The well-tuned blues: the role of structural colours as optical signals in the species recognition of a local butterfly fauna (Lepidoptera: Lycaenidae: Polyommatinae)

Abstract

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.

Substance specific chemical sensing with pristine and modified photonic nanoarchitectures occurring in blue butterfly wing scales

“…Our recent investigations showed that in the case of nine closely related blue Polyommatus butterfly species the spectral sensitivity of their eyes (sensor) and the dorsal structural coloration of the wings (measured object) are well tuned for safe mate / competitor recognition [11]. Based on our noninvasive spectral measurements [12] on numerous museum exemplars, we showed that the dorsal coloration is species specific and stable in time.…”

“…To evaluate the vapor sensing data we used the color representation of the Polyommatine butterflies based on the CIE tristimulus color matching [11,19]. This tridimensional color space was created using the sensitivity functions of the four photoreceptors occurring in the eyes of the Lycaenid butterflies [13].…”

“…Based on our noninvasive spectral measurements [12] on numerous museum exemplars, we showed that the dorsal coloration is species specific and stable in time. The species of the investigated exemplars can be distinguished based on the dorsal coloration even in the case of the three violet species appearing to the human eye as similarly colored [11,12]. This clearly shows that the eyes of these butterflies are well tuned for the detection of minor color differences between the species, and the enhanced spectral sensitivity of the eyes concentrates on the wavelength range corresponding to the structural coloration.…”

“…In the case of the Polyommatine butterflies investigated earlier [11], both the color differences between the species [11] and the color change caused by the vapors [4,16] originate from the alteration of the structural color in the same near UV -blue wavelength region and they are of similar magnitude. Earlier we showed [11] that minor color differences between the Polyommatine species dorsal side are originated from the slight structural differences of their photonic nanoarchitectures, all exhibiting the same general layout called "pepper-pot" type scales [17]. Similarly minor color changes of a butterfly wing could be caused by the changes of the surrounding vapor mixtures: the refractive index contrast changes between the empty (chitin -air) and filled (chitin -vapor and condensed vapor -due to capillary condensation in the nanovoids [4]) state of the structure.…”

“…Similarly minor color changes of a butterfly wing could be caused by the changes of the surrounding vapor mixtures: the refractive index contrast changes between the empty (chitin -air) and filled (chitin -vapor and condensed vapor -due to capillary condensation in the nanovoids [4]) state of the structure. These two analogous behaviors suggests the use of tridimensional color space representation to monitor the changes caused by vapors, as it was used with success for the species discrimination [11].…”