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In my former lectures on pollination biology I also dealt with Diptera. Their main reason to visit flowers is to feed on nectar, stigmatic secretion, and pollen. Some taxa take carrion-mimicking or fungus-mimicking flowers and inflorescences for breeding sites; these structures also serve as lekking arenas. Pseudocopulatory pollination by fungus gnats is known from Lepanthes (Orchidaceae). Many flies are hairy, even their eyes, so pollen sticks easily. Plant taxa that display rhinomyiophily are pollinated by flies with long proboscises, e.g. Nemestrinidae. Even if the flies visit different taxa, pollen transfer to a wrong species is avoided by the morphology of the flowers, depositing the pollen on different body parts that all are, of course, hairy. On the other hand, Orchid pollination (the transfer of stuck pollinaria) demands smooth surfaces. Pollinaria stick best on proboscises but also on smooth abdomen parts.
As flies do not collect pollen for their offspring, their legs or other body parts do not possess special adaptations. Their hairiness is an efficient tool in unintentional pollen transfer. As with other hairy pollinators, electrostatic forces play a certain role in pollen adhesion, but this is not Diptera-specific.
You can easily google texts about myiophily and its subcategories in which the kind of adhesion to the pollinator is mentioned. If you have more questions, send me a private message.
Regards, Sundew