Ant scolding aphid
Not "scolding"... but rather "herding"!
See http://insects.about.com/od/coolandunusualinsects/f/antsandaphids.htm for more information.
I wrote a blog post ( http://gorga.org/blog/?p=150 ) with a couple of photos about "ants herding aphids" a while back.
Nice photo, by the way!
Frank ( http://www.gorga.org/blog )