Ants attending Imperial Blue Butterfly caterpillars

Ants on plants! We spotted this cluster of hundreds of ants at the top of a bush while walking near Mount Tibrogargan in the Glasshouse Mountains. I'm not sure what kind of ants they were, but after doing some research on the Imperial Blue Butterfly which I photographed on the same bush, I discovered that Imperial Blue caterpillars hang about on acacia leaves and are always attended by ants as the caterpillars secrete a substance that attracts them. Looking very closely at this pic again, I believe I can see caterpillar shapes under the clusters of ants. Fascinating!!! Imperial Blue Butterfly spotting: http://www.projectnoah.org/spottings/8975089