  In spite of the crucial role it is believed to play in nature, fluorescence
in natural organisms remains under-investigated from optical, chemical and
biological perspectives. One example is the transparent wings of insects from
the order Hemiptera: fluorescence emission has so far not been reported in any
of the 3,000 described species of the superfamily Cicadoidea. The phenomenon
has however been investigated in several butterfly and moth species, in which
the coloured wing scales are known to embed fluorophores such as papiliochrome
II. Photonic structures present in these scales may mediate the associated
light emission. Fluorescence has also been described in the vein joints and the
membrane of the transparent wings of damselflies and dragonflies. This emission
was attributed to the presence of autofluorescent proteins such as resilin that
gives rise to low stiffness and high strain in the biological tissues, allowing
passive wing deformations. In this article, we characterised the fluorescence
emission from the transparent wings of the grey cicada \textit{Cicada orni}
(Linnaeus, 1758) and the common cicada \textit{Lyristes (Tibicen) plebejus}
(Scopoli, 1763), using linear and nonlinear optical methods. We compared their
properties with the fluorescence emission from two other insects exhibiting
transparent wings, namely, \textit{Hemaris fuciformis} (Linnaeus, 1758)
commonly known as the broad-bordered bee hawk-moth and the Bornean damselfly
\textit{Vestalis amabilis} Lieftinck, 1965. These fluorescent behaviours were
optically characterised by one- and multi-photon microscopy and spectrometry,
allowing us to infer the symmetry of the material as well as the biological
role of the fluorophores in the visual perception of insects and to explain the
high absorption by the material in the range 300-400~nm.
