DOI: 10.5061/dryad.qnk98sfnx
title: A meta-analysis of butterfly structural colors: their color range, distribution, and biological production

description: Butterfly scales are among the richest natural sources of optical
 nanostructures, which produce structural color and iridescence. Several
 recurring nanostructure types have been described, such as ridge
 multilayers, gyroids, and lower lamina thin films. While the optical
 mechanisms of these nanostructure classes are known, their phylogenetic
 distributions and functional ranges have not been described in detail. In
 this Review, we examine a century of research on the biological production
 of structural colors, including their evolution, development, and genetic
 regulation. We also create a database of more than 300 optical
 nanostructures in butterflies and conduct a meta-analysis of the color
 range, abundance, and phylogenetic distribution of each nanostructure
 class. Butterfly structural colors are ubiquitous in short wavelengths but
 extremely rare in long wavelengths, especially red. In particular, blue
 wavelengths (around 450 nm) occur in more clades and are produced by more
 kinds of nanostructures than other hues. Nanostructure categories differ
 in prevalence, phylogenetic distribution, color range, and brightness. For
 example, lamina thin films are the least bright; perforated lumen
 multilayers occur most often but are almost entirely restricted to the
 family Lycaenidae; and 3D photonic crystals, including gyroids, have the
 narrowest wavelength range (from about 450 to 550 nm). We discuss the
 implications of these patterns in terms of nanostructure evolution,
 physical constraint, and relationships to pigmentary color. Finally, we
 highlight opportunities for future research, such as analyses of subadult
 and Hesperid structural colors and the identification of genes that
 directly build the nanostructures, with relevance for biomimetic
 engineering.
description: Several intersecting approaches
 were used to search for articles, book chapters, and theses reporting
 nanostructures that produce structural colors in butterflies. First,
 Google Scholar searches were run with combinations of these keywords:
 structural color, butterfly, Lepidoptera, iridescence, scale, and
 Ghiradella. Database searches brought up tens of thousands of hits, many
 of which were not pertinent, so we used high-quality results to find cited and citing references and
 noted every species that was mentioned in connection with structural
 color, iridescence, or derived scale morphology. Finally, a database
 search was run on each species name that had been mentioned in any prior
 included reference. When a search on species name returned many results,
 it was searched again in combination with the keywords. If the search on
 species name returned no relevant results, we tried searches with only the
 genus name and checked for alternate nomenclature. We continued
 snowballing references and running database searches on species until we
 could no longer find any new taxa mentioned in connection with structural
 color. Criteria for inclusion were that the article must include either
 (1) reflectance measurements or (2) electron microscope images of
 non-transparent (i.e. colored) optical nanostructures in a butterfly. We
 also included studies which provided additional characterizations (e.g.
 absorption measurements, mathematical modeling,
 scatterometry) for structures
 that had been included on the basis of (1) or (2).
 This literature review strategy
 yielded 187 included references which described 421 potential structures
 from 378 species, all of which are included in this dataset. Before
 further analysis, we secondarily excluded entries that were presented as
 non-photonic comparisons to structurally colored specimens. Only seven
 reported structures occurred outside adult wing scales, which were all
 multilayer broadband reflectors in the pupal cuticle (Neville, 1977; Steinbrecht, 1985; Steinbrecht et
 al., 1985). We therefore narrowed our focus to structures located in
 scales or bristles on the adult, which can be homologized and directly
 compared in subsequent analyses. After filtering, there was pairwise
 complete data on both color and morphology for 314 optical nanostructures
 from 287 species. Some species had multiple structural colors on different
 body parts (e.g. blue dorsal and green ventral wings in
 <em>Cyanophrys</em> <em>remus</em> and
 <em>Albulina</em> <em>metallica</em>; Biró et al.,
 2007).  To
 compare color between structures, we recorded the peak reflected
 wavelength (i.e. hue) and the percent reflectance at that wavelength (i.e.
