SWALLOW, J.G.*; RIBAK, G.; WILKINSON, G.S.; Univ. of S. Dakota; Univ. of S. Dakota; Univ. of Maryland: Comparative Analysis of Wing Beat Frequency and Morphology in Stalk-Eyed Flies

Sexual selection often results in exaggeration of morphological and behavioral characters when individuals of one sex experience a mating advantage through female choice or male competition. Stalk-eyed flies have eyes placed laterally from the head on elongated peduncles. Sexual dimorphism in eyestalk length has arisen several times in the family, leading to dramatic variation between species and sexes. We hypothesized that elongation of eye stalks could create selectively important variation in flight performance limiting further elaboration as recent studies suggest. To identify links between eye stalk elongation and flight performance we undertook a comparative analysis of wing beat frequency and wing morphology in14 taxa that varied in average eye span (range 2.0 � 14.2 mm) and degree of sexual dimorphism (male � female eye span; range = -0.44 � 7.84 mm). Wing beat frequencies were measured using OFIDIS (Qubit Systems). Morphological variables measured were eye span, body length, thorax width, wing length and wing area. After normalizing all morphological dimensions for the size differences between species we found that an index for eye stalk elongation (eye span/body length) was not correlated with wing area, wing length or wing loading (body mass/wing area). However, species and sexes with longer eye stalks had significantly increased wing aspect ratio and reduced wing beat frequency. Thus, changes in wing shape but not wing size appear to be associated with eye span elongation. The observed reduction in wing beat frequency may reflect altered aerodynamic properties of the higher aspect ratio wings. These observations support the idea that eye span elongation affects flight performance, creating a tradeoff between sexual and natural selection. Supported by NSF IOB-0448060.