here's life

I was playing frisbee the other night with some people from church.  It was pretty windy, but we had fun, and lots of turn-overs.  As evening approached, some gnats materialized and made a cloud above one guy who had a white shirt. It was very bright white, almost glowed, probably UV brighteners in the fabric (they take the UV in sunlight and convert it into visible light, making colors brighter).  A girl had on a white shirt, but the gnats seemed to ignore her.  It was not as bright--just a normal white.  Then as the sun fell and we continued to run up and down the field in the deepening red light of the sunset, the Pigpen-style cloud continued to track the guy, but then the girl started to notice that the gnats were over her too.  Interesting.  I think it has to do with defraction. 
While the sun was away from the horizon, we got the UV light from the sun, which activated the guy's shirt, making it show up as more white to the gnats.  Then, as the sunligh began to come more at an angle through the atmosphere, the UV got bent away, but the lower frequencies made it through.  This made the two shirts closer to the same brightness to the gnats. 
We lost, and went and ate homemade icecream. Now I wonder, what is the range of light that is visible to gnats?