The Collaborative International Dictionary of English v.0.48:

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.
   [Webster 1913 Suppl.]