Optical Modelling and Phylogenetic Analysis Provide Clues to the Likely Function of Corneal Nipple Arrays in Butterflies and Moths

Abstract

The lenses in compound eyes of butterflies and moths contain an array of nipple-shaped protuberances, or corneal nipples. Previous work has suggested that these nipples increase light transmittance and reduce the eye glare of moths that are inactive during the day. This work builds on but goes further than earlier analyses suggesting a functional role for these structures including, for the first time, an explanation of why moths are attracted to UV light. Using a phylogenetic approach and 3D optical modelling, we show empirically that these arrays have been independently lost from different groups of moths and butterflies and vary within families. We find differences in the shape of nipples between nocturnal and diurnal species, and that anti-glow reflectance levels are different at different wave-lengths, a result thereby contradicting the currently accepted theory of eye glow for predator avoidance. We find that there is reduced reflectance, and hence greater photon absorption, at UV light, which is probably a reason why moths are attracted to UV. We note that the effective refractive index at the end of the nipples is very close to the refractive index of water, allowing almost all the species with nipples to see without distortion when the eye is partially or completely wet and providing the potential to keep eyes dry. These observations provide a functional explanation for these arrays. Of special interest is the finding that their repeated and independent loss across lepidopteran phylogeny is inconsistent with the explanation that they are being lost in the ‘higher’, more active butterflies.

Substrate Selection of Ascidian Larva: Wettability and Nano-Structures

“…This lower reflection simultaneously indicates higher transmission of light into the eyes. This is also an advantage in low-light conditions, and comparative studies of nocturnal and diurnal lepidopterans have suggested that greater transmission of UV light from the moth-eye structure may be important for lepidopterans and cause attraction to UV lights [38]. Moreover, some other functions have been demonstrated for this structure.…”

Pale tussock moth, Calliteara pudibunda (Linnaeus, 1758) (Lepidoptera; Erebidae) with a focus on its situation in Türkiye

“…Additionally, Cordyceps militaris (L., 1758) (Hypocreales, Cordycipitaceae) was identified from C. pudibunda larvae and pupae (Nilsson, 1978;Sylvén, 1942). Spalding et al (2019) found that the corneal protrusions, which are thought to be an ancestral character of diurnal butterflies, are atrophied in C. pudibunda, suggesting specialization for night and a nocturnal life style adapted to low light intensity. This means that C. pudibunda is attracted to light.…”

“…Spalding et al (2019) found that the corneal protrusions, which are thought to be an ancestral character of diurnal butterflies, are atrophied in C. pudibunda , suggesting specialization for night vision and a nocturnal life style adapted to low light intensity. This means that C. pudibunda is attracted to light.…”

A Comparative Study of Crystallography and Defect Structure of Corneal Nipple Array in Daphnis nerii Moth and Papilio polytes Butterfly Eye

“… 34 In this direction, the analysis of chitin nanostructure in the eyes of butterflies and moths needs to be understood in detail. 35 − 38 …”

“…Better anti-reflective structures have paved the way for advanced manufacturing of moth-eye structures for various applications such as solar cells, light-emitting diodes, and display screens . In this direction, the analysis of chitin nanostructure in the eyes of butterflies and moths needs to be understood in detail. − …”