The Effects of Diet Quality on Developmental Plasticity of Size and Flight Energetics in the Hawk Moth Manduca sexta

Abstract

Energy metabolism is one of the most widely measured structural backbones of physiological and life history traits, where variation in body size and morphology significantly impact flying insects' flight energetics. Both size and energy metabolism are a central feature of a species and individuals' ecology and evolution. In holometabolous insects, the larval stage of development is crucial in determining the final size of adults but also various body proportions. Generally, larger individuals will beat its wings during flight at a lower frequency. However, variations in wing proportions can affect wingbeat frequency, therefore affecting the energy that is required for flight. The flight muscles associated with the metabolic properties of flight vary with body mass and wing proportions, connecting morphological variation with muscle metabolic properties. To assess this, we experimentally induced developmental plasticity by subjecting Manduca sexta fifth instar larvae with varying intermittent starvation treatments: (1) controls being fed ad libitum; (2) our 'mild' group subject to intermittent feeding with 24 hour starvation periods between rations where animals are subject to intervals of one full day of feeding following one full day of starvation; (3) our 'severe' group subject to 48-hour starvation treatment where animals are subject to intervals of two full days of starvation followed by one full day of feeding. Our results show a significantly longer developmental period in mild and severe groups, along with significantly smaller sized larvae at wandering, pupae, and adults. Sexual dimorphism was observed in treated males showing morphological variation of body to wing proportions, along with reduced thorax size while females' morphological variation remained unchanged throughout nutrient restriction. Although males showed higher flight wingbeat frequency than females, wingbeat frequency did not change among treatments, along with showing no relationship between wingbeat frequency and body mass. Further, flight metabolic rate was not significantly different between treatment and sex. These findings suggest that individuals under nutrient-restricted developmental conditions can have a lasting impact on morphology, which may contribute to reduced flight capability, where males are more affected. Further research is needed to understand the metabolic and biomechanical thresholds that are responsible for successful flight under nutrient restricted individuals. These future investigations can provide insight into how developmental plasticity impacts flying insects in terms of energy balance and flight performance.