Sexual Dimorphism in the Sensory Organs of Monochamus saltuarius Gebler (Coleoptera: Cerambycidae): A Morphometric Analysis of Compound Eyes and Stridulatory Files

The Sakhalin pine sawyer, Monochamus saltuarius Gebler, 1830 (Coleoptera: Cerambycidae), is a newly discovered insect vector of the pine wood nematode (Bursaphelenchus xylophilus) in China. Despite its ecological importance, the detailed morphology of its sensory systems remains largely unexplored. This study presents comprehensive ultrastructural analysis of the compound eyes and stridulatory organs in adult M. saltuarius, with a focus on sexual differences. Our morphometric results revealed pronounced sexual dimorphism: males possessed significantly larger compound eyes in terms of total area and perimeter. Furthermore, all three types of ommatidial facets (hexagonal, pentagonal, and quadrilateral) exhibited significantly greater area, perimeter, and diameter in males. Interestingly, while the total number of ommatidia and the counts within the dorsal and ventral eye regions showed no significant sexual difference, they were strongly positively correlated with body length in both sexes. Regarding the stridulatory organs, key morphological features including the transverse diameter, longitudinal diameter, and width of the lateral sulcus of the stridulatory file were also significantly greater in males. In contrast, no sexual dimorphism was detected in the width or density of the stridulatory teeth. These findings collectively indicate a significant investment in sexually dimorphic sensory organs in M. saltuarius, which we hypothesize is closely linked to sex-specific behavioral roles, particularly in active mate searching, courtship displays, and intrasexual competition. This research provides a crucial morphological foundation for understanding the species’ visual and acoustic communication, supporting future work on its sensory ecology and the development of behavior-based control strategies for pine wilt disease.