Variability of the Structural Coloration in Two Butterfly Species with Different Prezygotic Mating Strategies

Abstract

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.

Changes in structural and pigmentary colours in response to cold stress in Polyommatus icarus butterflies

“…Cooling for 10 days did not produce significant deviations in the peak position of the blue reflectance. Cooling for longer durations caused measurable deviations from the normal biological variation of the blue colour of the male P. icarus 24 and induced the appearance of blue scales on all the females, which have brown dorsal wing surfaces in the wild population. The possible cause of the shift in the spectral position of the reflectance maximum may be related to the slight alteration of the dimensions of the photonic nanoarchitecture or to changes in the scale arrangement.

Figure 3Normalized reflectance spectra averaged over all four wings of the exemplars.

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“…In Hungary, the typical female is dominantly brown on the dorsal wing surface, whereas in Northern Europe, females with partial blue colouration may be found 33 . We used our earlier results from the investigation of natural variations in the blue colouration of the males within a given population of P. icarus 24 to discriminate between the natural colour variation and the effects induced by the prolonged cooling of the pupae. The reflectance of the dorsal wing surfaces (all four wings of each individual) was measured using UV-VIS spectroscopy.…”

Pretreated Butterfly Wings for Tuning the Selective Vapor Sensing

“…The detailed investigation of the vapor-sensing properties of the pretreated Polyommatus icarus butterfly wings was preceded by the control of the possible color modification caused by the pretreatment method. The spectral characteristics were investigated using an integrating sphere optical setup [ 29 ] and the reflectance spectra of the untreated and the soaked wings were measured. For each treatment we used all four wings of P. icarus males, two of which were untreated and two of which were soaked for 14 days.…”

“…One can see that the normalized spectra of all four wings (two pretreated vs. two pristine) are highly similar for each of the volatiles used for pretreatment: the maximal difference in the peak wavelength values is 10 nm in the case of the ethanol pretreatment ( Figure 1 A) and all the other measured spectral differences are lower than that. It clearly shows that the spectral variance of the measured color is in the range of the natural color variability of these insects which is typically ±15 nm using this measurement method [ 29 ], meaning that structural changes caused by the volatile pretreatment of the photonic nanoarchitecture can be excluded. Furthermore, this color variability is significantly lower than the spectral shift which is measured during vapor exposition ( Figure 1 D).…”

“…The wing coloration of the blue polyommatine butterflies constitutes a sexual communication channel [ 23 ] which affects the photonic nanoarchitectures, producing the structural color. We showed in our recent work that the photonic nanoarchitectures occurring in the wing scales of polyommatine butterflies are species-specific and are stable over time; therefore, the structural color they generate only has minor natural variability [ 29 ], making them suitable as prototypes of photonic nanoarchitectures with 3D porous nanostructures.…”