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Stigmergy is thought by many to be the principal mechanism whereby complex and coherent structures can arise "from the bottom up", that is through self-organized behavior of multitudinous agents that themselves have no conception of what they are building. As such, stigmergy is one of the foundation stones for an atomist philosophy of biology: all the complex and wonderful contrivances of the living world are explainable by the low-level interaction of "atoms", autonomous and indivisible units that interact according to a discernible and limited set of rules.
While it is true that self-organization can generate marvelously complex structures (see Philip Ball's Self-Made Tapestry), the question still nags: is it sufficient? This page explores the extent to which it probably is, and the extent to which it might fall short.
Stigmergy & the queen cell
Gallery architecture
The positive feedback problem
Stigmergy and the queen cell
Possibly the best example of stigmergy explaining a complex structure involves the queen cell constructed in Macrotermes nests. This was establshed by the pioneering work of O H Bruinsma, which was unfortunately never published.
This structure, which houses the queen, the king and attending workers, is generally located slightly above the nest center. The queen is confined to her queen cell, which opens to the broader nest environment through a series of small portholes, though which workers bring food to the queen, and remove her eggs (produced at the prodigious rate of about one every three seconds) and waste. A queen cell in situ is shown in the photograph above, showing the portholes. The photograph to its right shows the queen cell opened, with the queen attended by numerous workers and a few soldiers.
Bruinsma's model, which has impressive experimental support, posits that the queen cell is produced by an interaction between the attractive pheromones that emanate from the salivary glue, and a pheromone secreted by the queen which, above a certain threshold concentration, inhibits termites from depositing soil. The queen's pheromone cloud therefore serves as a template for constructing the queen cell. Where the queen pheromone falls below the threshold concentration, termites engages in stigmergic building normally. Where queen pheromone rises above the threshold, stigmergic building is suppressed. The threshold concentration isobar, therefore, represents a kid of bubble that can serve as a template for construction of the queen cell, as shown in the cartoons to the right.
Bruinsma's original work exists only in his PhD dissertation:
Bruinsma, O. H. (1979). An analysis of building behaviour of the termite Macrotermes subhyalinus (Rambur). Wageningen, The Netherlands, Landbouwhogeschool te Wageningen: 85.
Fortunately, his work has been described fully by others. Perhaps the best synopsis is found in:
Camazine, S., J.-L. Deneubourg, N. R. Franks, J. Sneyd, G. Theraulaz and E. Bonabeau (2001). Self-Organization in Biological Systems. Princeton, New Jersey, Princeton University Press.
Stigmergic building: Gallery achitecture
Other structures built by termites have the mark of self-organization. Termite nests have a stereotypical architecture of galleries that is reminiscent of the spongy build. The photoraph below shows a cross section through a nest of Trinerviermes trinervoides, which inhabits the winter-rainfall region of South Africa. The similarity to the spongy build is clear. Similarly, the fossil termite nest shown in the photograph to the right (found along Namibia's Skeleton Coast) also shows a clear similarity to the spongy build.
This basic nest architecture can be modified in various ways, each consistent with simple stigmergic processes. The plaster cast to the far right, for example, shows a cross section through the edge of a Trinervoides mound: the convoluted passageways of the spongy build are obvious. Deeper in the nest, though, this architecture shifts to a network of parallel galleries, as shown in the plaster cast immediately to the right. Both pillars and walls are outcomes of stigmergic building.
Stigmergic building: The positive feedback problem
While stigmergic building can produce complex structures, there is a fundamental problem that makes it impossible for stigmergy alone to be a sufficient explanation for complex architecture in termite mounds. We might termi this the positive feedback problem.
The positive feedback problem arises because it is driven by the attractive pheromone in the salivary glue. Each droplet of salivary glue stimulates the local deposition of more attractive pheromone, which elicits more intense soil deposition, and along with it, a more intense focus of attraction, and so on and so on.
This process can be demonstrated by confining stigmergic building into an enclosed space, like a PVC pipe implanted into the mound. The sequence of photographs to the right shows the process over a period of about 12 weeks. The tunnel networks have been cast in plaster. The photograph shows a sawn section through the pipe. Stigmergic building at first produces a conventional spongy build, but over the space of a few weeks, the pipe comes to be completely filled in.
We think the process operates as outlined in the diagram below. As soil is deposited within the pipe, it establishes a gradient in pheromone concentration that elicits a net vector for soil movements to the end of the pipe. As soil is moved to the end of the pipe, it concentrates the pheromone there, intensifying the gradient, and vectoring soil ever more intensely to the end of the pipe. Eventually, the soil comes to be packed solid at the end. If soil is continually fed into the pipe from the left, this process will continue until the pipe is completely packed.
Forcing termites to build in a pipe like this is essentially allowing stigmergic building to proceed with out any intervening influences. This means that stigmergy alone can never produce anything but a solidly packed mass of soil. For a complex structure to emerge, stigmergy must be interrupted or disrupted somehow. In the case of the queen cell, for example, the disruption comes from the inhibitory pheromone cloud produced by the queen. In the case of complex galleries, as in the Trinervitermes nest, something else, perhaps a fungal or bacterial film on the gallery surface may disrupt the building. In the case of the Macrotermes mound, it appears to be contact with the mound atmosphere. In all instances, stigmergic building can, at most, produce a foundational structure for other complex architectures. Allowing these to emerge means stopping stigmergic building somehow. TOP
