Seeing the invisible: Evolution of wing interference patterns in Hymenoptera, and their application in taxonomy

Sammanfattning: The remarkably thin transparent wing membranes in tiny wasps may appear to have a simple structural design, but hide a largely unexplored complex of micro-morphological features that serve aerodynamics and may also function in visual signaling. I found that when such small transparent wings are viewed against a dark background they display vivid structural color patterns due to thin film interference, and named them Wing Interference Patterns (WIPs). Areas of different thickness across the wing membrane reflect specific interference colors and all together produce a specific color pattern, offering a new way to map the wing micro-morphology through direct observations. The color sequence is very characteristic and lacks pure red but may contain UV light. Hence, it fits the UV-blue-green trichromatic color vision of most small insects, strongly suggesting that the biological significance of WIPs lies in visual signaling. WIPs are optically stabilized by corrugations in the wing membrane and are essentially noniridescent over a large range of light incidences. These patterns show a high diversity in small Hymenoptera and are often species-specific, which makes this new morphological character useful in taxonomy. Several sympatric species of parasitic wasps were found to display sexually dimorphic WIPs, suggesting sexual selection as one of the driving forces for their evolution. The significance of wing membrane micro-morphology and the origin of the specific color sequence observed in WIPs are discussed, using Achrysocharoides and Omphale as model taxa. Several new findings are reported in addition to those in my five publications. A comprehensive study of the wing cuticle ultra-structure, based on analyses of wing membrane cross-sections by transmission electron microscopy, revealed asymmetrical organization of the dorsal and ventral cuticles. Presence of ultraviolet light reflections in WIPs is indirectly demonstrated through fluorescence microscopy, further strengthening the signaling function of WIPs.