Morphometric analyses of sexual dimorphism in sound-emitting structures in adults of the bess beetle Vindex agnoscendus (Coleoptera: Passalidae)

In insects, the sexually dimorphic body size, color, and horns are very conspicuous. Other not so obvious characteristics, such as behavioral traits related to reproduction, are also dimorphic (courtship dances and sounds). Among these, the stridulation, stridulatory apparatus, and hearing systems could also be different between sexes. Passalids are subsocial beetles with acoustic communication in the interactions of larvae-adult and adult-adult. Most of the species do not have morphological traits with sexual dimorphism, but sounds are dimorphic under reproductive and aggression contexts. We studied sexual dimorphism in the stridulation organs (plectrum) in Vindex agnoscendus (Percheron) as an important step towards understanding its functioning. We acquired SEM micrographs of the plectrum for 14 specimens per sex, for measuring size, density and shape of the spines. We performed traditional and geometric morphometric analyses to test for sexual differences in the spines of three zones (Z1-Z3) of the plectrum. Allometric analyses showed spine variation is not related to body size in either sex. Our univariate and multivariate analyses uncovered sexual dimorphism in spine size (length and width of spines), spine density (distance between spines and number of spines), and spine shape (landmark coordinates). Spines were wider in males than in females in the apical zone (Z1). The spines were further apart in females than in males in two zones (Z1, Z2) and the spine numbers were higher in females than in males in the apical zone (Z1). The shape of spines was different between sexes in two zones (Z1, Z2). Moreover, our disparity analyses showed different variability patterns in size and density of spines. Variability of spine shape was similar between sexes, and variability of spine density was higher in females than in males, while for spine size it was higher in males than in females. These findings for the plectrum suggest further research for sexual dimorphism in the pars stridens and acoustic signals, and for the possible roles of natural and sexual selection in the patterns of variability of spines.