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

Abstract

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 ability relies on a complex of features, including insect behavior and effective tarsal structures. The detailed male and female tarsal design, dimension, geometry, and performance on the counterpart substrates were contents of the present comparative study. The presence of tarsal adhesive setae with lanceolate, filamentous, spatulate, and discoid terminals was confirmed. 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 terminal and short, mushroom-shaped with circular terminal). The mushroom-shaped one appeared to be extraordinary, which has not been found in other leaf beetles so far. Summarizing the widths of all thin-film elements (adhesive terminals) potentially participating in contact formation resulted in the overall peeling line of all adhesive pads of 1.1 cm and 1.0 cm in males and 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 occurred between (i) sex and glass surface geometry, (ii) glass surface wettability and geometry, (iii) sex, glass surface wettability and geometry, as well as sex and plant substrate geometry. Pull-off force depended significantly on sex and substrate. Reliable attachment in D. virgifera virgifera on host plants and during mating depends on (i) dimension and geometry of adhesive structures, (ii) substrate dimension and geometry, (iii) involved secretions (tarsal fluid, epicuticular grease), (iv) leg and feet postures (angles of segments to each other and the substrate), as well as (v) beetles' motions and behaviour. We suggest balanced attachment-detachment procedures that suit the mobile lifestyle of the species studied.