Evidence for adaptive constraints on size of marginal wing spots in the grayling butterfly, Hipparchia semele

Butterfly wing ocelli (spots) are argued to have adaptive functions (anti-predator, mate enhancement). However, little attention has been given to differences between individual spots, sexes and populations. Hipparchia semele is a lateral basker, rarely opening its wings when settled other than during courtship. This provides a potential for uncoupling the functions of ventral and dorsal ocelli. Data from 725 specimens belonging to 12 North Wales populations are used to examine the consistency in wing spots in relationship to wing expanse, sexes and biotopes. Given that predators tend to prefer heavier, larger and slower prey, spot size should increase at a faster rate than wing expanse and should be larger in females. Spot size should also reflect sex differences in behaviour, resource use and vulnerability to predators, in addition to sexual selection. Given that females expose the dorsal wings during courtship, the forewing dorsal wing spots should be larger than the forewing ventral wing spots, and the forewing anal spot should act as a more prominent signal than the apical spot compared with males. These observations are supported by this study. The results are discussed in relationship to strategies (crypsis and active defence) to combat predation and for sexual selection.