This is a new one on me!

http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=lot&storyid=96161&source=0

"The mayflies tricked the radar algorithm into thinking they were large rain drops because the reflectivity values were higher than normal for insects as were the correlation coefficients. Also, large rain drops tend to flatten out like pancakes when they fall due to friction, so the large differential reflectivity value... were consistent with those that would be expected with large rain drops."

That's a lot of bugs, I wouldn't like to have got caught up in it, though.

Pity anyone wearing white at the time.....