with specialization for reproduction, nutrition, communication, defense, and often thermoregulation. Seemingly autonomous individuals are actually workers whose function appears directed entirely to the whole, such as a worker who fans the colony to cool it or one who lives her life as a living honey storage pot (Seeley, 1989). The earliest analogies with multicellular organisms focused on these physiological processes and led to Wilson’s physiologically oriented definition of a superorganism (Wheeler, 1911; Wilson, 1971; Hölldobler and Wilson, 1990; Bonner, 2006). But the analogy could not be pushed too far, perhaps because of fundamental differences between the physiology of a divided organism (with separately mobile individuals) and a multicellular organism. The mobility of individuals means information and resources can be walked throughout the colony with no need for specialized structures.
Mobility may therefore underlie the relatively small number of castes in social insects. Castes are in some ways analogous to cell types in multicellular organisms. Each caste or cell type specializes in certain tasks, with the division of labor aiding the whole. All social insects have functional reproductive and worker roles, but only some are morphologically differentiated into queen and worker castes. A fraction of these species have multiple worker castes, with the primary distinction being between small foragers and large soldiers (Wilson, 1971). Even highly specialized functions, such as being a honey storage vessel in honeypot ants or using one’s head to block the colony entrance in Colobopsis ants, are usually performed by castes that also have more general functions.
Fig. 8.1 shows the complexity, measured as the number of types of subunits, of social insect colonies, compared with multicellular individuals. In one sense, of course, social insect colonies are more complex than multicellular individuals because the colonies include all of the complexity of their constituent individuals and then add more complexity at the colony level. But it is still interesting to compare the degree of complexity added by the specialization of parts in the two cases. Following Bonner (2006), as a measure of the complexity of specialization, we use cell-type number and caste number to represent the complexity of individuals and colonies, respectively. We are unable to use phylogenetically independent contrasts, but Fig. 8.1 well illustrates how depauperate in specialized castes social insects are compared with cell specialization in organisms, a pattern that is unlikely to disappear when analyses are performed with accurate phylogenies. Complexity increases with the number of units, the units being cells for organisms and individuals for colonies (Fig. 8.1). The lower complexity of colonies can be explained partly by size. On average, social insect colonies do not have as many units as multicellular animals; colonies rarely have more than a million individuals, whereas large organisms have billions of cells. But that is not the complete explanation. The