Effects of natural wing damage on flight performance in Morpho butterflies: what can it tell us about wing shape evolution?

Flying insects frequently experience wing damage during their life. Such irreversible alterations of wing shape affect flight performance and ultimately fitness. Insects have been shown to compensate for wing damage through various behavioural adjustments, but the importance of damage location over the wings has scarcely been studied. Using natural variation in wing damage, we tested how the loss of different wing parts affects flight performance. We quantified flight performance in two species of large butterflies, Morpho helenor and Morpho achilles, caught in the wild and displaying large variation in the extent and location of wing damage. We artificially generated more severe wing damage in our sample to contrast natural versus higher magnitude wing loss. Wing shape alteration across our sample was quantified using geometric morphometrics to test the effect of different damage distributions on flight performance. Our results show that impaired flight performance clearly depends on damage location over the wings, pointing to a relative importance of different wing parts for flight. A deteriorated forewing leading edge most critically affected flight performance, specifically decreasing flight speed and the proportion of gliding flight. In contrast, the most frequent natural damage, deteriorated wing margin, had no detectable effect on flight behaviour. Damage located on the hindwings - although having a limited effect on flight - was associated with reduced flight height, suggesting that the forewings and hindwings play different roles in butterfly flight. By contrasting harmless and deleterious consequences of various types of wing damage, our study highlights different selective regimes acting on morphological variations of butterfly wings.