Microstructural characteristics and mechanisms underlying dynamic light adaptation in the compound eyes of Exolontha
Xiankun Shang1, Jili Wei2, Wei Liu3
1Sugarcane Research Institute, Guangxi Academy of Agricultural Sciences/Key Laboratory of Sugarcane Biotechnology and Genetic Improvement (Guangxi), Ministry of Agriculture and Rural Affairs/Guangxi Key Laboratory of Sugarcane Genetic Improvement, Nanning, China.
Abstract:
Exolontha castanea (Coleoptera: Melolonthinae) larvae are a major subterranean pest of sugarcane in China. The adults exhibit strong flight capacity and positive phototaxis. This study investigated the external morphology, internal microstructure, and light-induced adaptations of the compound eyes in both female and male E. castanea using light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The compound eyes feature a peninsula-shaped canthus that extends deeply into the basement membrane region. A marked sexual size dimorphism of the eye was observed, with females having significantly larger eyes and a greater number of facets (10,780.92 in females vs. 9,901.30 in males). The eyes were identified as being of the optical superposition type. The ommatidial length is approximately 800 μm. Moreover, the cornea was significantly thicker in female adults than in males. The crystalline cone is eucone and bullet-shaped, consisting of three to four Semper cells. Each ommatidium contains eight retinula cells (R1-R8). The cell bodies of R1-R7 form a retinular tract that traverses the clear zone, with their nuclei located in a distally swollen region. R8 is a basal cell situated beneath the rhabdom near the basement membrane. The retina was classified as the scotopic type. The proximal ends of R1-R7 extend inward to form a rhabdomere, which collectively constitute an open, seven-lobed rhabdom. The inter-rhabdom spaces are densely filled with tracheoles. Under different light conditions, including darkness, ultraviolet, violet, blue, red, and white light, the eyes demonstrate dynamic adaptations by modulating the length of the primary pigment cells (PPCs) and the migration of pigment granules. Under dark adaptation, the retinula cells are closely apposed to the crystalline cone, and pigment granules accumulate around it. Upon light exposure, the PPCs surrounding the cone elongated, the retinula cells shortened radially, and the pigment granules migrated into the elongated PPCs region beneath the cone, thereby reducing light entry. Furthermore, the degree of PPCs elongation is positively correlated with light intensity.
