In the Tibetan language, caterpillar fungus is called yartsa gumbu or ‘summer grass winter worm’. This name captures the complex nature of this organism and the seasonal metamorphosis it undergoes. As the pastoralists say, in winter it is mbu, or an insect, and in summer is rtsa, or grass. Tsering Drölma explained:

This organism is an entomophagous fungus called Ophiocordyceps sinensis, which parasitizes the larvae of ghost moths, largely from the genus Thitarodes. These caterpillars (the ‘worm’ from the quotation above) are attacked by the fungal spores, the fungus develops inside the host organism and then causes its death. Upon reaching maturity, it produces a stroma, or fruiting body (the ‘horn’), which protrudes from the larva's head. On the top of it a sporacia, or spore-producing tissue, develops (the ‘flower’), containing spores that attack more larvae. A large part of this process takes place in the soil, where the larvae spend the winter. In spring, caterpillar fungus already has the hybrid look of a larva with a blade- or horn-like stroma. The larva has an almost lifelike appearance but is completely filled with mycelium. It rests vertically in the ground. The only part visible above the ground is the stroma; it is by locating this that the caterpillar fungus can be found.

This sort of caterpillar fungus is endemic to the Tibetan plateau and the Himalayas. It grows in China, Nepal, Bhutan, and India. In China, its distribution reaches up to the Qilian Mountains in northern Qinghai and to Nagchu Prefecture in the TAR (Map 4). It covers five provinces or autonomous regions: the eastern part of the TAR, Qinghai, south-western Gansu, western Sichuan, and north-western Yunnan – which in total accounts for one tenth of China's territory (Yao 2010). Three factors shape the extent of this distribution zone: altitude, humidity, and temperature.