 brightness) for each structural color. Due to iridescence, quantification
 of structural color is extremely sensitive to the measurement protocol,
 specifically illumination and detection angles, light source, reference
 sample, and spot size (Meadows et al., 2011). Spectroscopy methodologies
 were variable among the included studies making comparisons imperfect;
 nevertheless, the data are useful to show broad trends. When multiple
 spectra were available, we used the following rules for consistency. When
 reflectance was reported from more than one angle, the peak wavelength at
 the maximally reflective angle was used. If comparable reflectance data
 was reported from more than one study or from replicated specimens, we
 took their average. When reflectance data was found for both an isolated
 scale and the intact wing, both values were noted, but the intact wing
 reflectance was preferentially used in comparative analyses for
 consistency, because single-scale reflectance measures were uncommon. In
 cases where structures produced two reflectance peaks – as in
 <em>Chrysozephyrus</em> species with both a UV and a green
 peak (Imafuku, Hirose and Takeuchi, 2002) – the brighter peak was used in
 graphical summaries, but both were listed in the spreadsheet. Peak
 wavelengths were typically estimated by eye from graphs, which limited
 precision to a 5–10 nm window around the measured peak. This precision
 limit is similar to the magnitude of inter-individual variation (Imafuku,
 Gotoh and Takeuchi, 2002; Bálint et al., 2008). We also recorded percent
 reflectance at the maximally reflective wavelength (i.e. spectral
 intensity or ‘brightness’). When no reflectance spectra were available but
 a color image or a qualitative color descriptor (e.g. ‘blue’, ‘UV’) was
 given, the qualitative descriptor was recorded. Broadband reflectors have
 a similar reflectance intensity across many wavelengths, so the maximally
 reflecting wavelength is not a good summary of the reflector’s properties
 and may not be identifiable from a visual inspection of a graph.
 Therefore, for broadband reflectors, we only recorded a qualitative
 descriptor, such as ‘white’, ‘silver’, or ‘gold’. Additionally, some
 reddish lamina thin films that reflected in both violet and red, without a
 peak wavelength in either region, were handled as qualitatively ‘magenta’.
 Note that many reflectance spectra were likely influenced by co-occurring
 pigments as well as the nanostructures. 
 To compare scale morphological
 modifications, we noted which kind of optical nanostructure was present.
 Generally, we followed the author’s conclusion as to which scale component
 caused the optical properties. If the author’s description was brief but a
 micrograph was provided, we assigned the structure to the same category as
 the well-studied examples that it most resembled. In a few cases when the
 proposed mechanism seemed questionable, we noted the explanation in the
 spreadsheet but dropped that structure from comparative analysis (for
 example, the proposed nanostructure was not present in the provided
 micrograph, or the mechanism was disputed across studies). Filled-in
 windows and crossrib air columns likely involve modifications to both the
 crossribs and microribs, and reflectance in these scales also requires the
 lower lamina; for simplicity, we have summarized them as crossrib bilayer
 structures. Data was processed using R.
description: This README file was generated on 2023-09-22 by Rachel C. Thayer. GENERAL
 INFORMATION 1. Title of dataset: A meta-analysis of butterfly structural
 colors: their color range, distribution, and biological production 2.
 Author for correspondence: Name: Rachel C. Thayer Institution: University
 of California, Davis Email: thayerr@berkeley.edu 3. Data was derived from
 the following sources: * 187 previously published studies from the years
 1940-2023. See the dataset variable entitled 'References' for
 details. 4. Information about funding that supported the data curation:
 NSF-DEB-1601815. SHARING/ACCESS INFORMATION 1. Licenses/restrictions
 placed on the data: CC0 1.0 Universal (CC0 1.0) Public Domain 2. Links to
 other publicly accessible locations of the data: None 3. Was data derived
 from another source? YES A. If yes, list source(s): See dataset variable
 entitled 'References' 4. Recommended citation for this dataset:
 Thayer, R.C. and Patel, N.H. (2023). A meta-analysis of butterfly
 structural colors: their color range, distribution, and biological
 production. Journal of Experimental Biology.
