Constructing an aerodynamic explanation for the forces

generated during the flapping flight of insects is an important challenge in the study of both animal locomotion and fluid mechanics. Two complimentary experimental approaches have been used to study animal aerodynamics; one that focuses directly on the forces generated by flapping wings and another that attempts to reconstruct these forces by careful analysis of the resulting wake (Brodsky, 1994; Ellington, 1984; Rayner, 1979; Spedding et al., 1984). Due to their small size and rapid stroke frequency, direct measurement of forces on insect wings has not been possible. Although researchers have succeeded in capturing whole-body forces on tethered insects, such measurements are difficult to interpret because of contamination by wing mass inertial forces (Cloupeau et al., 1979; Dickinson and Götz, 1996; Wilkin and Williams, 1993; Zanker and Götz, 1990). Although several studies have documented the flow pattern around the flapping wings of tethered insects (Brodsky, 1994; Dickinson and Götz, 1996; Ellington et al., 1996; Grodnitsky and Morozov, 1993; Willmott et al., 1997), these studies have not yet yielded quantitative measures of sufficient spatial and temporal resolution to permit estimates of either mean or instantaneous flight force. Even if fluid motion could be quantified to sufficient resolution, the reciprocating stroke pattern seen in insects still creates complex time-dependent flows that are difficult to quantitatively interpret. Currently, two approaches attempt to circumvent these difficulties in measuring force production in living insects. The first is through the use of dynamically scaled robots programmed with kinematics derived from studies of flying or 2257 The Journal of Experimental Biology 206, 2257-2272 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00381