Termite castes are defined by their specific characteristics. It is said that juvenile hormone (JH) plays an important role in caste differentiation and JH has been shown to highly influence imaginal-organ development, as well as soldier-specific morphogenesis in some species. To provide additional comparative data for interpreting the effects of JH on the development of alate and soldier characteristics, we performed morphological investigations on newly molted individuals from different nymphal instars of Reticulitermes speratus induced by various concentrations of JH III. As a result, each organ (especially wing (-bud) and mandible) development was shown to be strongly affected by JH III in each nymphal instar. Moreover, depending on the JH III concentration, there was a trade-off-like relationship between wing formation (alate character) and mandibular elongation (soldier character) only in molted individuals from final instar nymphs. Our results suggest that the endogenous JH titer or the threshold titer for caste-specific morphogenesis changes via nymphal development in R. speratus. This study shows that highly developmental plasticity is retained in the derived species of termites, and that it must be a driving force of caste evolution.