Fly amanitaFly amanita\, Fly fungus \Fly fungus\ . (Bot.) A poisonous mushroom (Amanita muscaria, syn. Agaricus muscarius), having usually a bright red or yellowish cap covered with irregular white spots. It has a distinct volva at the base, generally an upper ring on the stalk, and white spores. Called also fly agaric, deadly amanita.