If I were to mention an ant-fungus mutualism- that is, an ecological partnership between an ant and a fungus that benefits both- most biologically literate people might think of the famed leafcutter ants and the edible mycelia they cultivate. But that is just one example.
Several other fungi have entered into productive relationships with ants, assisting especially in ant architecture. Consider:
Lasius umbratus walking in the galleries of an underground carton nest (Illinois)
A larger view of the same nest. The intricate galleries are made from fungal mycelia growing through a matrix of ant-chewed wood pulp.
Myrmecos Blog 