When a Cordyceps fungus attacks a host, the mycelium invades and eventually replaces the host tissue, while the elongated fruit 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 current and former Cordyceps species are able to affect the behaviour of their insect host: Ophiocordyceps unilateralis (formerly 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 that maximal distribution of the spores from the fruit body that sprouts out of the dead insect is achieved.[4] Marks have been found on fossilised leaves that suggest this ability to modify the host’s behaviour evolved more than 48 million years ago.[5]