Ordinary house flies can land on upside down glass surfaces and still stay where they land. This phenomenon is related to their amazing legs. At the end of their legs, they have tiny claw-like structures that help them grab on to the smallest roughnesses on any surface. These claws are about 300 micrometers across and about 10 micrometers thick.
While the claws are useful in grabbing onto surface roughness, wherever the surface is perfectly smooth, they have two sticky nets on each side of their claw that use electrostatic forces to adhere to any surface.