The hyphal wall of fungi is rigid owing to the presence of chitin. Chitin is a tough, protective, and semi-transparent polysaccharide that makes up the exoskeletons of insects and crustaceans, as well as the cell walls of fungi. It is made up of long chains of a modified sugar called N-acetylglucosamine, which form a tough, fibrous structure that provides support and protection to the fungal cell. The presence of chitin in the hyphal wall gives it its characteristic rigidity.