Social organisation and the status of workers in termites

Abstract

Whereas the soldier caste is clearly ancestral to all extant termite lineages and constitutes the hallmark of termite eusociality, the origin and evolution of workers is much more difficult to comprehend. Here, we first review the status of working individuals throughout the Isoptera, insisting on the need for a consistent terminology. Pseudergates sensu stricto are defined ontogenetically as individuals stepping aside the straight egg-to-alate pathway, through regressive or stationary moults. They reveal an ancestral developmental flexibility, since they are common in small-colony wood-dwelling termites, but may also occur in large-colony Rhinotermitidae or Termitidae. They may participate in social tasks but never constitute a functional category by themselves. Pseudergates sensu lato, or false workers, are defined as a functional category of individuals performing work, while remaining developmentally flexible. This latter property distinguishes them from true workers, which result from an early and irreversible developmental bifurcation, and have lost the possibility to resume alate development. We emphasize three major kinds of social organisation, with respect to the worker caste. In the drywood-dwelling Cryptotermes (Kalotermitidae), all immatures (pseudergates sensu lato) participate in colony work in a cooperative rather than altruistic manner, with very little if any direct fitness loss. In Prorhinotermes (Rhinotermitidae) and Glossotermes (Serritermitidae), wing bud development is postponed until the last pre-alate nymph instar and the preceding instars form a large functional caste of pseudergates (sensu lato), performing partially altruistic colony work. In the Termitidae, tasks are done by a specialized, altruistic true worker caste. After reviewing the proximate developmental and regulatory mechanisms determining the fate of termite immatures, we discuss the ultimate causes of worker evolution. The potential role of kin-selected benefits of helping versus direct benefits of inheritance is examined in detail. Both approaches still face some difficulties and it is probably a combination of both that explains the early social evolution of termites. We stress the need for new theoretical models as well as broader comparative data on life history and social behaviour. © Springer Science+Business Media B.V. 2011.