 #########################################################################
 DATA-SPECIFIC INFORMATION FOR: Database\_butterfly\_structural\_colors.txt
 1. Number of variables: 20 2. Number of cases/rows: 420 3. Field
 delimiting character: This is a tab-delimited file 4. Description: This
 database organizes previously published data on optical nanostructures
 that produce structural color in butterflies. It can be used to identify
 structures that are good candidates for further characterization, to
 explore the evolutionary history and realized functional limits of several
 different types of optical structure, and to identify butterfly taxa whose
 optical nanostructures have been understudied to date, among other uses.
 Each row describes one putative optical nanostructure, occuring in a
 butterfly species, as reported by a broad literature survey. 5. Variable
 List: * Species: Species from which the optical nanostructure was
 isolated. A species may occur in more than one row within the dataset if
 more than one optical nanostructure has been characterized from the
 species, such as scales sampled from differently colored wing regions, or
 sexually dimorphic scales from each sex. * Family: Family-level taxonomic
 classification to which the specimen belongs * Subfamily: Subfamily-level
 taxonomic classification to which the specimen belongs. Missing data means
 the specimen was not precisely identified. * Structure\_type: This column
 gives the unifed term used to categorize the structure in figures 2-4 in
 the corresponding publication. "lumen\_PF" = perforated
 multilayer in the lumen; "lumen\_3D" = 3D photonic crystal in
 the lumen; "lamina" = lower lamina thin film reflector;
 "crossrib" includes crossrib bilayers with and without air
 columns and one instance of a crossrib multilayer;
 "lumen\_smoothmulti" = continuous multilayer in the lumen;
 "ridge" = ridge multilayer; for "other" see the
 structure\_notes variable or the accompanying publication for details.
 Scales with more than one optical nanostructure have two values separated
 by a comma in this field, see publication also for further explanations.
 Missing data means that there is insufficient data to confidently assign
 the structure to one of the aforementioned categories. * structure\_notes:
 Additional commentary on the structure type and its optical mechanism. The
 commentary may include justification for the assignment in the
 structure\_type variable, further details on the structure type especially
 if it belongs to the "other", or "crossrib" category,
 or a brief description of putative optical structures turned up by the
 literature review that could not be categorized. This field contains
 free-form text and may include space and punctuation characters. *
 color\_notes: Notes about the color of the structure. May give more detail
 than the variable 'Color\_qualitative'. This field includes
 free-form text and may include space or punctuation characters. *
 Color\_qualitative: Unified terms describing the qualitative color of the
 structure, if a qualitative description is the best color data available.
