One of the basic concepts of flight is that lift must be generated to get into the air and maintain flight. Many animals and even flying vehicles have specific processes to generate their lift and take flight, processes that are very planned out and must be done properly. One such example being a hang glider taking off, where on the ground speed must be picked up to get a fast air speed over the wings to generate lift.
Take off for your average fly consists of two actions. First they extend their wings upwards together, setting up for a large flap to generate lift. That lift isn’t enough to set the fly off into the air though. They also at the same time compress their legs so that they can extend them and launch off the ground. The legs extend while the wings flap and flight is achieved.
Flies will do this natural take off, also called voluntary flight, when they want to change where they are, when they want to do something they can’t get done in wherever they currently are. But there are other times when the fly doesn’t have the time for a voluntary take off. When they need to get out of the way of something, such as a tree they’re flying into or a predator coming at them (animal or fly swatter). At these times the fly won’t have time to get it’s wings up to that extended position so they just jump off wherever the are and into the air.
Opo_Terser_-_Female_Tabanus_Horse_Fly_(by)_(1).jpg
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No matter where they are jumping from, the fly manages to recover. They can hit the ground even and continue their flight. It is definitely a faster take off, which makes the fact that there are two different take offs for a fly bizarre. It could be because the “escape” take off is very unorthodox, or maybe that it expends more energy trying to stabilize flight after the free fall. Either way, the fly can make it into the air multiple ways.
Sources:
http://upload.wikimedia.org/wikipedia/commons/0/0d/Opo_Terser_-_Female_Tabanus_Horse_Fly_(by)_(1).jpg
http://blnnewsdotcom.files.wordpress.com/2012/09/onwall.gif
http://www.microscope-microscope.org/gallery/Kenn/More2/BB%20fly%20supermacro-2a.JPG