12.2. Eusociality in insects
Eusocial insects have a division of labor in their colonies, involving a caste system comprising a restricted reproductive group of one or several queens, aided by workers — non-reproductive individuals that assist the reproducers — and in termites and many ants, an additional defensive soldier group. There may be further division into subcastes that perform specific tasks. At their most specialized, members of some castes, such as queens and soldiers, may lack the ability to feed themselves. The tasks of workers therefore include bringing food to these individuals as well as to the brood — the developing offspring.
The primary differentiation is female from male. In eusocial Hymenoptera, which have a haplodiploid genetic system, queens control the sex of their offspring. Releasing stored sperm fertilizes haploid eggs, which develop into diploid female offspring, whereas unfertilized eggs produce male offspring. At most times of the year, reproductive females (queens, or gynes) are rare compared with sterile female workers. Males do not form castes and may be infrequent and short-lived, dying soon after mating. In termites (Isoptera), males and females may be equally represented, with both sexes contributing to the worker caste. A single male termite, the king, may permanently attend the gyne.
Members of different castes, if derived from a single pair of parents, are close genetically and may be morphologically similar, or, as a result of environmental influence, may be morphologically very different, in an environmental polymorphism termed polyphenism. Individuals within a caste (or subcaste) often differ behaviorally, in what is termed polyethism, either by an individual performing different tasks at different times in its life (age polyethism), or by individuals within a caste specializing on certain tasks during their lives. The intricacies of social insect caste systems can be considered in terms of the increasing complexity demonstrated in the Hymenoptera, but concluding with the remarkable systems of the termites (Isoptera). The characteristics of these two orders, which contain the majority of the eusocial species, are given in Boxes 12.2 and 12.3.