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The second phase of mound repair involves a self-organizing process termited stigmergy.
The literal meaning of the term, first coined by Pierre-Paul Grassé in the 1950s, means "driven by the mark." In this instance, the mark is a grain of soil set into place with a salivary glue that contains an attractive pheromone. This substance then attracts other termites to glue grains of soil on top. This amplifies the attractive signal (two or more dollops of attractive pheromone where once there was one), which stimulates more deposition at the orginal site, and so on. This process is called stigmergic building, and it produces a characteristic set of structures, which are described below.
This page outlines what is known about stigmergy and how it can produce complex structures in the nest. It also discusses a basic property of stigmergic building that ensures it cannot be the sole explanation for the complex Macrotermes mound.
Pillar construction & the roofed gallery
Wall production & space enclosure
The spongy build
Stigmergic building: pillar construction and the roofed gallery
Followiing the recruitment phase, termites begin to deposit grains of soil onto surfaces. Each deposited grain represents a focus for new building.
At its simplest, stigmergic building produces pillars (sequence to the right). On a flat surface, the pillars grow to about the height of a termite standing on its hind legs. At this point, new soil grains are added laterally, producing a shelf that extends from each pillar. Eventually, these shelves merge to form a roof, producing one of the basic architectureal motifs of the termite mound, the roofed gallery.
The roofed gallery is often found around foraging sites. The roofed gallery is a kind of caisson, which provides termites a sheltered environment to work as they gather grass stems of dismantle twigs and logs. The images below show two examples of roofed gallery construction at foraging sites.
You can see stigmergic building in the video clip to the right.. TOP
Stigmergic building: Wall construction and enclosure of space
Pillars are built when new grains of soil are added to the top of already existing sites of stigmergic building. When new grains of soil are added laterally, stigmergic building produces walls, as depicted in the sequence to the right.
One often finds wall construction at sites of repair for holes in the mound, as shown below.
When walls get too high, they can become unstable and collapse onto one another. This is evident in the time-lapse video below. This sequence depicts a developing site of stigmergic building within the mound, which we are observing under glass (which mists over periodically. Use the slider to observe how the building walls collapse onto one another.
Stigmergic building: The spongy build
In mound repair, stigmergic building produces a space-filling network of walls and pillars called a spongy build.
The spongy build is the first step in closure of a breach in the mound. The confined space within the spongy build also helps confine the attractive pheromone that drives stigmergic building, which intensifies the building. The end point of this positive feedback process is solid packing of soil, as shown in the pipes below. The top pipe represents spongy build after a day. The bottom pipe shows a pipe after about a week, filled with a nearly solid pack of soil.
