Why you can't stop army ants from going across a river by dynamiting the bridges?

What do you do at home if you have bottle of some sherbet or sweet drink and ants keep crowding towards the top of that bottle so as to lick of droplets of sweet drink. Generally you keep the bottle in a bowl of water as ants can’t cross or swim across water. But you will be wrong if the ants in question are Army ants. Army ants cross rivers or bodies of water by climbing on top of each other and clustering together into a little ball that floats. Many of them fall off and sink, and naturally the ants on the bottom of the ball drown. The ones who are quick and vigorous enough to keep clawing their way to the top survive. A lot of them make it across, and that's why you can't stop army ants by dynamiting the bridges.

References:-
http://lib.ru/INOFANT/STEFENSON/snow_crash_engl.txt
http://www.angelfire.com/nc/isoptera/cretaceous.html

Ramneek Maan Singh