"Weaver ants (Oecophylla spp) live in canopies where their territories span multiple trees. They therefore need to travel between trees and for this they have developed ability to bridge gaps between twigs and leaves. They grab each other with their mandibles and build a bridge of connected ants. In this present case a horisontal bridge where they are fighting against gravity. See also the other video on this channel where the ants are faced with a vertical gab which is easier to bridge as they get help from gravity. In that case they can build longer bridges. They use the same technique when they build their nests by pulling leaves together (see nest building video on this channel). This footage was shot with a smartphone by students at the Department of Bioscience at Aarhus University, Denmark, during their third year course 'Ecology and Diversity of Arthropods'."

From http://www.reddit.com/r/ants/comments/es0rqg/weaver_ants_build_a_horisontal_bridge/.