Revealing natural fluorescence in transparent insect wings by linear and nonlinear optical techniques

For most natural organisms, the physical, chemical and biological aspects of fluorescence emission are poorly understood. For example, to the best of our knowledge, fluorescence from the transparent wings of any of the 3000 known species of cicadas has never been reported in the literature. These wings are known to exhibit anti-reflective properties arising from quasi-periodic arrays of nipples. Our study, using linear and nonlinear optical techniques, including spectrofluorimetry, two-photon fluorescence spectroscopy and Second Harmonic Generation (SHG), reveals the fluorescence properties in the wings the grey and the common cicadas (Cicada orni and Lyristes (Tibicen) plebejus, respectively), as well as the broad-bordered bee hawk-moth (Hemaris fuciformis). The study suggests that fluorescence would be more widespread in transparent insect wings than what was previously believed. Comparing this result to the fluorescence emission from the wings of the Bornean damselfly (Vestalis amabilis), we inferred that this emission probably arises from resilin, a protein reported to enhance wing flexibility. Moreover, the nonlinear optical investigation of the insects’ wings provided further insight into wing structure, indicating that multiphoton techniques add valuable information for the analysis of insect integuments. The strong SHG signal detected from the wing veins implies that these veins are materially organised in a non-centrosymmetric and hence non-random fashion.