| Summary: | Insect visual systems are notable for their ability to operate in an impressive range of photic environments. Whilst insect eyes exhibit a high degree of evolutionary conservatism they exhibit diversity in structure, wavelength sensitivity and polarization sensitivity. Evolutionary adaptations for different visual conditions have been well studied and many different optical, physiological and neural specialisations have been discovered within the visual organs of this invertebrate class. In spite of this, little is known about the adaptive features of aquatic hemimetabolous insect visual systems whose larval and adult developmental stages occupy freshwater and terrestrial environments, respectively. The larval stages of these insects experience different visual conditions from the terrestrial adults. For example, the light in the freshwater environment is spectrally attenuated, causing a shift towards longer wavelengths, intensity levels are lower overall and the polarization of light differs. Thus, with the aim to investigate potential visual adaptations to the aquatic and terrestrial environment, the visual system of the Emperor dragonfly Anax imperator was explored using a range of methods. Firstly, behavioural studies were carried out to test the polarization sensitivity of larval A. imp era tor. Evidence suggests that the larval eye is sensitive to the polarization of light and possibly adapted to reduce the contrast degrading effects of polarized scatter underwater. Secondly, the potential morphological basis underlying this visual capability was investigated, using transmission electron microscopy, and found to differ from that of the adults. Whilst the mechanism for polarization analysis in adult A. imperator is in agreement with other adult dragonflies, namely an orthogonal array of R1 cell microvilli, such a structural arrangement was not found in the larval eye. Despite examining the structure of the larval eye in some detail the mechanism behind their polarization sensitivity is not clear. Finally, the opsins expressed in adult and larval stages were investigated using transcriptomics. A unique set of opsins is expressed at each developmental stage, adult and larval. There were many amino substitutions between opsins expressed at each life stage suggesting that they may exhibit functional differences possibly shifting the peak sensitivity of the visual pigment. Spectral tuning sites were identified suggestive of such shifts. There is also compelling evidence to suggest that the larval stage lacks a UV-sensitive visual pigment. This may be an adaptive feature of the visual system in a freshwater environment where short wavelengths are strongly scattered and attenuated. This study has begun to shed light on the changes that occur in the visual systems of A. imp era tor from the transition from a freshwater to aquatic environment. Results suggest that both the polarization and wavelength sensitivities of these stages may be unique and possibly adapted to their visual environments.
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