Abstract
Insect antennae are covered in hairlike sensilla that detect diverse environmental cues. Selection on these functions has produced a bewildering variety of antennal forms, including many examples of sexual dimorphism (SD). Antenna length SD is particularly common, but poorly understood, in longhorned beetles (Coleoptera: Cerambycidae). Extremely elongate male antennae may extend the reach of individuals searching for mates, enabling rapid recognition via antennal contact. Alternatively, they may increase sensitivity to airborne pheromones by bearing more olfactory sensilla. We tested these hypotheses by modeling sensillum distributions and abundances across species and sexes of Anoplistes, a cerambycid genus with extensive variation in antenna length and SD. We found limited evidence that olfactory sensillum abundance scales with antenna segment length; instead, mechano- and contact chemosensory sensilla cluster near the antenna tip, consistent with contact-mediated mate recognition. If the tip segment serves an important tactile role, that may explain why it is exceptionally elongated in males of several species with long, sexually dimorphic antennae. In other Anoplistes species with strong antennal SD, however, all segments exhibit similar levels of dimorphism. Collectively, our results suggest that alternative pathways to antenna SD evolved rapidly in Anoplistes, perhaps due to different patterns of selection on tactile sensation.