Free flight maneuvers of stalk-eyed flies: do eye-stalks affect aerial turning behavior?

Abstract

The eyes of stalk-eyed flies (Diopsidae) are positioned at the end of rigid peduncles projected laterally from the head. In dimorphic species the eye-stalks of males exceed the eye-stalks of females and can exceed body length. Eye-stalk length is sexually selected in males improving male reproductive success. We tested whether the long eye-stalks have a negative effect on free-flight and aerial turning behavior by analyzing the morphology and free-flight trajectories of male and female Cyrtodiopsis dalmanni. At flight posture the mass-moment-of-inertia for rotation about a vertical axis was 1.49-fold higher in males. Males also showed a 5% increase in wing length compared to females. During free-flight females made larger turns than males (54 +/- 31.4 vs. 49 +/- 36.2 degrees , t test, P < 0.033) and flew faster while turning (9.4 +/- 5.45 vs. 8.4 +/- 6.17 cm s(-1), ANOVA, P < 0.021). However, turning performance of both sexes overlapped, and turn rate in males even marginally exceeded turn rate in females (733 +/- 235.3 vs. 685 +/- 282.6 deg s(-1), ANCOVA, P < 0.047). We suggest that the increase in eye-span does result in an increase in the mechanical requirements for aerial turning but that male C. dalmanni are capable of compensating for the constraint of longer eye-stalks during the range of turns observed through wingbeat kinematics and increased wing size.

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“…For example, a comparative study on birds with ornamental elongated tails suggests that males compensate for the potential costs of reduced flight performance by increasing their wingspans (Balmford et al, 1994). Similarly, males of stalk-eyed fly that possess wider eye-spans than females and would be at a disadvantage for flight performance show a surprising lack of reduced flight capacity (Swallow et al, 2000;Ribak and Swallow, 2007). Any potential disadvantage is likely masked because of the correlational selection on increased wing size in relation to increased investment in eye span (Swallow et al, 2000;Ribak and Swallow, 2007;Husak et al, 2011b).…”

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