A hostile takeover
Deep in the high-altitude meadows of the Tibetan Plateau and the Himalayas, a parasitic fungus known as Ophiocordyceps sinensis has a remarkable and macabre life cycle. This organism is an entomopathogenic fungus, meaning it preys on insects, specifically the larvae of ghost moths (family Hepialidae). The infection begins when a microscopic spore lands on a caterpillar. These spores are often sticky, allowing them to adhere to their host. Once attached, the spore germinates, and its hyphae, tiny thread-like filaments, penetrate the caterpillar's exoskeleton.
Inside the host, the fungus begins to proliferate, growing a network of mycelia that fills the insect's body cavity. It methodically digests the caterpillar's non-essential tissues, keeping the host alive for as long as possible. During this phase, the caterpillar's behavior changes; it becomes sluggish as the fungus consumes it from within. The fungus produces a cocktail of bioactive compounds that it uses to manipulate its host, though the exact mechanisms are still under investigation. The fungal cells form a vast network throughout the caterpillar's body, wrapping around muscle fibers but avoiding the brain until the final stages.
The final summit
When the fungus is ready to reproduce, it initiates the final phase of its control. It directs the dying caterpillar to move upward, a behavior known as "summiting." This forces the insect to crawl to the soil surface or climb a short distance up a plant. This positioning is significant for the fungus's reproductive success. Once in an ideal location, the fungus induces a final, fatal action: the caterpillar clamps its mandibles onto the plant or soil, locking itself into place.
With the host secured, the fungus consumes the remaining essential tissues, including the brain, killing the caterpillar. Soon after, a stalk-like fruiting body, called a stroma, erupts from the caterpillar's head. This dark brown or blackish stalk can grow to be 4 to 10 centimeters long, longer than the caterpillar's own body. The head of the stroma is granular, covered in tiny sacs that produce and release millions of new spores into the environment, ready to infect the next generation of ghost moth larvae. the entire complex, the mummified caterpillar with the protruding fungal stalk—is what is commonly known as the caterpillar fungus.