Wing interference patterns of Chrysomya blowflies (Diptera: Calliphoridae)

Wing interference patterns (WIPs) are stable structural colours displayed on insect wings which are only visible at specific viewing geometries and against certain backgrounds. These patterns are widespread among flies and wasps, and growing evidence suggests that they may function as species- and sex-specific mating cues in a range of taxa. As such, it is expected that WIPs should differ between species and show clear sexual dimorphisms. However, the true extent to which WIPs vary between species, sexes, and individuals is currently unclear, as previous studies have only taken a qualitative approach, without considering how WIPs might be perceived by the insect. Using multispectral digital imaging and a tentative model of blowfly colour vision, we provide the first quantitative dataset of inter- and intra-specific variation in WIPs across seven Australian species of the blowfly genus Chrysomya. These data suggest that WIPs have diversified substantially in blowflies as a result of either sexual or ecological selection.

Methods
Wings were mounted with transparent UHU glue onto a custom rotating stage and positioned at a 45° angle to maximise WIP visibility. Photos were taken of both the left and right wing of each fly with a MZ16A stereomicroscope mounted with a Leica DFC295 digital microscope colour camera. All photos were taken at the same magnification, under standardised and uniformly diffuse lighting provided by a Leica LED5000 HDI illuminator.

Images were processed using the Multispectral Image Analysis and Calibration Toolbox for ImageJ (MICA toolbox). This produces linearized, calibrated images which allow for the measurement of relative reflectances. We calibrated our images against a 3% reflectance standard from an X-rite colour checker passport, which was placed 5 mm below the wing in the background of each photo. This resulted in a total of 231 multispectral images (visible spectrum only) of left and right wings across the seven Chrysomya species.   

From these multispectral images, we made measurements of the average values of red, green, and blue (RGB) channels (hereafter referred to as mean ‘colour’) and the standard deviation in RGB (hereafter referred to as ‘colour contrast’) across five individual wing cells (a1, b1, c1, d1, and e1) as well as a measurement of the entire wing (wing1). In addition to this viewer-independent analysis, we used a cone-mapping approach to convert the multispectral images into two viewer-subjective formats; the CIELab model of human colour sensation, and a receptor-based model of ‘blowfly vision’ based on the visual phenotype of Calliphora.