Tarantula infected with Cordyceps. Cordyceps is a genus of ascomycete fungi that includes about 400 described species. When a Cordyceps fungus attacks a host, the mycelium invades and eventually replaces the host tissue, while the elongated fruiting body(ascocarp) may be cylindrical, branched, or of complex shape. The ascocarp bears many small, flask-shaped perithecia containing asci. These in turn contain thread-like ascospores, which usually break into fragments and are presumably infective. Some Cordyceps species are able to affect the behavior of their insect host: Cordyceps unilateralis causes ants to climb a plant and attach there before they die. This ensures the parasite’s environment is at an optimal temperature and humidity and maximal distribution of the spores from the fruiting body that sprouts out of the dead insect is achieved.