The mouthparts of Musca domestica end in the labellum, a pair of fleshy lobes scored by rows of fine channels called pseudotracheae. Fluid drawn into those grooves is funnelled to a single opening and pumped into the gut. Nothing in the apparatus can pierce, cut or grind.
Solid food is therefore dissolved outside the body. The fly regurgitates saliva and crop contents onto the surface, allows enzymes to act, and takes the mixture back up. The habit leaves visible traces, since regurgitation spots and faecal specks are both deposited during ordinary feeding, and it explains why houseflies move so readily between refuse and prepared food.
Adults generally live a few weeks in warm conditions, less when cool or short of food. What is often overstated is the epidemiological arithmetic. Houseflies carry large numbers of bacteria on their bristles and in their gut, but showing that a particular human infection travelled by fly rather than by water, hands or food handling remains difficult.