Also cheese hopper, ham skipper.

The larva of Piophila casei, the cheese fly (also bacon fly, cheese skipper fly). (Piophila means "milk-loving"; casei "of cheese"). It bores into and infests ripening or stored cheeses and also cured meats. The larva gets its name from the fact that it can move by popping itself into the air. It does this by grasping its tail-end in its mouth-hooks, tensing its muscles, and then releasing the tail suddenly. It can also crawl by peristalsis as other larvae do, but it uses popping to escape when disturbed.

The cheese fly is a dipterous insect, one of the true flies, black and about half the size of a common house fly. It is considered synanthropic, meaning that it lives in close association with human beings. The female lays some 140-200 eggs, which develop to adulthood in 11-19 days. It has a short life-cycle after reaching adulthood. The larval form of the fly is better known than the adult; it holds an important place in one of the little-known corners of Italian cheese-making and also in forensic entomology.

Cheese infested by Piophila casei turns pink and creamy, with a strong smell. In parts of Italy this cream, made from pecorino and called cacio marcetto ("rotten cheese", in Sardinian, casu marzu), is blended for consistency and eaten spread on bread. It is common in mountain areas, and Italian gastonomes have been known to rhapsodize about its nutritional benefits, which they claim comes from some quality of the mountain grass eaten by the sheep. Marcetto is treated as a kind of strong-tasting cream cheese, although it is actually different from true cream cheeses, which are technically not ripened. Because the mouth-hooks of the larva sometimes fail to get removed and are blended into the cheese, careless processing can lead to damage of the human gastrointestinal tract. For this reason it is illegal to sell marcetto in Italy; it is only permitted to be made for private consumption. (See also the node maggot cheese)

Piophila casei larvae are frequently found in exposed cadavers, where they grow three to six months after the body's death, during the period of protein fermentation. Larvae stay in their food source for only about five days; they leave in order to pupate. The purplish brown pupal cases, which the pupae leave behind after emerging (i.e., they don't eat them) are important evidence of the date of death, because Piophila casei attacks cadavers much later than most other maggots.

Cheese skippers are also a known cause of myiasis, or maggot infestation of a living body. There is a somewhat amusing description of myiasis at men, maggots and medics, but in fact it can be accompanied by violent abdominal pain, nausea, vomiting, and diarrhea with bloody discharge.

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