Cheese flies are members of the family Piophilidae of flies (Diptera), including the members of two now-obsolete families, Neottiophilidae and Thyreophoridae. Most are scavengers in animal products and fungi. The best-known member of the family is Piophila casei. It is a small fly, about four mm (1/6 inch) long, found worldwide. The fly's larva infests cured meats, smoked fish, cheeses, and decaying animals. The larva is about 8 mm (⅓ inch) long and is sometimes called the cheese skipper for its leaping ability - when disturbed, this tiny maggot can hop up to 15 cm (six inches) into the air. Adults are also known as "bacon flies" and their larvae as "bacon skippers", "ham skippers", "cheese maggots", and "cheese hoppers". In Sardinia, Italy the larvae are intentionally introduced into pecorino cheese to produce casu marzu.
If eaten (accidentally or otherwise), the larvae can pass through the digestive system alive (human stomach acids do not usually kill them) and live for some time in the intestines. This is referred to as an enteric myiasis. Cheese fly larvae are a leading cause of myiasis in humans, and are the insect most frequently found in the human intestine. The larvae can cause serious intestinal lesions as they attempt to bore through the intestinal walls. Symptoms include nausea, vomiting, pain in the abdomen, and bloody diarrhea. Living and dead larvae may pass in the stool.
Forensic entomology uses the presence of Piophila casei larvae to help estimate the date of death for human remains. They do not take up residence in a corpse until three to six months after death.
The adult fly's body is black, blue-black, or bronze, with some yellow on the head, antennae, and legs. The wings are faintly iridescent and lie flat upon the fly's abdomen when at rest. At four mm (1/6 inch) long, the fly is one-third to one-half as long as the common housefly. Liopiopila is a typical genus.
The larvae accomplish their jumps by bending over, grabbing onto the rears of their own bodies with their mouth hooks, tensing their muscles, and quickly releasing the grip. Spring action propels them into the air. A series of photos illustrating this remarkable behaviour in larvae of the tiny piophilid Protopiophila litigata, commonly known as the "antler fly" (it breeds exclusively on discarded antlers of cervids such as moose and deer), can be seen at Prochyliza xanthostoma breeds on rotting carcasses and is highly sexually dimorphic, with males having greatly elongated heads, antennae and legs, relative to females. P. xanthostoma also has spectacular mating and fighting behaviours.
This is a small family of 67 species in 23 genera, mainly Holarctic in distribution. To identify them entomologists use an identification key as a first step. Then the fly must be compared with a full description. The two most recent works containing keys and descriptions are: