Think twice about eating something a fly has eaten. When a fly lands on food, it will first extend its mouth parts until the mop of a pseudo trachea touches the food. Then it gulps the food through the pores of this mop. The digestive juices start to dissolve the food until it can be swallowed like a liquid. However, the stomach content of the fly may contain nasty bacteria or other disease. 

Centre for Microscopy and Microanalysis