The ‘sparkle’ in fake eyes – the protective effect of mimic eyespots in lepidoptera. Ent
Many species of lepidoptera bear conspicuous circular patterns on their wings, known as eyespots, that are hypothesised to protect their bearers against predatory birds. In this study, we focus on a small but ubiquitous feature occurring naturally in lepidopteran eyespots, namely the so-called ‘sparkle’. The ‘pupil’ in an eyespot is often highlighted by a ‘sparkle’, which is hypothesised to mimic a natural corneal total light reflection evident as a highlight, twinkle, or sparkle in the vertebrate eye. In a study exploring the presence of such sparkles, we found that 53% of lepidopteran eyespots exceeding 1 mm in diameter have a central, pinpoint-like ‘sparkle’, 12% have a marginal, crescent-shaped ‘sparkle’, 13% have a semi-circular ‘sparkle’, and 22% have an intermediate semi-circular to crescent-shaped ‘sparkle’. In the lepidopterans’ natural resting position, the marginal ‘sparkles’ are positioned in the upper part of the eyespots’ ‘pupil’ and thus may create the illusion of a spherical eyeball. The ‘sparkles’ in lepidopteran eyespots do not only appear white to humans, but also reflect ultraviolet light. White and UV-reflecting ‘sparkles’ also appear ‘white’ for UV-sensitive viewers such as birds, and thus may effectively mimic the natural highlight in vertebrate eyes as an area of total light reflection. In field experiments using lepidopteran dummies baited with a mealworm, we show that the ‘sparkle’ is one of several components of eyespots eliciting a deterrent effect and that eyespots with a ‘sparkle’ in a natural position have a stronger deterrent effect than those with a ‘sparkle’ in an unnatural position. These findings support the eye mimicry hypothesis that better vertebrate eye mimicry improves the deterrent effect of eyespots.