MERRY, J.W.*; RUTOWSKI, R.L.; Arizona State University; Arizona State University: Butterfly Eye Structure: Measurement Error vs. Differences Among Individuals, Sexes, and Species

Interspecific variation in eye structure in insects has traditionally been explained as an adaptive product of differences in habitat and behavior between species. However, prerequisites for adaptive evolution (interindividual variation, trait heritability, and fitness consequences) have rarely been investigated (Chittka and Briscoe 2001). Moreover, the precision of techniques for studying eye characteristics that determine visual system performance have not been evaluated. In this study we evaluate the contributions of natural variation in eye structure and measurement error to variation in acuity of two species of butterfly that share similar light environments but differ in size: the Orange Sulphur, Colias eurytheme, and the Marine Blue, Leptotes marina. We inferred visual acuity from measurements of interommatidial angles and facet diameters from five regions of the eye. A nested ANOVA with body size included as a covariate was used to separate variation among individuals from measurement error. We found that a) both species featured significant regional variation in visual acuity, with the highest acuity occurring frontally; b) C. eurytheme, but not L. marina, was highly sexually dimorphic in visual acuity–males had more acute vision than females; and c) variation among individuals accounted for only 5-10% of residual variation in the models; measurement error accounted for the remainder. This suggests that our methods could be prone to error. However, both morphological measures gave similar estimates of interindividual variation, which indicates that interindividual variation in visual acuity is small within sexes in these two species. Therefore, opportunities for selection to act on these aspects of visual system structure may be very limited.