It looks like a lightning bug to me. If you lower the brightness and increase the contrast of the image, you can see a shape below the flash that looks like a body with wings. The body of the bug caused the dark streak, and he flashed us just as the shutter was closing.
Slashdot Mirror
It looks like a lightning bug to me. If you lower the brightness and increase the contrast of the image, you can see a shape below the flash that looks like a body with wings. The body of the bug caused the dark streak, and he flashed us just as the shutter was closing.