Morphological compensation and flight performance in stalk-eyed flies

Several species of stalk-eyed flies exhibit exaggerated sexual dimorphism where females favor males with longer eyespans. However, longer eyespan increases a fly's moment of inertia, and may impact maneuverability and predator evasion. Flies with longer eyespans tend to have larger thoraces and wings, suggesting co-selection for compensatory traits, potentially ameliorating the costs of bearing long eye-stalks. The purpose of this study was to investigate the flight performance of stalk-eyed flies and its relationship to body morphology and development. Sixteen groups each of 1-day old Teleopsis dalmanni and Diasemopsis meigenii were collected and housed in separate 3L chambers so that age was known at the time of the flight performance and morphometric analysis. The flight performance of male flies was assessed by presenting normoxic, variable-density mixtures of heliox (O₂, N₂ and He) in 10% increments ranging from air to pure heliox; the least-dense gas allowing flight represented maximal performance. Flies were then euthanized, photographed, dissected and weighed. D.meigenii (n=84) were larger and had greater flight capacity than T.dalmanni (n=123). Total body mass, thorax and abdominal mass increased across age in both species. Maximal flight performance was associated with thorax mass, and tended to increase with age as flies became heavier, as the thorax-to-body mass ratio increased with age. Although flies with longer eyespans were larger, maximal flight was independent of eyespan, presumably because younger flies had not yet accumulated sufficient flight muscle. Thus, flight performance appears associated with morphological compensation across age.