PUPAL COLOUR PLASTICITY AS A STRATEGY AGAINST DESICCATION

Abstract

O_LITerrestrial insects are vulnerable to desiccation due to their small body size. Because insects lose most water through cuticular evaporation, cuticular traits strongly influence desiccation tolerance. Individuals with greater cuticular melanisation, i.e., darker ones, are hypothesised to tolerate desiccation better than less melanised ones. C_LIO_LIIn many butterflies, pupal colour is plastic - individuals pupating on leaves tend to be greener, while those that pupate away from leaves (off-leaf), such as on tree bark or defoliated twigs, tend to be browner. Brown pupae are hypothesised to have more cuticular melanin and are expected to experience higher desiccation stress than leaf-borne green pupae. Thus, plasticity in pupal melanisation may be an adaptation against desiccation. We tested this in the butterfly Eurema blanda. C_LIO_LIWe demonstrate that individuals pupating on on-leaf substrates are greener than those pupating on off-leaf substrates, and that desiccation stress is higher in the off-leaf substrates, a microenvironment typical of brown pupae, than in typical green pupae. Using Raman spectroscopy, we show that brown, but not green, pupal cuticles contain melanin. C_LIO_LIFollowing this, we obtained greener and browner pupae by manipulating substrate colour. When subjected to desiccation stress, browner pupae survived better than greener ones. There was no correlation between pupal colour and survival in the absence of desiccation stress. Thus, melanisation appears to confer a survival advantage to pupae by increasing desiccation tolerance. C_LIO_LISurvival under desiccating conditions was inversely related to water loss. Interestingly, melanisation did not correlate with water loss, suggesting that melanisation helps tolerate desiccation through physiological mechanisms not directly related to water loss reduction. C_LIO_LIOur findings reveal an additional, crucial, adaptive value of pupal colour plasticity, a trait that has been studied primarily from an anti-predatory perspective. C_LI