Aerial performance of stalk-eyed flies that differ in eye span.

Stalk-eyed flies have eyes placed laterally away from the head on elongated peduncles. The elongation of eye span may increase the energetic cost of flight, reduce flight performance via aerodynamic effects or via increased load, or necessitate compensatory changes in other body dimensions. Body mass and body dimensions were measured to test the hypothesis that elongation of eye span is correlated with increased head mass in two closely related species of stalk-eyed flies. Cyrtodiopsis whitei is sexually dimorphic, with the eye span of larger males exceeding body length. Cyrtodiopsis quinqueguttata is sexually monomorphic with eye span substantially less than body length. Although eye span was significantly longer in C. whitei, head mass did not differ between species after accounting for differences in body mass. C. whitei males had longer wings, heavier thoraxes, and lighter abdomens in relation to body mass than did female C. whitei or C. quinqueguttata of either sex. Three-dimensional tracking of flight paths showed that path velocity and the horizontal component of velocity did not differ according to species or sex, but the long-eyed C. whitei males showed reduced overall aerial performance by flying at shallower ascent angles and reduced vertical velocity. Although increased mass loading does not occur in C. whitei males, increased drag, aerodynamic effects from the wake of the eye stalks, and constrained visual processing are possible mechanisms which could cause their reduced performance.