Antennomere numbers in fireflies (Coleoptera: Lampyridae): unique patterns and tentative explanations

Abstract

The plesiomorphic antenna with eleven articles is stunningly stable across the over 400.000 beetle species, despite the astonishing diversity of morphologies and ecological settings in which these organisms live. However, a few beetle lineages evolved different antennomere numbers, and these offer an interesting opportunity to understand how an otherwise fixed phenotype may eventually change, and to study what factors shape their patterns of variation. Here, we review for the first time, based on original and literature data, the unique patterns of variation in antennomere numbers in the firefly family (Coleoptera: Lampyridae), which has the greatest range (7–62) in antennomere numbers among beetles despite a relatively low species diversity (∼2000 species). We also tested the hypothesis that antennomere numbers are positively related to body size and explored patterns of antennal asymmetry across Amydetes spp, which have the greatest range in antennomere numbers (24–62). Out of the ∼100 firefly genera, nine show antennomere numbers lower or higher than 11. Among those nine genera with unusual antennomere numbers, five show intraspecific variation as well. In taxa whose antennae have less than 11 antennomeres, males seem to follow mainly light cues to find mates. In Amydetes, we found that antennomere numbers are positively related and negatively allometric to body size, suggesting that in this group the mechanism shaping antennal segmentation is sensitive to body size. Moreover, fluctuating asymmetry was found to be widespread in Amydetes spp., with a difference of up to six antennomeres between left and right antennae. The combined occurrence of unusual antennomere numbers and within-species variation suggests that in several firefly taxa a destabilization of the mechanism driving antennal segmentation has occurred.