Proceedings of The Physiological Society
University of Central Lancashire (2002) J Physiol 543P, S221
Targeted limb movements of insects
Tom Matheson and Volker Durr
Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK and Abteilung fur Biokybernetik und Theoretische Biologie, Fakult
Touching different sites on the body of a locust elicits scratching movements of one or more legs that are directed towards the stimulus (Berkowitz & Laurent, 1996; Matheson, 1998). To determine how these movements are generated requires an understanding of how spatial information signalling target location is used to drive groups of motor neurones that control muscles of several limb joints. To address this question we have analysed in detail the kinematics of targeted scratching movements of the locust Schistocerca gregaria. Mechanical activation of localised arrays of tactile hairs on a front wing elicits appropriately targeted movements of a hindleg, which scratches the target site. Single frame analysis of 839 videotaped scratches from eight animals has allowed us to quantify leg joint angles and tarsal trajectories in relation to the start posture of the leg, three different loading conditions, and the target position on the wing.
We show that touching stimulus sites along the length of the wings elicits a graded series of scratches that form a behavioural continuum. There is no evidence for discrete switching between alternative strategies, at least for the combinations of two start positions and five target locations that we used. When the leg was unloaded, the precision of scratching (computed as the minimum distance between the tarsus and the target) was similar for all except the most anterior stimulus site, where it was reduced. The mean direction of tarsal movement calculated over the first 200 ms of each scratch revealed a biased lift-off direction that depended on the initial leg posture. This part of the response therefore differed systematically from the optimal direction required to reach the target.
A detailed analysis of the two-dimensional 'response fields' of tarsal position for different stimulus sites along the wing provides powerful evidence for precise graded limb targeting in insects.
Loading the leg with a mass of 142 mg at the proximal femur, distal femur or distal tibia (unloaded leg mass is 117 mg) made no significant difference to the precision of targeting (ANOVA, P > 0.05, N = 3 animals). This suggests that position is a controlled factor in these targeted scratching movements.
This work was supported by a BBSRC Advanced Research Fellowship and a Royal Society Research Grant to T.M. and a FIF2 grant of the University of Bielefeld to V.D.
Where applicable, experiments conform with Society ethical requirements