How can some insects like housefly walk upside down on the ceiling?

When the foot of a fly is seen through the microscope, two membranes, or cushions, can be noticed. These are covered with tiny hairs, each with a disc at the end. (See the microscopic image below.) Some scientists believe that the discs act as suction pads or suckers, and that when a fly walks on the ceiling, holding the fly up there by the pressure of the air. Against this theory, however, is the fact that when the air is drawn out of a vessel in which the fly is walking upside down, the insect does not fall off.

Other scientists who have carefully studied the fly believe that it is able to hold on by means of a sticky substance coming out from the underside of the foot. Each hair would seem to be tiny tube for conveying this liquid from the little pouch in which it is produced, pouring it out as required. But however the fly performs the feat, it still seems to be defying gravity.

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