Escape flight in butterflies
Abstract: Flight is considered to be the overarching reason for the enormous diversity and world-wide abundance of insects. Not only does flight enable great distances to be covered and new areas to be colonised, flying has also evolved to be important in most adult life-history characteristics from reproduction to anti-predator strategies. However, despite its advantages, the costs of flight are high, particularly with regard to building a flight apparatus and staying in the air. Winged insects are popular prey for various predators from which they rely on flight to escape. However, because of their nutrient poor adult diet, butterflies are especially sensitive to the trade-off between flight and reproduction. Theory therefore predicts costs of physiological changes such as weight gain to be visible through altered aerial performance. Whereas insect flight has been extensively studied with regard to biomechanics, aerodynamics, dispersal and force production, little effort has been made to empirically study the relationship between escape strategies and weight loading, despite its value for survival and fitness. In this thesis a novel three-dimensional flight-recording set-up was used to study free flight ability in relation to natural weight loads in male and female Aglais urticae and Pieris napi butterflies. Weight loads consisted of ingested food, mate-carrying and reproductive mass, affecting wing loading and flight muscle ratio, key determinants of flight ability. Moreover, butterfly escape strategies were investigated through the use of model predators. The results showed that perceived predation risk affected butterfly flight behaviour, with greater speed being observed in attacked butterflies. Decreased flight muscle ratio after feeding resulted in slower escape flights in A. urticae, and impaired flight during mate-carrying in P. napi. Increased wing loading during reproduction in P. napi negatively affected male flight speed and female take-off angles. In summary, this thesis demonstrates that flight effort is context dependant and shows a trade-off between flight ability and longevity- and fitness related weight gain that may ultimately affect survival, mating success and energy expenditure.
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