  Upon illumination by ultraviolet light, many animal species emit light
through fluorescence processes arising from fluorophores embedded within their
biological tissues. Fluorescence studies in living organisms are however
relatively scarce and so far limited to the linear regime. Multiphoton
excitation fluorescence analyses as well as non-linear optical techniques offer
unique possibilities to investigate the effects of the local environment on the
excited states of fluorophores. Herein these techniques are applied for the
first time to the study of insects' natural fluorescence. The case of the male
Hoplia coerulea beetle is investigated because the scales covering the beetle's
elytra are known to possess an internal photonic structure with embedded
fluorophores, which controls both the beetle's colouration and the fluorescence
emission. An intense two-photon excitation fluorescence signal is observed, the
intensity of which changes upon contact with water. A Third-Harmonic Generation
signal is also detected, the intensity of which depends on the light
polarisation state. The analysis of these non-linear optical and fluorescent
responses unveils the multi-excited states character of the fluorophore
molecules embedded in the beetle's elytra. The anisotropy of the photonic
structure, which causes additional tailoring of the beetle's optical responses,
is confirmed by circularly polarised light and non-linear optical measurements.
