Subject: What are the glowing, gnat-like insects seen once a year?

A friend of mine has a house on a western North Carolina creek bank. Once a 
year for the past three years, for a very short period of time, we have noticed 
very small (the size of a gnat) flying bugs that light up like fireflies. No 
one can tell us what they are, though old-timers in the area say they are only 
out for 10 to 12 hours a year -- and only if the temperature is warm enough and 
there is no rain or wind.

Re: What are the glowing, gnat-like insects seen once a year?