Fig. 3

Measures of mating angle and tilting index in two different mating positions. a Tilting index and mating angle measurement protocols of this study (red lines) and Lang & Orgogozo 2012 (blue lines). The resulting angle α is the same in the two methods, as the blue lines are perpendicular to the red lines. Tilting index (see Materials and Methods section) approximates 1 when the male is positioned exactly on top of the female, as the measured distance between the bases of the wings of the male (denoted by d(M,t)) is similar to the measured distance between the bases of the wings of the female (denoted by d(F,t)), corrected for the difference in size between male and female. b Tilting results in a negative mating angle and in a decreased (<1) tilting index, as the measured distance between the bases of wings of the male d(M,t) is shorter than the measured distance between the bases of wings of the female d(F,t)