Sexual dimorphism in the compound eye of the moth Operophtera brumata (Lepidoptera, Geometridae)

Abstract

Abstract. Brachyptery in female moths is a common phenomenon. This article examines the eyes of the moth Operophtera brumata, in which only the males have wings. Both sexes possess eyes of the superposition type, have facets with corneal nipples, and exhibit eyeglow, but the eye of the male has 2174 facets, measures 725 μm in diameter, and has a clear zone of 112‐μm width. That of the female has 1352 facets, measures 557 μm across, and has a 77‐μm‐wide clear zone. There were no significant differences in rhabdom diameters (10 μm) and tracheal sheath development, but male rhabdoms were significantly longer (85 vs. 65 μm). Dark/light adaptational changes manifested themselves in the positions of the distal screening pigment granules: between the cones during darkness and into the clear zone during light adaptation. The combination of a larger radius of curvature, wider clear zone, and longer rhabdoms gives the males a sensitivity at least twice that of the female. The narrower interommatidial angles of the male (2.8° vs. 3.2°) together with the well‐developed tracheal rhabdom sheaths, furthermore, provide the eyes of the males with better resolution. Finally, eye‐glow intensity in males diminishes considerably faster in light than it does in females. The differences between males and females appear to reflect their different roles: males begin to search after sunset in and around stands of trees for the pheromone‐emitting females, which sit on the trunk of the tree under which they had emerged from their pupal case. Flying males encounter rapid light intensity changes and have to evade obstacles and enemies. The females are considerably more sedentary, but once mated, still seek suitable oviposition sites long after the males have ceased searching for them. During this activity, the females also need visual sensitivity and acuity, but less than the actively flying males.