 Possible values are [pearl, white, blue, black, silver, pearl, gold, grey,
 magenta, orange, pink, green, blue-green, UV, violet, yellow-green,
 bronze]. This field is missing if a quantitative color value is available
 in the variable max\_WL, or if there is no color information available at
 all. This variable is used in Fig. 2 in the corresponding publication. *
 max\_WL: The wavelength of maximal reflection, measured in nanometers,
 i.e. the "hue". See Methods for details on how this was
 determined when multiple spectroscopy measurements were found in the
 literature review. * brightness: The percent reflectance of the maximally
 reflected wavelength given in variable max\_ML, i.e. the brightness. *
 angle\_illum: The angle of illumination that is associated with the
 reflectance spectral data, measured in degrees. 'Normal' = 90
 degrees. Missing data means this information was not reported in the
 original data source. The field may contain a brief description if it
 could not be accurately summarized as a single number, for example
 "approximately normal but with a range of 15 degrees." If
 reported methods only stated that a microspectrophotometer was used, I
 assumed a 90 degree illumination angle. * angle\_obs: The angle of
 observation that is associated with the reflectance spectral data,
 measured in degrees. 'Normal' = 90 degrees. An
 "integrated" value in this field means reflected light was
 collected by an integrating sphere. An empty field means this information
 was not reported in the original data source. If reported methods only
 stated that a microspectrophotometer was used, I assumed a 90 degree
 observation angle. * reflectance\_notes: Reflectance notes summarizes in
 more detail the published reflectance spectra. Where applicable,
 information in this field includes the maximum wavelength and the
 brightness for each available measurement when the structure has been
 measured by multiple studies, from multiple specimens, or with alternate
 methods. For example, if a study by Wilts et al. measured a maximal
 reflectance of 15% at 450 nm, this field would include "Wilts:
 450x15". Where methodology was adequately described by the original
 study, these notes also include some methodology details, particularly
 whether spectra were taken from an intact wing region versus from a single
 isolated scale, and whether an integrating sphere ('int sphere')
 or a microspectrophotometer ('microspec') was used. * evidence:
 A semi-colon delimited list of the types of data available to characterize
 the structure, such as electron micrographs (SEM or TEM), reflectance
 measurements, modeling work to explore or confirm its optical mechanism,
 scatterometry, absorption measurements, light polarization measurements,
 or x-ray scattering (SAXS). Empty field means none of these data types
 were found. * in\_cover\_scale: Reports whether the nanosturcture occurs
 in cover scales as 'Y' = yes or 'N' = no. Empty field
 means the source data did not specify which scale type contained the
 nanostructure. There is one cell with a value "maybe weakly",
 meaning the nanostructure may occur in cover scales in a less developed
 state. * in\_ground\_scale: Reports whether the nanostructure occurs in
 ground scales. Possible values are 'Y' = yes, 'N' =
 no, or a brief statement in cases where the answer involves more nuance.
 Empty field means the source data did not specify whether the structure
 occurred in ground scales. * co-occurring\_pigment: Reports whether there
 is a co-occurring pigment in the scale, and if so, identifies what kind of
 pigment it is--by name, by color, by its wavelength of peak absorption, or
 otherwise. Possible values include but are not limited to [yes, no,
 probably not, maybe]. An empty field means the presence or absence of
 pigment has not been determined. This field can include punctuation and
 space characters. * Variation\_documented: Reports whether there is any
 documentation of variation in either the structure's morphology or
 its color. This could include data from more than one scale, more than one
 individual, or more formal comparisons between populations, sexes, etc.
 The field may simply give 'Y' for yes, or it may briefly give
 details on the sample size. 'N' or empty field mean the sample
 size was n=1, i.e. no documentation of variation. * Specimen\_metadata:
 Reports whether metadata on the butterfly specimens was provided, such as
 GPS locality data, collection region and altitude, collection date and
 time, and habitat description. The format is a list of values separated by
 ';'. An empty field means specimen metadata was not reported. *
 References: A list of studies that provide the original source data,
 separated by semi-colons. The literature list may also include studies
 that provide additional context, such as experiments testing the
 ecological function of the structure. Reference format is as follows:
 Surname of the first author, year, abbreviated journal title, e.g.
 'Stavenga 2011 J Exp Bio'. If the reference source is not a
 journal article, it is labeled "undergraduate thesis", "MS
 thesis" = master's thesis, (dissertation)= doctoral
 dissertation, or (book chapter). * other\_notes: Other notes. For species
 that occur in more than one row, i.e. species with multiple structures in
 different wing regions, this column clarifies which structure / wing
 region / sex is the focus of the row. This field may also mention any
 other point of interest or clarification regarding the structure. This
 field may include whitespace and punctuation characters. 6. Missing data
 codes: NA
 #########################################################################
 CODE/SOFTWARE 1. The R code used to analyze this dataset for the
 corresponding publication is provided at
 https://doi.org/10.5281/zenodo.8215997. It was run in R version 3.2.2.