The feeding habits of flies depend on their species. Some specialize in sucking nectar and plant juices, and some specialize in eating human and animal blood or animal wound blood and eye and nasal secretions. The common house flies, golden flies, green flies, blow flies, and lineflies are omnivorous flies, that is, they widely feed on human food, livestock and poultry secretions and excrements, kitchen scraps, and organic matter in garbage. They have a strong tendency towards sugar, vinegar, ammonia, and fishy smells. According to research, if female flies are simply supplied with water, sugar and carbohydrates, they can grow, but their ovaries cannot develop and lay eggs. Only by feeding them protein food or a variety of amino acids can they lay eggs normally. If royal jelly is used to feed female houseflies, the pre-oviposition period can be shortened and the amount of eggs laid can be increased.

The feeding habits of flies are very mixed, and they like aroma, sweet, sour, and smelly. When feeding, they spit out crop fluid to dissolve the food. Their habits are eating, spitting, and pooping. Someone has observed that when food is abundant, flies defecate 4 to 5 times per minute.

Flies will also eat bacteria that are not good for them when eating, so the "eating and spitting" method can help quickly eliminate bacteria. Generally, it only takes 7 to 11 seconds for a fly to process, absorb nutrients, and excrete waste from the body. When encountering bacteria with the ability to reproduce rapidly, the fly's immune system will emit two globulin proteins, BF64 and BD2. Once in contact with the bacteria, it will "explode" and "die" with the bacteria. The emission of these two types of globulin is always in tandem, in pairs, and never messed up. It is worth pointing out that the bactericidal ability of BF64 and BD2 is hundreds of times stronger than penicillin.