Caterpillar Fungus

Ophiocordyceps sinensis is not a mushroom in the ordinary sense but a parasitic ascomycete that develops inside the buried larva of a ghost moth. The fungus digests the caterpillar from within, leaving a rigid, mummified but perfectly shaped larval husk, and then produces a single slender fertile stalk — a stroma — that emerges from the head end and rises a few centimetres above the soil.

The result is one of the most unmistakable objects in fungal biology: a pale ochre-brown, segmented caterpillar 3–6 cm long, hard and dry, joined seamlessly to a dark brown or blackish club. The pairing gives the fungus its Tibetan name yartsa gunbu, 'summer grass, winter worm', which describes exactly how it was long understood — as something that was an insect in winter and a plant in summer.

The genus Ophiocordyceps (Ophiocordycipitaceae) contains many insect-parasitic species worldwide, but O. sinensis is restricted to high alpine grassland, where it is tied to a narrow band of host moths and cold soil conditions.

Stroma (the club)

The fertile stalk is 3–10 cm long and only 2–5 mm thick, cylindrical to narrowly club-shaped, unbranched, and dark brown to blackish, often paler toward the base and slightly swollen and roughened in the upper fertile portion. Under a lens the fertile head is minutely pimpled, because the flask-shaped spore-producing chambers (perithecia) are sunk into the surface with only their necks projecting.

Host body

Below ground, still attached to the base of the stroma, lies the larva: 3–6 cm long, ochre to yellowish-brown, rigid and dry, retaining its segmentation, head capsule, leg stubs and even fine cuticle detail. It is completely filled with dense white to pale fungal tissue, and snaps rather than bends when flexed. The larva lies horizontally or obliquely a few centimetres down, so the club appears to rise out of the turf on a thin dark stem.

Microscopic and other features

There are no gills, pores or true cap, and no spore print is produced; spores are long, thread-like and multi-septate, released from the perithecia and breaking into segments. The whole structure has a faint mushroomy or slightly musty odour, and the tissue does not change colour when cut, showing only white fungal flesh inside a brown shell.

This fungus is confined to high alpine meadow and shrub grassland on the Tibetan Plateau and the Himalaya, typically between about 3,000 and 5,000 m in Tibet, Qinghai, Sichuan, Yunnan and Gansu in China, and in Nepal, Bhutan and northern India. It requires short alpine turf with cold, moist, well-drained soil, and it is absent from forest and from lower elevations.

The stromata push through the turf shortly after snowmelt, from roughly April to June depending on altitude and aspect, and are visible for only a few weeks before withering. They stand barely above the surrounding grass and are extremely hard to see, since a dark brown club a few millimetres thick reads as a dead grass stem. Populations are patchy, following the distribution of the host larvae in the soil.

Ophiocordyceps sinensis is an obligate parasite of the soil-dwelling larvae of ghost moths in the genus Thitarodes (Hepialidae), which spend several years underground feeding on the roots of alpine plants. Spores or fragments reaching a larva germinate and penetrate the cuticle, and the fungus grows through the body cavity, gradually replacing all internal tissue with fungal biomass while leaving the outer cuticle intact. The infected larva typically dies positioned vertically with its head upward near the surface.

The fungus then overwinters as a filled mummy and, after snowmelt, produces the stroma from the larval head. Spores released from the perithecia disperse in the alpine turf to encounter further larvae, completing a cycle that takes several years from infection to fruiting. Because both the host moth and the fungus depend on intact alpine grassland and cold soils, populations are highly sensitive to grazing pressure, ground disturbance and warming.

Cordyceps militaris also grows from a buried insect, but its stroma is bright orange to orange-red and club-shaped with a clearly swollen, roughened head, and its hosts are the pupae and larvae of various moths and butterflies in woodland and grassland worldwide, not alpine ghost moths. Ophiocordyceps robertsii of New Zealand and Australia is much larger and darker with a long whip-like stroma from a big hepialid larva.

Tolypocladium ophioglossoides (formerly Elaphocordyceps) produces a similar dark olive-brown club with yellow mycelial cords at the base, but if traced downward those cords lead to a buried false-truffle (Elaphomyces) rather than to an insect — digging carefully settles it immediately. Simple grass stems and the small black clubs of Xylaria and Geoglossum species are separated the same way: nothing is attached below except soil, wood or litter, never a rigid mummified caterpillar.