Phenotypes of organisms are not determined only by genes, but vary according to environmental factors (phenotypic plasticity). Some organisms express several discrete adaptive phenotypes (polyphenism). Polyphenism can be explained as the modification of postembryonic development to produce alternative phenotypes. Social insects possess a few types of individuals (castes) in their colonies, to which specific tasks are allocated. In a colony, different castes possess different characteristic morphologies, such as the enlarged mandibles of soldiers for defense, or the wings of alates for dispersal. These caste-specific body parts are exaggerated or reduced during postembryonic development by responding to extrinsic cues, such as physical environments and/or social interactions among colony members. In my laboratory, we are studying on the polyphenism in ants, termites and aphids, in terms of the developmental mechanisms of phynotype-specific characters. We are working on these topics, in terms of the alteration of body plan in response to environmental signals, and trying to understand the evolutionary process of the interaction between ontogeny and environment.