Final answer:
Chitin is the structural polysaccharide found in the exoskeletons of arthropods and in the cell walls of fungi.
Step-by-step explanation:
The polysaccharide that is the main structural fiber in the external skeletons of arthropods, and a structural material in the cell walls of fungi is chitin. Chitin is a complex carbohydrate that forms the tough exoskeletons of insects, crustaceans, and other arthropods. It is composed of units of N-acetylglucosamine, a modified form of glucose. Unlike starch or cellulose, which are made of glucose units, chitin includes a nitrogen-containing group attached to its glucose subunits, making it unique among the biologically important polysaccharides.