hiroyuki Yoshitomi. Although part of the species group testudo, it is also related to the compactus-group species C. chayuensis and C. thai. It is distinguished from these due to its straight parameres, which are curved in the other two species. Specimens were collected across the Chinese provinces of Guizhou and Yunnan, the latter of which gave the species its name.

The Limnichidae (minute marsh-loving beetle) genus Caccothryptus, within the subfamily Limnichinae, was first described by the entomologist David Sharp in 1902.^[1] In 2014, the genus was divided into five species groups by Carles Hernando and Ignacio Ribera based on similarities in genital shape and some external features. Reclassification and reanalysis of Caccothryptus specimens under the new system brought the total number of species to 23, up from a previous total of seven.^[1][2]

In 2018, a distinct form of the C. testudo species group was identified by entomologist Hiroyuki Yoshitomi from specimens collected in southwestern China which displayed novel genital characteristics. This species was named Caccothryptus yunnanensis, after Yunnan, the province where many of the specimens were found. The holotype of the species is a male specimen collected in Xima, Yingjiang County, Yunnan, by the entomologist Yu-Tang Wang in 2014. This is joined by large numbers of paratypes collected by Wang in Xima and several other areas of Yunnan, including Ruili, Mengla, Daweishan. Three specimens were also collected in Leishan County in Guizhou province.^[3]

Despite its presence in the C. testudo group, it shares a genetic relation with the C. compactus group species C. thai and C. chayuensis due to the shared innovation of minuscule serrae (saw-like indentation) along the lower margin of the median lobe of the aedeagus (male genital organ).^[3]

Caccothryptus beetles are commonly found in piles of dead, water-logged wood located along forest streams.^[4] C. yunnanensis has an oblong body, with a total length ranging from 3.90–6.05 mm (0.15–0.24 in), averaging at 5.04 mm (0.20 in). Its elytral width ranges from 2.00–3.13 mm (0.079–0.12 in), with lengths of 3.10–4.85 mm (0.12–0.19 in). Its body is shiny and is densely covered with silvery setae of varying lengths. It has short antennae, only about one-eighth the length of the elytra—about 0.25 mm (0.01 in). It is almost uniformly black except for its brown antennae and legs, with two antenna segments and its tarsis a light brown. It exhibits very minor sexual dimorphism.^[3]

It is distinguished from related species through genital characteristics. Its presence in the species group C. testudo through its parameres, which are separated and emarginate (with concave edges that appear notched). Although related to the compactus species C. thai and C. chayuensis due to its median lobe serrae, C. yunnanensis's parameres project straight out, instead of the curves seen on the other two species. Additionally, the notches along the underside of the parameres are V-shaped, unlike the deep U-shaped notches of C. thai.^[3][5][6]