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

Abstract

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.

Abstract (Swedish)

Popular Abstract in Undetermined

The transparent wings of tiny parasitic wasps and small flies are remarkably thin, only a few hundreds nanometers thick, and may appear to have a simple structural airfoil design. The thesis deals with new findings about structural color reflections and the ultrastructure of such tiny transparent wings of parasitic wasps. The main discovery is that when seemingly transparent insect wings are viewed against a dark background they display vivid and stable structural color patterns due to optical interference. The patterns are coined with a new term as Wing Interference Patterns (WIPs). It is suggested that small wasps may have evolved specific color patterns that are utilized for visual signaling, for instance by displaying their wings to each other during courtship much in the same way as larger insects with colorful wings (e.g. butterflies) do. Such WIPs lack pure red but may contain ultraviolet light and fit very well to the color vision of most small insects. The visible (to humans) colors displayed are characteristic and identical to the colors one can see on the surface of a soap bubble. But unlike the constantly changing color reflections from soap bubbles WIPs displayed on insect wings are optically stabilized by the thickness profile and specific microstructures of the wing membrane. These factors ensure that the wings display the same color and pattern under different angles of view. Since the WIPs are very diverse among small wasps and often seem to be specific for a species they provide new characters useful for species identification and classification of insects, especially among closely related and otherwise very similar species that live together in nature. In several such cases tiny parasitic wasps appear to have evolved distinct patterns only in the male wings, while the females are similar between species. This suggests that the females chose mating partner based on their WIPs and that these color patterns are an important but previously largely overlooked part of insect biology and evolution.

• author: Shevtsova, Ekaterina
• supervisor: Christer Hansson; Jostein Kjaerandsen
• opponent: Johnsen, Sönke, Professor
• organization: Department of Biology
• publishing date: 2012
• type: Thesis
• publication status: published
• subject: Biological Sciences
• keywords: Chalcidoidea, parasitic wasps, structural colors, visual signaling, cryptic species, sexual dimorphism, wing membrane thickness, wing micro-morphology, wing cuticle ultrastructure
• pages: 148 pages
• defense location: Biology Building, Sölvegatan 35, room D205
• defense date: 2012-02-24 10:00:00
• ISBN: 978-91-7473-268-9
• language: English
• LU publication?: yes
• id: 6fcceb5d-e41b-455c-b3d9-52d7ba6562f2 (old id 2301518)
• date added to LUP: 2016-04-04 11:48:49
• date last changed: 2018-11-21 21:07:22