Bodenheimer (Animal Life in Palestine, 1935) has this to say:
The Fungus-gnats develop in late winter on mushrooms (Macrocera), and only the Sciophila feed on their surface. The larvae of the numerous species of Sciara may be found in vegetable mould. They apparently are never sufficiently numerous in Palestine to form large processions ("army-worm"). [N.B. Neither Sciara nor Macrocera are in the following list. Sciaridae is a separate family.]

From Wikipedia:
Mycetophilidae is a family of very small flies, forming the bulk of those species known as fungus gnats. There are approximately 3000 described species in 150 genera but the true number of species is undoubtedly much higher. They are generally found in the damp habitats favoured by their host fungi and sometimes form dense swarms.
Adults of this family can usually be separated from other small flies by the strongly humped thorax but identification within the family between genera and species generally requires close study of microscopic features. The terrestrial larvae usually feed on fungi, especially the spores and hyphae, but some species have been recorded on mosses and liverworts. The larvae of some species, while still being associated with fungi, are at least partly predatory.