DOI: 10.5061/dryad.gqnk98sm7
title: Wing interference patterns of Chrysomya blowflies (Diptera: Calliphoridae)

description: 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.
description: 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
 <i>Chrysomya</i> 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
 <i>Calliphora</i>. 
description: CIELab is a perceptually uniform model of human vision, whereby
 ‘L’ represents lightness, ‘a’ represents values on a green-red axis, and
 ‘b’ represents values on a blue-yellow axis. We measured the average L, a,
 and b pixel values (hereafter referred to as human ‘colour’) and standard
 deviation in L, a, and b pixel values (hereafter referred to as human
 ‘colour contrast’). For the blowfly visual model, we
 were unable to measure UV reflectance due to the limitations of our
 digital microscope camera. Importantly however, UV and visible scattering
 are correlated in simple thin films, so even though we cannot measure UV
 directly, we are still capturing most of the variation in WIP appearance.
 As such, we created a simple receptor-based model of blowfly colour
 vision, based on the long-wavelength sensitivities of
 <i>Calliphora</i> as there are no published receptor
 sensitivities for <i>Chrysomya</i> species. We assumed
 involvement of the R8p (Rh5 opsin) and R8y (Rh6 opsin) receptors, which
 partly mediate colour vision as well as the R1-6 receptors (Rh1 opsin)
 which are sensitive around both 360nm and 490nm and contribute to both
 colour and luminance vision in flies. We estimated the mean quantum catch
 of Rh5, Rh6 and Rh1 (hereafter blowfly ‘colour’) as well as their standard
 deviation (hereafter blowfly ‘colour contrast’) across each of five
 individual wing cells, as well as the entire wing. This blowfly model was
 formatted as per the <i>Drosophila </i>cone catch data
 provided by MICA Toolbox. Missing values represent wing
 cells which were not measured due to one or two damaged cells. Severely
 damaged wings (damage to several wing cells) were not included in the
 dataset.