The mouth parts or 'proboscis' of a Blowfly have a very special structure. Flies evolved many specialised mandibles. Some flies have stilettos for killing other insects, others have trunks to eat pollen. The Blowfly eats liquid food. And if it isn't liquid the Blowfly will make it liquid. It's mouth parts have a large sucking device at the end (like a mop). It consists of many hollow tubes called pseudo trachea (because they resemble the breathing system of insects) The image shows a close up of the pseudo trachea.
When a fly lands on your cake it will first extend it's mouth parts until the mop of pseudo trachea touch the food. Than it gulps his stomach content through the pores. The digestive juices start to dissolve the food until it can be swallowed like a liquid. This all sounds quite fascinating but don't forget that the stomach content of the fly may contain nasty bacteria or other disease bringing content. The fly's food before your cake might be a rotting corpse or some fresh excrements. Therefore it is not completely without hazard to invite flies to dinner.
Click the thumbnail to see of the whole Fly proboscis.