Sexual dimorphism and allometry of secondary sexual character in Morimus asper (Coleoptera: Cerambycidae)

The longhorn beetle Morimus asper exhibits the typical sexual dimorphism of cerambycid beetles: males have longer antennae than females and the antennal length also varies greatly among males of the same population. This study explores the sexual dimorphism in M. asper and the scaling relationship between antennal length and body size in males. Our aim was to assess the allometric pattern of these two traits and their potential effects on individual eco-ethological features. Sexual dimorphism was observed in antennal length and in other morphological traits: males showed longer pronota and females larger and longer elytra, suggesting that males allocate more resources to the anterior body traits, while in females more resources are invested in the development of post-prothoracic body traits. The allometric relationship between antennal length and elytra length of males was best described by a segmented regression, which identified a switch point dividing the male population into two different groups. The analysis of covariance showed that smaller males (with elytra length below the switch point) exhibit a positive allometry, while larger males (above the switch point) showed isometric scaling relationship. Our findings showed that morphometric traits were correlated with aspects of male behaviour and ecology: larger males were more frequently found mating, probably because they have an advantage in defending mates and in winning fights against other males.