The photonic nanoarchitectures responsible for the blue colour of the males of nine polyommatine butterfly species living in the same site were investigated structurally by electron microscopy and spectrally by reflectance spectroscopy. Optical characterization was carried out on 110 exemplars. The structural data extracted by dedicated software and the spectral data extracted by standard software were inputted into an artificial neural network software to test the specificity of the structural and optical characteristics. It was found that both the structural and the spectral data allow species identification with an accuracy better than 90 per cent. The reflectance data were further analysed using a colour representation diagram built in a manner analogous to that of the human Commission Internationale de l'Eclairage diagram, but the additional blue visual pigment of lycaenid butterflies was taken into account. It was found that this butterfly-specific colour representation diagram yielded a much clearer distinction of the position of the investigated species compared with previous calculations using the human colour space. The specific colours of the investigated species were correlated with the 285 flight-period data points extracted from museum collections. The species with somewhat similar colours fly in distinct periods of the year such that the blue colours are well tuned for safe mate/competitor recognition. This allows for the creation of an effective pre-zygotic isolation mechanism for closely related synchronic and syntopic species.

Butterfly wing scales containing photonic nanoarchitectures act as chemically selective sensors due to their color change when mixing vapors in the atmosphere. Based on butterfly vision, we built a model for efficient characterization of the spectral changes in different atmospheres. The spectral shift is vapor specific and proportional with the vapor concentration. Results were compared to standard principal component analysis. The modification of the chemical properties of the scale surface by the deposition of 5 nm of Al 2 O 3 significantly alters the character of the optical response. This is proof of the possibility to purposefully tune the selectivity of such sensors.

While numerous papers have investigated the effects of thermal stress on the pigmentary colours of butterfly wings, such studies regarding structural colours are mostly lacking, despite the important role they play in sexual communication. To gain insight into the possible differences between the responses of the two kinds of colouration, we investigated the effects of prolonged cold stress (cooling at 5 °C for up to 62 days) on the pupae of Polyommatus icarus butterflies. The wing surfaces coloured by photonic crystal-type nanoarchitectures (dorsal) and by pigments (ventral) showed markedly different behaviours. The ventral wing surfaces exhibited stress responses proportional in magnitude to the duration of cooling and showed the same trend for all individuals, irrespective of their sex. On the dorsal wing surface of the males, with blue structural colouration, a smaller magnitude response was found with much more pronounced individual variations, possibly revealing hidden genetic variations. Despite the typical, pigmented brown colour of the dorsal wing surface of the females, all cooled females exhibited a certain degree of blue colouration. UV-VIS spectroscopy, optical microscopy, and scanning and transmission electron microscopy were used to evaluate the magnitude and character of the changes induced by the prolonged cold stress.

Structural coloration variability was investigated in two Blue butterfly species that are common in Hungary. The males of Polyommatus icarus (Common Blue) and Plebejus argus (Silver-studded Blue) use their blue wing coloration for conspecific recognition. Despite living in the same type of habitat, these two species display differences in prezygotic mating strategy: the males of P. icarus are patrolling, while P. argus males have sedentary behavior. Therefore, the species-specific photonic nanoarchitecture, which is the source of the structural coloration, may have been subjected to different evolutionary effects. Despite the increasing interest in photonic nanoarchitectures of biological origin, there is a lack of studies focused on the biological variability of structural coloration that examine a statistically relevant number of individuals from the same species. To investigate possible structural color variation within the same species in populations separated by large geographical distances, climatic differences, or applied experimental conditions, one has to be able to compare these variations to the normal biological variability within a single population. The structural coloration of the four wings of 25 male individuals (100 samples for each species) was measured and compared using different light-collecting setups: perpendicular and with an integrating sphere. Significant differences were found in the near UV wavelength region that are perceptible by these polyommatine butterflies but are invisible to human observers. The differences are attributed to the differences in the photonic nanoarchitecture in the scales of these butterflies. Differences in the intensity of structural coloration were also observed and were tentatively attributed to the different prezygotic mating strategies of these insects. Despite the optical complexity of the scale covered butterfly wings, for sufficiently large sample batches, the averaged normal incidence measurements and the averaged measurements using an integrating sphere are in agreement.

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