An important question in developmental biology is how organ proportion is regulated by the interplay of genetic and environmental stimuli. Termites display striking phenotypic plasticity, as manifested in, for example, the enlarged mandibles of the soldier caste. Juvenile hormone is one of the essential endocrine signals that regulate soldier differentiation. However, how this signal feeds into the genetic networks controlling mandible proportions has remained elusive. In a new study, Toru Miura and co-workers discover the factors important for mandible development in the termite Hodotermopsis sjostedti. To find genes preferentially expressed in mandibles upon induction of soldier differentiation, they perform a candidate RT-qPCR screen and identify dachshund (dac) as a likely regulator of mandible development. They show that Dac localises primarily to the apical part of the mandible. Furthermore, knockdown of dac reduces mandible size in solders and leads to a stronger curvature of the outer edge of the mandibles. Depletion of juvenile hormone receptor, insulin pathway components or particular Hox genes all lead to reduced dac expression, shedding light on the networks regulating Dac activity. Together, these findings show that dac is a key factor in the control of mandible size in termites and provide insights into how organ proportion is developmentally and hormonally regulated.