Why do flies bite before a rain?

Question

Why do flies bite before a rain?

Answer 1

I'm not aware of any documented cases of flies biting more frequently before a rain.  If it gets cooler right before a rain on a hot day, it is possible the conditions become more favorable for biting flies to be active (and bite), but the oncoming rain itself does not affect whether or not a fly will bite.  

There is a very old wives' tale about being able to tell when it is going to rain because "the house flies start biting."  The tale is completely false.  House flies cannot bite - they have sponging mouthparts, not biting or piercing mouthparts.  It is believed that the tale originated by people mistaking stable flies for house flies (stable flies do bite.)  They look very similar (except for the totally different mouth types).  It is thought (but not proven) that stable flies may sense the drop in pressure with an incoming storm and seek shelter.  More of them would come inside a house, increasing the chance of people in the house being bit by a fly before a rain (the wives' tale pre-dates the screened door and window).  Again, it is all conjecture, and the wives' tale is simply that - an old wives' tale.  It has not been proven.