Yllenus desertus

The spider is small. The female has a carapace that is typically 2.48 mm (0.098 in) long and 2.05 mm (0.081 in) wide. It is brown or dark brown and densely covered with white scales or hairs. The clypeus is brown with white hairs. White hairs also adorn the sternum, which is very dark brown, almost black. The labium is also dark brown, as are the toothless chelicerae. The spider has an almost-spherical abdomen that measures between 3.2 and 3.4 mm (0.13 and 0.13 in) long and 2.33 mm (0.092 in) wide. Some examples are brownish-black and have dense short white hairs and a vague pattern of three light triangulär patches longways down the middle with irregular light patches along the sides. Others are light grey with interrupted transverse stripes formed of pale brown patches. The underside is dark or grey-yellow respectively. Spinnerets are brown to brown-yellow. The legs are short, stumpy, yellow with brown patches and covered with brown and white hairs. The tarsus has a spade-like appendage. The spider has a large oval heavily-sclerotized epigyne that has a very narrow tube-shaped pocket. Two narrow crescent-shaped and widely spaced copulatory openings lead to broad ducts and many-chambered spermathecae. …

The species is endemic to Mongolia. The female holotype for the species was found in the Jarantai in 1975. The site is a military station near the Chinese border on the Bulgan Gol river. The species thrives in semi-desert areas that have a semi-arid climate.