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Termite nest architecture evolved along with changes in lifestyle, the basic building behaviour being transmitted through the genes of reproductives. The main selective pressure is thought to have been defence against predators. In more advanced termites the maintenance of homeostasis in temperature and humidity may appear to dominate nest architecture. Nest defence involves nest structures in combination with the morphology and behaviour of the sterile castes. The soldier caste, characteristic of termites, is specialized for defence, with a variety of mechanical and chemical weapons, but this caste was lost in some advanced genera, especially the Apicotermitinae. Workers are always involved in defence (by aggression, emergency building, evacuation of the brood). Morphology and behaviour of the sterile castes, (including nest architecture) evolved in a co-ordinated manner through selection at the colony level.

Summary. The hemimetabolous termites have a very different caste system from social Hymenoptera in which only true imagoes participate in all social tasks. In termites, the imagoes are restricted to reproduction. The termite solidiers have no equivalent in other social insects. They are unique both in their post-embryonic development and their exclusively defensive function. At maturity they keep their molting gland and therefore should be considered as stabilized, differentiated immatures. They appeared monophyletically, early in the evolution of termites. The definition of the worker caste and the current theories explaining its evolution are reexamined after a critical reappraisal of data on post-embryonic development. On the basis of ontogenic, morphologic and functional criteria, we define termite workers as individuals diverging early and irreversibly from the imaginal development, with a morphology typical of their caste but largely of larval appearance, and taking part in most of the social tasks. Such a truly eusocial worker caste is observed in all advanced phyla: Termitidae, Hodotermitidae, and Schedorhinotermes (Rhinotermitidae), but apparently also in one morphologically primitive termite, Mastotermes darwiniensis. This distribution can only be explained by postulating a polyphyletic origin of the worker caste in termites. In all other primitive termites, most of the work is done by temporary helpers (late larvae and nymphs). In these societies, pseudergates are redefined as individuals separating late from the imaginal line, functioning as workers, but not constituting the main working force in the natural society. The particularity of the caste system in termites results from two fundamental peculiarities of their post-embryonic development:-A certain dissociation of molt, growth, and differentiation.-A relative independence in the ontogeny of their different organs.

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