I'm not sure if these are scale insects or aphids. Would love to hear from an expert on this [UPDATE: I did. See below]. Last week, exploring Red Hills salamander habitat with TNC and USFWS people, we ran across this sight. NOTE: The date on the opening title is wrong; it should say 2009, not 2007.
The rattling sound is from the autofocus on my Canon camera, which is really a still camera but with a video option.
UPDATE, 7/14/09: Thanks to entomologist Charles Ray at the Alabama Cooperative Extension System in Auburn for identifying these as the Beech Blight Aphid (Grylloprociphilus imbricator). They range from Maine to Florida, and are found on American Beech (Fagus grandifolia). Charles provided this helpful link (pdf) from Massachusetts. A quote:
"it will raise the posterior end of its body and sway when it is disturbed. This action produces a dance-like effect that occurs throughout the colony. This phenomenon has led some to refer to this species as the 'Boogie-Woogie Aphid.' It is a unique experience to see hundreds, if not thousands, of these perform this defensive, yet highly entertaining, behavior."