Amanita caesarea, commonly known in English as Caesar's Mushroom, is a highly regarded edible mushroom in the genus Amanita, native to southern Europe and North Africa. It has a distinctive orange cap, yellow gills and stem. Similar orange-capped species occur in North America and India. It was known to and valued by the Ancient Romans, who called it Boletus, a name now applied to a very different type of fungus.
This mushroom has an orange-red cap, initially hemispherical before convex and finally flat. The surface is smooth, and margins striated, and it can reach 15 (6 in) or rarely 20 cm (8 in) in diameter. The free gills are pale to golden yellow, as is the cylinder-shaped stipe, which is 8–15 cm (3–6 in) tall and 2–3 cm ( around 1 in) wide. The ring hangs loosely and is lined above and smooth below. The base of the stipe is thicker than the top and is seated in a greyish-white cup-like volva, which is a remnant of universal veil. The spores are white.
It could be confused with the poisonous fly agaric (Amanita muscaria). Though A. muscaria has a distinctive red cap dotted with fluffy white flakes, these tend to fall off as the carpophor ages and the bright red tends to fade to a yellowy orange. The latter mushroom will always have white gills and stalk with a ringed volva rather than a yellow stalk and is typically associated with spruce (Picea), pine (Pinus) or birch (Betula).Certain varieties ( eg. Amanita muscaria var. guessowii ) are close to yellow even at the juvenile stage.
The common name comes from its being a favorite of the Roman emperors, who took the name Caesar as a title. However, the most famous killing involving Amanita took place in ancient Rome circa 50-60 A.D. The Emperor Claudius had ascended to the throne after the assassination of his nephew Caligula. We've all heard about the decadence (and Roman porno movies, such as "Veni, Vidi, Veni") associated with Caligula. Anyway, Claudius had several wives, but finally married his fourth wife Agrippina, who was also his niece. Agrippina already had a son from a previous marriage, Nero, for whom she had great plans. She persuaded Claudius to adopt Nero, putting him in line for the throne, should something happen to Claudius. (You can see where this is going already.) Agrippina was an impatient woman, and could not wait for a natural death for Claudius; she plotted to kill him by feeding Claudius his favorite meal, Amanita caesarea, laced with extracts from Amanita phalloides, the death cap. When the symptoms set in the next day, a co-conspirator doctor Xenophon administered an enema of colocynth, a potent toxin from a plant called bitter apple, which together with the mushroom toxin killed Claudius. Nero and his fiddle thus became emperor, and the rest, as they say, is history.