August 28, 2006: Fire ant raft

[Undertoad]

This page from a bug identification giude was linked to by Digg the other day. The photo was taken in Kissimmee, Florida.

Apparently this variety of ants do this to survive when their nest is flooded. As if a flood wasn't scary enough. But in case you're still underwhelmed, guess what happens to you if you encounter one of these things?

According to this document (bold emphasis mine):

Colonies of the fire ant, Solenopsis invicta, can survive flood conditions by forming a raft of workers that floats on the water’s surface until the flood recedes or higher ground is found. Forced from the protection of their nests and left without retreat, rafting colonies are both exposed and cornered, and are thus more vulnerable to damage than they would be otherwise.

As a logical corollary, I tested the hypothesis that rafting colonies would compensate for their elevated vulnerability through an increase in worker defensiveness. I measured defensiveness using the amount of venom workers deliver per sting, since the pain and tissue damage caused by fire ant venom (i.e. its repellency) is dose-dependent. In the lab, I assayed the venom doses delivered by S. invicta workers before and after flooding them from their nests with water. Workers delivered significantly higher venom doses while rafting than they did defending their nests pre-flood. Mechanistically, the unusual concentration of workers during rafting may result in a concomitantly unusual concentration of alarm pheromones, and thus the increase in defensiveness. Functionally, the increase in venom dose during rafting should serve to better protect the exposed colony from molestation. From a practical standpoint, human encounters with fire ants during flood conditions have the potential to be unusually dangerous; not only are large concentrations of workers exposed and available for defense, but they deliver significantly larger venom doses when they sting.

It's a giant mass of painful death. Hope you can swim.