Ants using stigmergy
Below are two excerpts that describe in different ways how ants use stigmergy to create physical order and organization (Note: these are distinct from the way ants use stigmergy to create message trails to sources of food).
1) Stan Franklin
From: Coordination without Communication (See original link)
Institute for Intelligent Systems and
Department of Mathematical Sciences
University of Memphis
Another robotic example of coordination without communication was modeled on the behavior of ants (Deneubourg et al, 1990). Noting that dead ants were carefully removed from an ant colony's nesting area, an entomologist strewed the area with several thousand dead ants. On the following day he observed numerous small piles of dead ants. The second day after strewing brought a smaller number of large piles. By the third day there was only one (or sometimes two) piles. He conjectured that a probabilistic explanation of this behavior as follows: Faced with a dead ant, an ant picked it up with probability inversely proportionate to the number of other dead ants in the vicinity. While carrying a dead ant, an ant put it down with probability directly proportional to the density of dead ants in the vicinity. Thus the coordinated behavior of piling dead ants in a single pile resulted from a simple local rule controlling the behavior of a single ant. The piling behavior is accounted for without postulating communication between the ants.
Hoping to model this complex global behavior emerging from simple local rules, Beckerts, Holland and Deneubourg (1994) produced small, puck-piling robots. Picture a fifteen to twenty foot in diameter circular area inhabited by two or three shoe box size robots and very many hockey-puck sized wooden discs, uniformly distributed. Each robot is equipped with a scoop in front. When turned on, a robot moves forward until it accumulated three discs in its scoop, at which time it backs up, turns in a random direction, and again moves straight ahead. Hitting a wall also produces this backing and turning behavior, as does sensing (infrared) an impending collision with another robot. This simple local behavior results in global behavior modeling that of the ants, going through the stages of many small piles of discs, fewer larger piles, and eventually a single pile. All this again with no communication between the robots. The robots essentially ignore each other except for avoiding collisions. A single robot will go through the same stages of piling up discs.
Observing the nest-building behavior of termites led entomologist P. P. Grasse (1959) to define the notion of stigmergy, referring to actions of one agent being influenced by the effects of prior actions of other agents. Here's a description (Beckerts, Holland and Deneubourg, 1994, page 181) of the kind of collective, coordinated behavior that led to this concept.
When they start to build a nest, termites modify their local environment by making little mud balls and placing them on the substrate; each mud ball is impregnated with a minute quantity of a particular pheromone. Termites deposit their mud balls probabilisticly, initially a random. However, the probability of depositing a mud ball at a given location increases with the sensed presence of other mud balls and the sensed concentration of pheromone. The first few random placements increase the other termites probability of putting their loads at the same place. By this blind and random game little columns are formed; the pheromone drifting across from neighboring columns causes the tops of the columns to be built with a bias towards the neighbouring columns, and eventually the tops meet to form arches2, the basic building units.
2) Lewis Thomas
From his book 'Lives of the Cell' (1974):
"Stigmergy" is a new word, invented recently by Grasse to explain the nest-building behavior of termites, perhaps generalizable to other complex activities of social animals. The word is made of Greek roots meaning "to incite to work," and Grasse's intention was to indicate that it is the product of work itself that provides both the stimulus and instruction for further work. He arrived at this after long observation of the construction of termite nests, which excepting perhaps a man-made city are the most formidable edifices in nature....
The interior of the nests are like a three-dimensional maze, intricate arrangements of spiraling galleries, corridors, and arched vaults, ventilated and air-conditioned. There are great caverns for the gardens of fungi on which the termites depend for their nourishment, perhaps also as a source of heat. There is a rounded vaulted chamber for the queen, called the royal cell. The fundamental structural unit, on which the whole design is based, is the arch.
....Grasse placed a handful of termites in a dish filled with soil and fecal pellets...and watched what they did.....they all simply ran around, picking up pellets at random and dropping them again. Then, by chance, two or three pellets happened to light on top of each other, and this transformed the behavior of everyone. Now they displayed the greatest interest and directed their attention obsessively to the primitive column, adding new pellets and fragments of earth. After reaching a certain height, the construction stopped unless another column was being formed nearby; in this case the structure changed from a column to an arch, bending off in a smooth curve, the arch was joined and the termites set off to build another. (p. 133-134) "