Imperial’s campus in Berkshire, Silwood Park, is a fabulous place to go fungus spotting. The fly agaric (Amanita muscaria) is very common there as there are a lot of birch trees around, and this fungus forms a symbiosis with the roots of those trees:
Fly agarics are rather poisonous (for some value of ‘rather’), by virtue of containing muscimol, and ibotenic acid; the former is a psychoactive dissociative that can cause visual and auditory hallucinations. There is an enormous amount of folklore (i.e. lies) associated with this mushroom that I shan’t bother repeating. There’s not even much evidence that it’s any use for killing flies.
Although fly agarics are toxic, they are nothing like as poisonous as their amanitin-containing relatives, the cheerily named death cap (Amanita phalloides) and the delightfully named destroying angel (Amanita virosa), whose toxins don’t merely bugger up your nerves, but poison every cell in your body at an intimate molecular level. Fungi probably contain toxic compounds for the same reason as many plants and corals do: even if you can’t bite or run away from your predators, you can still make them very, very sorry.