Fly amanita \Fly 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}.