Division of labor is fundamental to the ecological success of social insects. In termites, both sexes engage in social tasks, and the sexual division of labor is common in many taxa. Each caste consisting of both sexes is supplied from the population of newly hatched larvae. To understand their social system, it is necessary to investigate the influence of larval sex composition on their developmental fate. However, no method currently exists for sexing young larvae non-invasively, which is essential for experimental manipulation of the sex composition in a society. Here, we report on sex-specific characteristics of the first and second instar larvae of the subterranean termite Reticulitermes speratus. Male larvae possess bristles near the center of the posterior margin of the eighth abdominal sternite that are absent in females. The bristles are detectable under a stereomicroscope without damaging the young instar larvae. The validity of morphological sexing was confirmed by a known sex-specific genetic marker. The sex-specific bristles were also useful to identify the sex of first and second instar larvae of the damp-wood termite Zootermopsis nevadensis, suggesting that morphological sexing is possible for a wide range of termite species. The morphological sexing presented here has broad applicability in studies addressing sex differences in development, caste differentiation, and behavior. These approaches will contribute to understanding why both sexes co-exist in a termite society and allocate tasks, and which tasks are completely compensable by the other sex, thereby deepening our understanding of social systems where both sexes engage in tasks.