Behavioral analysis of polarization vision in tethered flying locusts

Abstract

For spatial navigation many insects rely on compass information derived from the polarization pattern of the sky. We demonstrate that tethered flying desert locusts (Schistocerca gregaria) show e-vector-dependent yaw-torque responses to polarized light presented from above. A slowly rotating polarizer (5.3° s⁻¹) induced periodic changes in yaw torque corresponding to the 180° periodicity of the stimulus. Control experiments with a rotating diffuser, a weak intensity pattern, and a stationary polarizer showed that the response is not induced by intensity gradients in the stimulus. Polarotaxis was abolished after painting the dorsal rim areas of the compound eyes black, but remained unchanged after painting the eyes except the dorsal rim areas. During rotation of the polarizer, two e-vectors (preferred and avoided e-vector) induced no turning responses: they were broadly distributed from 0 to 180° but, for a given animal, were perpendicular to each other. The data demonstrate polarization vision in the desert locust, as shown previously for bees, flies, crickets, and ants. Polarized light is perceived through the dorsal rim area of the compound eye, suggesting that polarization vision plays a role in compass navigation of the locust.

Appendix

Calculation of periodicity score P

The periodicity in the locusts yaw-torque response is quantified by the periodicity score P (for further details see von Philipsborn and Labhart 1990). It takes into account several parameters from the yaw torque histograms:

$$ \begin{array}{*{20}l} {{{\text{P}} = x \cdot q^{{\mathbf{z}}} } \hfill} & {{} \hfill} \\ {{} \hfill} & {{x = \frac{{{\left( {a + b} \right)}^{2} }} {{{\left( {a - 18} \right)}^{2} + {\left( {b - 18} \right)}^{2} + {\left| {b - a} \right|} + c + m + n}}} \hfill} \\ {{} \hfill} & {{} \hfill} \\ {{} \hfill} & {{q = \frac{{{\sum\limits_{}^A {RG_{i} + {\sum\limits_{}^B {{\left| {RG_{i} } \right|}} }} }}} {{{\sum\limits_{}^{36} {{\left| {RG_{i} } \right|}} }}},\,\,\,0 \leqslant q \leqslant 1} \hfill} \\ {{} \hfill} & {{} \hfill} \\ {{} \hfill} & {{z = \frac{{36 - {\left( {a + b} \right)}}} {6}} \hfill} \\ \end{array} $$

where a is the maximum number of positive yaw-torque responses (RG_i) following each other in one string (A) of the histogram, b is the maximum number of negative yaw-torque responses following each other in one string (B) of the histogram, c is the number of sign alterations (+/−) in the histogram, m is the number of maxima in string A, n is the number of minima in string B, ${\sum\limits_{}^A {RG_{i} } }$ the sum of positive RG_i values in string A, ${\sum\limits_{}^B {{\left| {RG_{i} } \right|}} }$ the overall sum of negative RG_i values in string B, and ${\sum\limits_{i = 1}^{36} {{\left| {RG_{i} } \right|}} }$ the overall sum of all 36 RG_i values.