Contribution of individual legs to overall attachment in the adult ladybird Harmonia axyridis depends on the relative leg orientation to an external force

Abstract

This study investigates the attachment ability of harlequin ladybird, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), focusing on the synergic action of opposite legs, the anisotropy of adhesive organs and sexual dimorphism. Contrary to expectations, experiments showed that beetles with fewer legs sometimes exhibited higher attachment forces, challenging the hypothesis that collective leg action enhances attachment. This result is attributed to differences in experimental set-ups, where our centrifugal method highlighted the importance of pad orientation relative to external forces. The anisotropy of adhesive pads, characterized by the directional dependence of adhesion and friction, significantly influenced attachment performance. Results demonstrated that inward-oriented legs enhanced attachment due to lower peeling angles, whereas outward orientation reduced effectiveness. Sexual dimorphism was evident in the structure of adhesive pads, with males possessing discoidal setae on fore- and midlegs. No significant difference in attachment was observed between intact males and females on smooth surfaces, although ablated males sometimes showed higher forces due to the presence of discoidal setae. These findings underscore that attachment strength in H. axyridis is not solely dependent on pad area but also on the orientation and anisotropic properties of adhesive organs, as well as the specific role of sexual dimorphism in leg morphology.