High in the trees on the Fiji islands, ants in the species Philidris  nagasau are doing something extraordinary. They've brought in seeds from  several species of a large, lumpy fruit from a plant known as  Squamellaria and carefully planted them in the nooks and crannies of the  tree bark. Once the plant takes root in the tree and begins to grow,  the ants climb inside its young stalks and fertilize it. But then the  real action starts. As the fruit swells, the ants move inside, carving  tunnels and rooms into the fleshy interior. When the colony expands, it  may include dozens of these fruits, which look like strange tumors  sprouting from tree branches. (...)