This very common little parasite of Mycena species (and of several other mushroom genera) is known as a pin mould. It appears on the caps of larger fungi as erect pale brownish-yellow sporangiophores (specialised hyphae) on which its zygospores are produced.

Bonnet Mould and the other members of the Spinellus genus are able to reproduce when 'pins' from the same mycelium contact one another; such fungi are termed 'homothalic'.