Scaling Relationships and Sexual Size Dimorphism Among the Body Parts of Holotrichia oblita (Coleoptera: Scarabaeidae).

Sexual dimorphism is common among insects. However, whether dimorphism influences the wing loading (i.e., body mass per unit wing area) and scaling relationships among body parts in beetles has seldom been explored. Here, we examined Holotrichia oblita (Coleoptera: Scarabaeidae) gender differences in body mass, total hind wing area, leg length, and wing loading, and quantified the scaling relationships between body mass and body length, between head-and-prothorax mass and non-head-and-prothorax mass, and between total elytron mass and total hind wing area. Results revealed that (i) females exhibited significantly greater body mass and wing loading, (ii) males showed a relatively larger hind wing area and longer legs (including front, mid, and hind legs), and (iii) scaling analysis demonstrated that the 95% confidence interval of the scaling exponent of body mass versus body length included 3 in both genders. In addition, (iv) the data indicated an isometric relationship between head-and-prothorax mass and non-head-and-prothorax mass, and an allometric relationship between total elytron mass and total hind wing area. These results are interpreted to indicate that sexual dimorphism in H. oblita likely reflects different selective pressures on gender: the smaller wing loading of males enhances flight maneuverability, potentially aiding in predator avoidance and procuring mates, whereas the larger wing loading and body mass of females provide support for a larger reproductive investment (egg mass). Our study quantifies how sexual dimorphism and tissue-specific investment alter scaling in beetles. While prior work on leaves and eggs established the role of density heterogeneity, such analyses are scarce in insects, particularly for segmented bodies (head-thorax-abdomen) and paired structures (elytra-wings). By integrating allometry with functional ecology, we reveal how trade-offs between protection (elytra mass), mobility (wing area), and reproduction (female-biased size) drive non-isometric growth, potentially advancing predictions of insect performance under selective pressures.