Why do house flies rub their front legs together?

 Flies sense their surroundings with the help of their eyes and bristles on their bodies and legs. The flies run their legs all over their body and wings to pick up the dust particles and then rub their legs together so as to clean the dust and dirt off their legs. Thus, by cleaning these stimulus receptors, they can have a better sense of feel and test receptors, they can have a better sense of feel and taste than with a dirty body.