Colour Pattern Plasticity in the Hoverfly, Episyrphus balteatus: The Critical Immature Stage and Reaction Norm on Developmental Temperature.

Episyrphus balteatus is phenotypically plastic with respect to abdominal colour pattern. It was hypothesized that developmental temperature was the environmental cue governing this plasticity. The length of different immature stages was manipulated by altering the rearing temperatures. It was shown that all stages of the pupal developmental period were equally important in influencing adult phenotype. Lengthened pupal period, achieved by reducing rearing temperature, resulted in darker individuals. Adjusting the length of the larval period had no affect on adult colour pattern. Colour pattern was quantified using image analysis. Reaction norms of black pigment on each tergite against pupal developmental temperature were constructed. All reaction norms were approximately linear. Females were darker than males at comparable temperatures. In both sexes tergite 2 was the most plastic, whereas tergite 3 was the least plastic. The results are discussed in connection with the colour plasticity having a possible thermoregulatory function.