Researchers have found that the distinctive black and white stripes keep the flies away. Horseflies have been found in tests to be less attracted to zebra's stripes than to a white, brown or black horse coat. Such protection is vital, as horseflies transmit diseases.
When a sunlight is reflected off a dark brown horse, the light waves tend to vibrate horizontally rather than vibrate in all directions as they do in sunlight or off the coat of white horses. This phenomenon is called 'linearly polarised light' and are also produced by water, where horseflies can mate and lay eggs. Originally, the zebras were black and were introduced the white stripes to destroy the signal of the reflected linearly polarised light.
Horsefly |
Source : BBC Knowledge, Vol.4 Issue 6, page 16 "How the zebra got its stripes"