The morphological structure of mouthparts and trophic relationships have been studied in the larvae of sciaroid dipterans developing in decaying wood and fruiting bodies (carpophores and basidiomes) of various fungi. Six structural types of mandibles are distinguished, and their specific features are analyzed in relation to the larval mode of life and type of feeding. It is shown that predation is not common in this group. The majority of epibiontic larvae have mandibles that operate like a scraper, collecting various small particles from the substrate surface, which accounts for a mixed type of feeding in these larvae.