Taxonomy and naming
The name of the mushroom in many European languages is thought to derive from its use as an insecticide when sprinkled in milk. This practice has been recorded from Germanic- and Slavic-speaking parts of Europe, as well as the Vosges region and pockets elsewhere in France, and Romania.:198 Albertus Magnus was the first to record it in his work De vegetabilibus some time before 1256, commenting vocatur fungus muscarum, eo quod in lacte pulverizatus interficit muscas, "it is called the fly mushroom because it is powdered in milk to kill flies."
Showing the partial veil under the cap dropping away to form a ring around the stipe
The 16th-century Flemish botanist Carolus Clusius traced the practice of sprinkling it into milk to Frankfurt in Germany, while Carl Linnaeus, the "father of taxonomy", reported it from Småland in southern Sweden, where he had lived as a child. He described it in volume two of his Species Plantarum in 1753, giving it the name Agaricus muscarius, the specific epithet deriving from Latin musca meaning "fly". It gained its current name in 1783, when placed in the genus Amanita by Jean-Baptiste Lamarck, a name sanctioned in 1821 by the "father of mycology", Swedish naturalist Elias Magnus Fries. The starting date for all the mycota had been set by general agreement as January 1, 1821, the date of Fries's work, and so the full name was then Amanita muscaria (L.:Fr.) Hook. The 1987 edition of the International Code of Botanical Nomenclature changed the rules on the starting date and primary work for names of fungi, and names can now be considered valid as far back as May 1, 1753, the date of publication of Linnaeus's work. Hence, Linnaeus and Lamarck are now taken as the namers of Amanita muscaria (L.) Lam..
The English mycologist John Ramsbottom reported that Amanita muscaria was used for getting rid of bugs in England and Sweden, and bug agaric was an old alternative name for the species. French mycologist Pierre Bulliard reported having tried without success to replicate its fly-killing properties in his work Histoire des plantes vénéneuses et suspectes de la France (1784), and proposed a new binomial name Agaricus pseudo-aurantiacus because of this.:200 One compound isolated from the fungus is 1,3-diolein (1,3-Di(cis-9-octadecenoyl)glycerol), which attracts insects.
It has been hypothesised that the flies intentionally seek out the fly agaric for its intoxicating properties.
An alternative derivation proposes that the term fly- refers not to insects as such but rather the delirium resulting from consumption of the fungus. This is based on the medieval belief that flies could enter a person's head and cause mental illness. Several regional names appear to be linked with this connotation, meaning the "mad" or "fool's" version of the highly regarded edible mushroom Amanita caesarea. Hence there is oriol foll "mad oriol" in Catalan, mujolo folo from Toulouse, concourlo fouolo from the Aveyron department in Southern France, ovolo matto from Trentino in Italy. A local dialect name in Fribourg in Switzerland is tsapi de diablhou, which translates as "Devil's hat".:194
Amanita muscaria is the type species of the genus. By extension, it is also the type species of Amanita subgenus Amanita, as well as section Amanita within this subgenus. Amanita subgenus Amanita includes all Amanita with inamyloid spores. Amanita section Amanita includes the species with patchy universal veil remnants, including a volva that is reduced to a series of concentric rings, and the veil remnants on the cap to a series of patches or warts. Most species in this group also have a bulbous base. Amanita section Amanita consists of A. muscaria and its close relatives, including A. pantherina (the panther cap), A. gemmata, A. farinosa, and A. xanthocephala. Modern fungal taxonomists have classified Amanita muscaria and its allies this way based on gross morphology and spore inamyloidy. Two recent molecular phylogenetic studies have confirmed this classification as natural.
is now a synonym for Amanita muscaria
Amanita muscaria varies considerably in its morphology, and many authorities recognize several subspecies or varieties within the species. In The Agaricales in Modern Taxonomy, German mycologist Rolf Singer listed three subspecies, though without description: A. muscaria ssp. muscaria, A. muscaria ssp. americana, and A. muscaria ssp. flavivolvata.
However, a 2006 molecular phylogenetic study of different regional populations of A. muscaria by mycologist József Geml and colleagues found three distinct clades within this species representing, roughly, Eurasian, Eurasian "subalpine", and North American populations. Specimens belonging to all three clades have been found in Alaska; this has led to the hypothesis that this was the centre of diversification for this species. The study also looked at four named varieties of the species: var. alba, var. flavivolvata, var. formosa (including var. guessowii), and var. regalis from both areas. All four varieties were found within both the Eurasian and North American clades, evidence that these morphological forms are polymorphisms rather than distinct subspecies or varieties. Further molecular study by Geml and colleagues published in 2008 show that these three genetic groups, plus a fourth associated with oak–hickory–pine forest in the southeastern United States and two more on Santa Cruz Island in California, are delineated from each other enough genetically to be considered separate species. Thus A. muscaria as it stands currently is, evidently, a species complex. The complex also includes at least three other closely related taxa that are currently regarded as species: A. breckonii is a buff-capped mushroom associated with conifers from the Pacific Northwest, and the brown-capped
A. gioiosa and
A. heterochroma from the Mediterranean Basin and from Sardinia respectively. Both of these last two are found with Eucalyptus and Cistus trees, and it is unclear whether they are native or introduced from Australia.
Amanitaceae.org lists four varieties as of May 2019 , but says that they will be segregated into their own taxa "in the near future". They are:
||Amanita muscaria var. muscaria 
||Euro-Asian fly agaric
||Bright red fly agaric from northern Europe and Asia. Cap might be orange or yellow due to slow development of the purple pigment. Wide cap with white or yellow warts which are removed by rain.
Known to be toxic but used by shamans in northern cultures. Associated predominantly with Birch and diverse conifers in forest.
||Amanita muscaria var. flavivolvata
||American fly agaric
||red, with yellow to yellowish-white warts. It is found from southern Alaska down through the Rocky Mountains, through Central America, all the way to Andean Colombia. Rodham Tulloss uses this name to describe all "typical" A. muscaria from indigenous New World populations.
||Amanita muscaria var. guessowii
||American fly agaric (yellow variant)
||Amanita muscaria var. formosa
||has a yellow to orange cap, with the centre more orange or perhaps even reddish orange. It is found most commonly in northeastern North America, from Newfoundland and Quebec south all the way to the state of Tennessee. Some authorities (cf. Jenkins) treat these populations as A. muscaria var. formosa, while others (cf. Tulloss) recognise them as a distinct variety.
||Amanita muscaria var. inzengae
||Inzenga's fly agaric
||it has a yellow to orange-yellow cap with yellowish warts and stem which may be tan.