Sexual dimorphism of tarsal structures and attachment forces in Diabrotica virgifera virgifera LeConte (Coleoptera, Chrysomelidae, Galerucinae)

Agile Diabrotica virgifera virgifera beetles reliably attach and walk on a wide range of substrates, including soil, waxy host plants, non-host plants, and female elytra. This capability relies on a complex of features, including insect behavior and effective tarsal structures. This comparative study focuses on the detailed design, dimensions, geometry, and performance of male and female tarsi across different counterpart substrates. The research confirmed the presence of tarsal adhesive setae with lanceolate, filamentous, spatulate, and discoid terminals. Male-specific adhesive setae occurred in ovoid patches on the first tarsomere of the fore- and midlegs, including two subtypes: long, spatulate with discoid-oval terminals and short, mushroom-shaped, having circular terminals. The mushroom-shaped setae appeared to be extraordinary, as they had not been observed in other leaf beetles so far. Summarizing the widths of all thin-film elements (adhesive terminals) that potentially participate in contact formation resulted in the overall peeling line length of all adhesive pads to be 1.1 cm for males and 1.0 cm for females, respectively. The attachment force normalized by the total peeling line length (N m^-1) in males (0.9) outperformed that of females (0.2). According to linear models, statistical interactions between (i) sex and glass surface geometry, (ii) glass surface wettability and geometry, (iii) sex, glass surface wettability, and geometry, and (iv) sex and plant substrate geometry. The pull-off force depended significantly on sex and substrate. Reliable attachment in D. virgifera virgifera on host plants and during mating depends on (i) the dimension and geometry of adhesive structures, (ii) the dimension and geometry of the substrate, (iii) the secretions involved (tarsal fluid, epicuticular grease), (iv) the postures of the legs and feet (angles of segments relative to one another and to the substrate), as well as (v) the motions and behaviour of the beetles. We propose that the attachment and detachment mechanisms are balanced to suit the mobile lifestyle of this species.