Flies (Diptera) are part of Mother Nature's
Sanitation Crew. They are primarily influenced
by what they smell.
The real mystery is what you spilled, smeared, or
intentionally put on your leg.
Flies can't resist certain smells:
"Yes, it's true. A fly could smell meat over 7 miles awsy"
"How do flies manage this? Well, it involves no magic - simply, a very good sense of smell. Flies like all other insects detect odours with their antennae which are densely colour in hair-like structures containing olfactory sensory nerve cells. These nerve cells are extremely sensitive to volatile chemicals, odours that is. And in the case of flies that feed on the dead and decaying flesh, so-called carrion flies, their olfactory cells are also optimised towards detecting carrion."