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The most recent issue of The Journal of Experimental Biology provides insight into why zebras have evolved stripes. Using a horse model, researchers have shown that horseflies find zebra stripes less attractive than homogeneous colors. It turns out that horseflies are attracted to linearly polarized light whereas the pattern of wide black and white stripes on zebras reflect different polarizations making them less attractive to the biting insects.

Too bad mosquitoes are not attracted to light patterns. I would wear zebra-striped pajamas all summer long if they were.

Source:
Egri A, et al., Polarotactic tabanids find striped patterns with brightness and/or polarization modulation least attractive: an advantage of zebra stripes. Journal of Experimental Biology. J Exp Biol 215: 736-745, 2012.