Morph-specific and sex-specific temperature effects on morphology in the colour polymorphic damselfly Ischnura elegans
Colour polymorphic species with extensive ranges often exhibit large-scale geographic patterns of morph frequency variation. Because colour polymorphism is associated with correlated differences in multiple traits, such as thermal performance, a likely proximate explanation for such patterns is morph-specific responses to temperature variation. The colour polymorphic Blue-tailed damselfly Ischnura elegans exhibits large-scale geographic variation in morph frequencies, but the possibility that temperature is a proximate explanation for the latitudinal cline in morph frequencies has only ever been tested within a single developmental stage (egg survival and hatching time), where no difference between the morphs was found. I therefore carried out a temperature manipulation on larvae of I. elegans which I raised to maturity in the laboratory. I found that individuals exhibited incomplete compensatory growth after being exposed to cold temperatures, and that individuals which did not emerge successfully and those that experienced cold temperatures had more juvenile morphology in the last instar. In addition, there were sex-specific and morph-specific effects of temperature on adult morphology, such that sexual size dimorphism was increased when individuals experienced warm temperatures throughout the larval stage, and that cold temperatures tended to result in larger size of androchromes and their offspring compared to the other morphs. These results are generally consistent with the large-scale geographic variation in morph frequencies found in this species.