Structural morphology and light-induced changes in the apposition eye of Tomicus minor (Hartig) (Coleoptera: Curculionidae: Scolytinae).

Tomicus minor, commonly known as the transverse shot hole borer, is the most destructive pest of Pinus yunnanensis in southwestern China, causing substantial tree mortality. Behavioral studies have confirmed its strictly diurnal flight patterns, suggesting a reliance on visual cues for host detection. We conducted a detailed examination of the compound eye morphology and optics of T. minor using light, scanning electron, and transmission electron microscopy. Each apposition eye contains about 230 ommatidia, showing slight sexual and lateral variations. Ommatidial facet shapes vary dorsoventrally: quadrilateral in the dorsal region, hexagonal and rounded in central-to-ventral regions, and pentagonal peripherally. Each ommatidium comprises a large corneal lens, an acone-type crystalline cone with four cells, and an open rhabdom with eight retinular cells (central R7-R8 surrounded by R1-R6). Changes in dark/light adaptation influence cone size/shape and rhabdom cross-section, thereby regulating photoreceptor light exposure. Key optical parameters, such as the F-number (1.17), interommatidial angle (2.70°), eye parameter (1.09 μm rad), and acceptance/interommatidial angle ratio (4.52), indicate high sensitivity and contrast perception in daylight, supporting the insect's diurnal behaviors. This research enhances understanding of coleopteran vision, provides a foundation for Tomicus photoreception investigations, and may guide pest management strategies that incorporate both olfactory and visual cues.