Opposing headings of excavating and depositing termites facilitate branch formation in the Formosan subterranean termite

Subterranean termites forage through a network of tunnels. The topology of the networks can be characterized in terms of branch angles and branching rate. When termites encounter depressions in the walls of tunnels, they may excavate the soil from the depression, which could result in the formation of new branches, or they may deposit previously excavated soil from elsewhere, which could fill in the depression. Thus, the likelihood of a branch forming is determined by the balance between soil arriving and filling the void and soil being excavated to enlarge it. We examined the tunneling behaviour of Coptotermes formosanus Shiraki in experimental arenas and found that excavation and deposition do not take place evenly across the face of tunnel wall depressions: excavation (by unloaded termites moving towards the digging face at the tunnel tip) is more likely at the proximal face (relative to the tunnel tip), while deposition (by loaded termites returning from the tunnel tip) is more likely at the distal face. This differential focus of labour within depressions facilitated branch formation by reducing localized competition between loaded and unloaded termites. Once a branch was established, biased excavation and deposition led to a shift in branch heading from about 90° relative to the main tunnel to about 50–60°, close to the theoretical optimum for the efficient flow of termites through tunnels. The basic elements of the excavation/deposition system and feedback between the two may be selected in evolution to optimize the tunnel network and thus the cost/benefit balance of feeding.