Unraveling the Compound Eye Design of the Diurnal Moth Histia flabellicornis (Lepidoptera: Zygaenidae).

Lepidoptera typically exhibit a dichotomy in compound eye design: diurnal butterflies possess apposition eyes for high resolution in bright light, whereas nocturnal moths have superposition eyes for enhanced sensitivity under dim conditions. However, exceptions, particularly among diurnal moths, challenge this pattern and offer insights into the evolution of compound eyes in Lepidoptera. In this study, we investigated the compound eye design of the diurnal moth Histia flabellicornis (Fabricius) using light, scanning, and transmission electron microscopy to determine whether it has superposition or apposition eyes, and to quantitatively analyze the structural features and visual acuity. Our results reveal that H. flabellicornis possesses apposition-type compound eyes composed of over 2000 ommatidia, each comprising a cornea, a crystalline cone, nine retinula cells forming a fused rhabdom, and a few isolated tracheoles. The calculated interommatidial angles (Δϕ = 4.08°) and the eye parameter P (P = 1.74) suggest a visual system adapted to moderate light conditions, balancing spatial resolution with photon capture. These findings confirm the presence of apposition eyes in H. flabellicornis, supporting the moth's adaptation to diurnal behavior and contributing to understanding the evolutionary diversification of compound eye designs in Lepidoptera.