Sexual ornaments do not impair predator evasion in stalk-eyed flies

Sexually selected ornaments can impose biomechanical constraints on locomotion and potentially affect predator avoidance. In stalk-eyed flies, males possess elongated eyestalks that increase head width and may elevate rotational inertia during flight. Compensatory changes in wing and thorax morphology may offset these costs. We tested whether ornament size and compensatory morphology influence escape performance in response to a simulated predator. We quantified evasive behavior and flight performance by measuring reaction time, changes in velocity, and flight trajectory curvature. Individuals were categorized by response type (yaw vs. non-yaw), sex, and compensation status based on their position relative to the relationship between eyespan and wingspan. Our results showed that flies-initiated escape responses within approximately 82 ms and frequently performed yaw-turn maneuvers. Despite males exhibiting approximately 41% greater eyespan than females, escape performance did not differ between sexes. Compensation groups differed in ornament size relative to wing size but exhibited little difference in escape performance. Response type appears to be associated with the centrifugal force due to the flight trajectory prior to the predator interaction.