Conglomerations of ants were considered rafts when multiple layers of workers were observed within the structure and the majority of individuals were free floating on the water with no attachment to solid substrate. During trials, colonies were not allowed to escape from the water. Any colony that successfully escaped the enclosure was removed from the data set. Rafts not maintained for ≥ 12h were considered a failed raft. - Raft Formation by the Red Imported Fire Ant, Solenopsis invicta

The researchers tested the movement, or cycling, of ants within the raft – by marking various groups of worker ants with modeling paint! They counted marked individuals every few hours, flipped the raft over, and continued to count. The researchers then froze the rafts and dissected them for microscope observation and photography. Through microscope observation, the researchers could examine for example how bubbles formed on the underwater worker ants and larvae, and how these bubbles might enhance the floating raft.

Finally, the researchers examined how long the rafts could remain intact, and how this longevity depended on the presence of various ant life stages (e.g. brood) in the raft.

So what did the researchers find out about raft formation in red imported fire ants?

First of all, at some point the collective of ants in the raft began to “pitch” extra ants overboard. The first to be pitched overboard to drown were the male “drones” or winged reproductive male ants. So much for being “king!” Wingless reproductive queen ants, however, were keep safe in the middle of the rafts. Brood was located on the bottom of the raft except for early stage larvae or eggs, which the workers keep in their mandibles.

The raft formation experiments in this study showed that the raft is a dynamic structure, with worker ants cycling from the bottom to the top of the raft, presumably to prevent drowning of individual ants. Red imported fire ants form the raft structure primarily by hooking and curling around each other’s hind legs in Velcro-like fashion. Score for teamwork!

The rafts could also remain floating for up to 12 days in undisturbed lab trials. The more brood in the raft, the longer it could stay afloat. In particular, curved bristles on the older larvae seemed to capture air bubbles and thus help the raft stay afloat on the surface of the water.