The societies of Isoptera and Hymenoptera differ in two general features. 1. The termite societies are bisexual. From a primitive symmetry between both sexes, many asymmetries appeared during the course of their evolution. These asymmetries are related either to a sexual dimorphism, or a biased sex ratio, or both, and are differently expressed in the separate castes of a given species. 2. The hemimetabolous development allows a termite to take part in the social tasks before the end of its postembryonic development, and even to reproduce at a larval stage (neoteny). Thus Isoptera exhibit a polymorphism of larvae, unlike Hymenoptera where a polymorphism of imagoes is observed. Moreover, an individual can change its physical caste: this temporal polymorphism never exists in Hymenoptera. These facts have important consequences for the caste system and its evolution. The differences between the social structures of Isoptera and Hymenoptera are related to the differences between their non-social ancestors.