The Four Principles
Summary table
Specialization
in computing
Polymorphic Messaging
in computing
Loading code
Interpreted code
in biology
Stigmergy
and "self"
in computing
in the Internet
Cell Suicide (Apoptosis)
in computing
Intertwined principles
Complexity
The problem
Out of control
Characterizing complexity
Dynamic complexity
Why the Biology Metaphor
Parallels with computing
Information processing
Encapsulation
Emergence
Example emergent systems
Multi-level emergence
in computing
in biology
Scale and emergence
Evolution
of computing
of multicellularity
Conclusions
Discussion & Comments
Stigmergy
is an organizing principle in which individual
parts of the system communicate with one another indirectly
by modifying and sensing their local environment. Termite mounds are a
classic
example but cells do it and now computers are doing it too.
The third of the four
principles of
multicellular systems
is that much of the communication between cooperating entities (cells,
social insects or computers) is indirect. The entities deposit
long-lived cues
in
external
structures -- connective tissue, termite mounds, or
databases as the case may be. The persistent information provide
by these cues helps to organize the collective behavior of the
cells/computers/insects. The term stigmergy
was coined in the
1950’s [1]
to put a name to these sorts of reciprocal relationships between social
insects and the
structures they build, e.g., termite mounds, ant hills, beehives and
even the
pheromone-marked ant trails of nomadic social ant species such as army
ants. More recently, the term has been adopted by other
disciplines including computer science.
Although the term
was coined to describe social insect behavior, the phenomenon
itself arose much earlier. The very bodies of all multicellular
organisms are stigmergy structures. Cells in a
multicellular
organism create a growing body whose
shape and
boundaries are defined by a non-living extracellular matrix created by
the cells. The cells deposit all sorts of cues, in the form of
messenger
molecules, in or on this matrix. In
the earliest and simplest multicellular organisms the extracellular
matrix may be nothing
more than a “slime” excreted by the cells that forms a clump or thin
film
in
which the cells live and through which messenger molecules diffuse from
one cell to its neighbors. More complex
multicellular organisms have much more complex extracellular matrix
structures that support more subtle complex communication. Plants
create rigid
stigmergy structures made largely of cellulose and other complex sugars.
Most complex animals create connective tissue that gives structure to
their various organs and generally holds their bodies together.
In
addition, mollusks (snails, clams, etc.) create
shells, insects create chitinous exoskeletons,
vertebrates create bone that is akin to coral (itself a
stigmergy structure). Unlike coral, bone is constantly reshaped by the
cells that create and maintain it to adapt to the changing stresses it
encounters.Social insects, cooperating cells, and cooperating computers
communicate both with signals and cues. The distinction is that
signals are active communication events in
real-time whereas cues are information embedded in the
stigmergy
structure to be read and reread many times. Both are specialized messages in the sense
that they mean different things to
different specialized receiving cells (or insects or computers). However
cues are further specialized
by their location -- that is, in addition to the information intrinsic
to their molecular form or digital content, there is also information
inherent in their location in the stigmergy structure. Because cues
have both a message content and a location, cues support
more complex kinds of communication than do signals and hence tend to
support more complex social organizations. For example, complex ant
societies rely more on cues whereas simple ant societies rely more on
signals[2].
Analogously, computing systems in complex human organization such as businesses rely on records (cues) deposited in databases (stigmergy structures), whereas loose organization, e.g., file-sharing, can work with real-time peer-to-peer messaging. Here again, multicellular computing recapitulates biology. Stigmergy is ever present in complex computing systems and many novel stigmergy structures are emerging in the Internet.
Finally, it is worth noting that stigmergy is intimately related to the notion of "self." A multicellular organism's "self" is more about the body than about the cells of which it is made. The body includes both the cells of the organisms and the nonliving extracellular matrix. However the extracellular matrix persists whereas most kinds of cells die and are replaced many times over during the lifespan of the body. Similarly an ant colony or bee hive outlives all the insects in it except for the queen, and the Internet, whose primary stigmergy structure is its root DNS servers, outlives all of its constituent parts. So, for example, protecting a computing system is more about protecting its stigmergy structure(s) than about protecting its individual computers.
[1] See “Self-organization in social insects.” Bonabeau, E., Theraulaz, G., Deneubourg, J.L., Aron, S. & Camazine, S., Trends in Ecology and Evolution, vol 12, pp. 188-193, 1997.
[2] “Individual versus social complexity, with particular reference to ant colonies,” Anderson, C & McShea, D. W. Biol. Rev., vol 76, pp. 211-237, 2001. p. 228
[3] Extracellular matrix controls myosin light chain phosphorylation and cell contractility through modulation of cell shape and cytoskeletal prestress.” Polte, TR, Eichler, GS, Wang, N, & Ingber, DE. Am J Physiol Cell Physiol 286: C518-C528, 2004.