Putative Photosensitivity-Associated Sexual Dimorphism in Compound Eye Structure of Lymantria xylina (Lepidoptera: Erebidae).

Lymantria xylina is a major pest in coastal casuarina shelterbelts and a species subject to quarantine regulations by countries to which it is non-native. Phototaxis is fundamental to the insect's surveillance and risk assessment analysis, and it exhibits pronounced sexual dimorphism in compound eye structure. This dimorphism was investigated using scanning and transmission electron microscopy. Males displayed significantly larger compound eyes, characterized by greater ommatidial areas and a higher total number of facets per eye compared to females. From the distal to proximal end, the ommatidium consists of the cornea, primary and secondary pigment cells, crystalline cones, retinula cells, a rhabdom bundle, and basal retinal cells (in a "7 + 1" arrangement). The internal ultrastructure of the ommatidia is similar in both sexes. However, males possess significantly thinner cornea and extremely elongated crystalline cones. Based on external morphology, both sexes generally exhibit a parallel-symmetrical compound eye form, minimizing optical asymmetry to optimize nocturnal vision. These differences are attributed to the distinct visual demands of males for mate-searching in low-light environments, while females, being more stationary, have reduced visual needs. Paraffin sections of Lymantria xylina compound eyes further revealed that, during dark adaptation, pigment granules aggregated within the crystalline cone region to enhance low-light capture. Conversely, following intense light stimulation, these granules translocated to the perinuclear region of photoreceptor cells, forming a light-shielding configuration.