F. costalis is the largest of the root-maggot flies on the beach, measured between 5.0mm and 9.0mm. Their cheeks are uniquely silver, which allows them to be easily identified when paired with their size. Their eyes are also wider than they are long and are shorter in height than their cheeks. Their legs are black, like F. separata. These flies are seen infrequently in patches. Their large larvae and pupae are seen under a single kelp wrack in patches. The larvae likely consume large amounts of kelp and are a factor in kelp removal. As adults, F. costalis is also a resource for predatory arthropods. Their diet is not well known, but may be limited to fluids or bacteria from kelp and bird feces.