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FIGURE 8.1 The number of specialized types (cell types or worker castes) as a function of the number of units (cells in an organism or number or individuals in a colony). Cell data (open circles) are from Bell and Mooers (1997). Caste data (crosses) are for ants and were compiled by Bonner (1993) from data in Hölldobbler and Wilson (1990) and from judgments of caste number by E. O. Wilson (Bonner, 1993). Mean colony sizes were used when available; when they were not available, we used the midpoint of the range.

specialization complexity of insect societies is lower for a given number of units than the complexity of multicellular organisms. This conclusion seems fairly clear despite the difficulties inherent in defining the number of castes or cell types, for which we have relied on the judgments of others (see Fig. 8.1). Social insects almost never have even as many as five castes, whereas many small multicellular organisms attain 10, or many more, cell types. We suggest that the mobility of the separate parts of a social insect colony reduces the need for specialized types at the level of the colony.

Despite the limitations of the physiological superorganism model, the superorganism view can be useful, not only for understanding divided organisms but also for stimulating new ways to view traditional organisms. Colonies may not have the same kinds of systems as animals and plants for fulfilling colony functions, just as in typical organisms the parts of a colony are subordinate to the whole. As Darwin himself noted, “… if on the whole the power of stinging be useful to the social community, it will fulfill all of the requirements of natural selection, although it may cause the death of some few members” (Darwin, 1872, p. 163). A colony can be viewed as an organism simply because it is highly adapted at the colony level. Of course, this logic does not apply to just any social community, so the social insects force the question of how some entities become organismal while others do not.

Although social insect colonies may not have physiologies that closely match those of multicellular organisms, they do have their own systems for



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