Michael Blackman, of King's High School, asks :-

How do flies walk on the ceiling upside-down?

Ruud Kleinpaste, an entomologist whose series "Buggin With Ruud" is on Animal Planet, responded.

I assume we are dealing with the 'common old' house fly, "Musca domestica". Great little insects.

If I were to give a really smart answer I could say that house flies rarely "walk upside-down" on the ceiling, but usually sit still. Mind you this in itself is quite a feat, especially when considering that these flies do not merely sit on the ceiling but also indulge in a spot of regurgitating at the same time! Oh, yes, the humble house fly does this frequently to aid digestion of its food. They are one of the few creatures that can successfully bring its food up while sitting upside-down on the ceiling!

Flies are able to cling to the ceiling with the aid of tiny hooks on their feet, known in the entomological trade as 'tarsal claws.' Think of them as minature crampons, attached to the front of their toes. Each foot has a couple of these claws and they manage to somehow grab hold of any small imperfection or rough area on the sometimes smooth surface.

How they land onto a ceiling is also one of Life's great tricks. The two-winged flies have modified their hind wings into small `halteres' which are formed like knobs on stalks. These halteres gyrate with some speed while the fly is airborne, and this gives them the necessary stability for completing an aerial loop, followed by a perfect three-point landing. The moment they are about to land, the legs are being extended, ready to grab the ceiling with their tarsal claws.