The compound eye functions as a pivotal visual organ for the majority of adult pterygote insects, and its distinctive morphological structure and optical design provide the foundation for the highly sophisticated visual capabilities of insects (Chapman, 2012). Scarab beetles, which constitute one of the largest groups within Coleoptera (Cave and Ratcliffe, 2008), display a substantial diversity in their compound eye structures (Meyer-Rochow and Horridge, 1975, Labhart et al., 1992, Gokan et al., 1998). A study by Gokan and Meyer-Rochow (2000), which examined the eye morphology and internal anatomy of 59 scarab beetle species (Coleoptera: Scarabaeidae), uncovered interspecific differences in the structure of ommatidial components, including the cornea, crystalline cone, retinula cell, and rhabdom. Notably, even within the same subfamily Melolonthinae, Ectinohoplia obducta and Sophrops formosana exhibit significant differences: the former has ommatidia approximately 134 μm in length, whereas the latter has ommatidia measuring about 500 μm. Correspondingly, their retinal types are the typical photopic type and the scotopic type, respectively, with rhabdom types identified as the axially fused type and the six-lobed type. This structural divergence is probably an adaptation to their respective diurnal and nocturnal activities. However, there are exceptions. Serica nigrovariata, which is also a diurnal species, has ommatidial length similar to E. obducta, yet it has a scotopic retina and an uncommon pseudomorphous rhabdom type (Gokan and Meyer-Rochow, 2000). Moreover, sexual dimorphism in compound eye structure has been demonstrated in several insect species, including fireflies (Lau and Meyer-Rochow, 2006) and moths (Lau et al., 2007a, Meyer-Rochow et al., 2008), which adds another layer of complexity. In conclusion, the pronounced diversity in insect compound eye morphology highlights the significance of research in this field for comprehending the structural evolution of the eye and its far-reaching implications in visual ecology.

Based on their internal anatomy and optical design, the compound eyes of insects are classified into three principal types: apposition eye, optical superposition eye, and neural superposition eye (Agi et al., 2014, Meyer-Rochow, 2015, Meyer-Rochow and Lindström, 2025). The apposition eye, which is commonly found in diurnal insects, functions with each ommatidium acting as an independent optical unit. This design prioritizes high spatial resolution under bright light conditions (Nilsson, 1989, Kittelmann and McGregor, 2024). In contrast, the optical superposition eye, which is often associated with nocturnal species, features a wide clear zone beneath the crystalline cones. This structure enables light to be collected across multiple facets, granting exceptional visual sensitivity and extraordinary night vision capability in dim environments (Warrant and Dacke, 2016, Warrant, 2017, Han and Chen, 2025). Consequently, this type exhibits greater sensitivity and light-capture ability compared to the apposition eye. The neural superposition eye, a type so far documented only in dipteran insects, attains enhanced sensitivity through the neural convergence of signals from adjacent ommatidia. This unique neural wiring preserves the high resolution of an apposition eye with the same optics, but significantly increases sensitivity (Agi et al., 2014). Furthermore, it is well-established that insects can dynamically adjust their eye structure in response to ambient light changes via mechanisms such as the lengthening and shortening of the crystalline cone tract, the migration of screening pigment cells, and the longitudinal movement of retinula cells (Warrant and McIntyre, 1996, Chapman, 2012, Xie et al., 2025). However, while current research on light adaptation has extensively explored the mechanisms underlying light/dark adaptation (Meyer-Rochow and Mishra, 2007, Hao et al., 2023, Han and Chen, 2025), studies investigating the structural changes and regulatory mechanisms in response to variations in light quality (e.g., wavelength/color) and intensity are still relatively scarce.

The scarab beetle Exolontha castanea (Coleoptera: Melolonthinae) is a significant subterranean pest of sugarcane, mainly distributed in sugarcane plantations across China and Vietnam (Gong and An, 2010, Shang et al., 2024). Guangxi Province, the largest sugarcane-producing region in China, serves as a primary area for its occurrence (Long et al., 2015, Shang et al., 2024). The larval stage (i.e., white grubs), which lasts for over 300 days, is the primary life stage responsible for crop damage. The larvae feed on sugarcane roots, causing symptoms such as stunted above-ground growth, lodging, and wilting, which severely impact both yield and sugar content, resulting in substantial economic losses (Shang et al., 2016, Shang et al., 2025b). Adults are nocturnal, emerging from April to June annually. They possess strong flight and reproductive capacities, making them the key stage for dispersal and population growth (Shang et al., 2025b). Our previous studies have shown that adults exhibit strong phototaxis (Shang et al., 2022b), and are particularly sensitive to ultraviolet and violet light within the 365–425 nm wavelength range (Shang et al., 2022a, Shang et al., 2025a). However, the morphology and internal microstructure of the adult compound eye, as well as its adaptive mechanisms in response to different light environments, remain unclear.

In this study, we utilized light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to investigate the external morphological characteristics and internal ultrastructure of the compound eyes in both female and male E. castanea adults. Taking a dark environment as the control, we studied the structural changes in the compound eye under low (50 lx) and high (1000 lx) light intensities across different wavelengths, including ultraviolet, violet, blue, red, and white light. The aim was to clarify the eye's adaptive mechanisms in response to varying light conditions. Our findings provide a foundational understanding of the evolutionary development and visual ecology of the compound eye in E. castanea adults.