Conditional expression of a sexually selected trait in the stalk‐eyed fly Diasemopsis aethiopica

Summary 1. An important assumption of many models of the evolution of sexually selected ornaments is that such ornaments should be conditionally expressed. This was investigated by rearing larvae of the stalk‐eyed fly Diasemopsis aethiopica (Diptera: Diopsidae) (Rondani) on two food treatments: normal and low‐quality food. 2. Female flies responded to better food by increasing both their body size and the relative length of their eyestalks. 3. Male flies responded by increasing only the length of their eyestalks. 4. This demonstrates that food quality can change the allometric relationship between body size and the size of a sexually selected character in this species, and gives an insight into the different developmental priorities of each sex in this species.