I was wondering, for a speculative evolution project, what could be the bare minimum amount of organisms the common housefly needs to survive? Could they just feed on grass? Or should I add a flowering or fruiting plant?
Common house flies would need something to break down grasses into some rotten form with enough nutrition support them. Maggots mostly eat bacteria in rotting material -usually meat, but fungus could work. This is why people sometime use maggots to clean wounds (gross, but true). I would probably say you would need a variety of bacteria and at least one kind of fungus to make the system tie together. There are fungi that attract various species of flies, but they're mostly related to fruit flies. Still, it's not impossible. Most people wouldn't even see or understand there was fungus or bacteria present.