Insect Flight

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Biol. Rev. (2001), 76, pp. 449471 " Cambridge Philosophical SocietyDOI: 10.1017\S1464793101005759 Printed in the United Kingdom449Mechanics and aerodynamics of insect ightcontrolGRAHAM K. TAYLORDepartment of Zoology, Oxford University, South Parks Road, Oxford, OX1 3PS, UK(E-mail : graham.taylor!zoo.ox.ac.uk)(Received 7 November 2000; revised 14 June 2001ABSTRACTInsects have evolved sophisticated ight control mechanisms permitting a remarkable range of manoeuvres.Here, I present a qualitative analysis of insect ight control from the perspective of ight mechanics, drawingupon both the neurophysiology and biomechanics literatures. The current literature does not permit aformal, quantitative analysis of ight control, because the aerodynamic force systems that biologists havemeasured have rarely been complete and the position of the centre of gravity has only been recorded in afew studies. Treating the two best-known insect orders (Diptera and Orthoptera) separately from otherinsects, I discuss the control mechanisms of dierent insects in detail. Recent experimental studies suggestthat the helicopter model of ight control proposed for Drosophila spp. may be better thought of as afacultative strategy for ight control, rather than the xed (albeit selected) constraint that it is usuallyinterpreted to be. On the other hand, the so-called constant-lift reaction of locusts appears not to be a reexfor maintaining constant lift at varying angles of attack, as is usually assumed, but rather a mechanism torestore the insect to pitch equilibrium following a disturbance. Dierences in the kinematic controlmechanisms used by the various insect orders are related to dierences in the arrangement of the wings, theconstruction of the ight motor and the unsteady mechanisms of lift production that are used. Since theevolution of insect ight control is likely to have paralleled the evolutionary renement of these unsteadyaerodynamic mechanisms, taxonomic dierences in the kinematics of control could provide an assay of therelative importance of dierent unsteady mechanisms. Although the control kinematics vary widely betweenorders, the number of degrees of freedom that dierent insects can control will always be limited by thenumber of independent control inputs that they use. Control of the moments about all three axes (as usedby most conventional aircraft) has only been proven for larger ies and dragonies, but is likely to bewidespread in insects given the number of independent control inputs available to them. Unlike inconventional aircraft, however, insects control inputs are likely to be highly non-orthogonal, and this willtend to complicate the neural processing required to separate the various motions.Key words : Insect ight control, steering, unsteady aerodynamics, kinematics, constant-lift reaction, turning,manoeuvre.CONTENTSI. Introduction ............................................................................................................................ 450II. Manoeuvre control versus reex stabilisation ........................................................................... 451III. General principles of control ................................................................................................... 452IV. Flight control in Diptera (true ies)........................................................................................ 454(1) Longitudinal control ......................................................................................................... 454(2) Lateral control .................................................................................................................. 456450 Graham K. TaylorV. Flight control in Orthoptera (locusts and crickets) ................................................................. 459(1) Longitudinal control ......................................................................................................... 459(2) Lateral control .................................................................................................................. 460VI. Flight control in other insects .................................................................................................. 461(1) Longitudinal control ......................................................................................................... 461(2) Lateral control .................................................................................................................. 463VII. Discussion ................................................................................................................................ 463(1) How many degrees of freedom do insects control?............................................................ 463(2) Evolution of insect ight control systems .......................................................................... 466VIII. Conclusions .............................................................................................................................. 467IX. Acknowledgements .................................................................................................................. 467X. References................................................................................................................................ 467I. INTRODUCTIONInsect ight control has been studied extensivelyfrom a physiological perspective, but its mechanicsare less well known. Even where the kinematicchanges elicited by a given stimulus have beendened, their consequences for aerodynamic forceproduction often remain obscure. Earlier work onight control was rmly rooted in the assumptions ofquasi-steady aerodynamics. However, since detailedquasi-steady analyses and direct force measurements(e.g. Weis-Fogh, 1973; Norberg, 1975; Cloupeau,Devillers & Devezeaux, 1979; Ellington, 1984c ;Ennos, 1989; Dudley & Ellington, 1990b; Wilkin,1990; Zanker & Go$ tz, 1990; Wilkin & Williams,1993) indicate that aerodynamic force production ininsects generally relies upon unsteady mechanisms(Willmott, Ellington & Thomas, 1997; Dickinson,Lehmann & Sane, 1999), our understanding ofinsect ight control must necessarily incorporateunsteady eects. Although our understanding ofunsteady mechanisms is still very limited, morerecent experimental studies (Go$ tz, 1987; Dickinson,Lehmann &Go$ tz, 1993; Dickinson, 1999; Dickinsonet al., 1999) have begun to investigate their role ininsect ight control.An experimental approach integrating physio-logical and mechanical observations is clearly de-sirable. For example, although electromyographicstudies have demonstrated that migratory locustsLocusta migratoria generate dierent motor patterns inresponse to roll and yaw stimuli (Zarnack & Mo$ hl,1977), they cannot tell us whether this is sucient toallow roll and yaw rotations to be producedseparately. This can only be resolved by directlymeasuring the torques and forces that the insectsproduce, or by analysing high-speed lm of theinsects in ight. Although the current literature doespermit some synthesis of aerodynamics and kin-ematics with neurobiology and muscle mechanics(see Kammer, 1985 for a review of this type), I havechosen to concentrate in detail upon the aero-dynamics and kinematics of ight control. Thisapproach is similar to that of most texts on aircraftight mechanics (e.g. Nelson, 1989; Etkin & Reid,1996; Cook, 1997; Vinh, 1993) in that it does notcomplicate the discussion of ight dynamics withdetails of the engineering mechanism that adjusts anaircrafts elevators or varies an insects wingbeatfrequency. Instead, I will concentrate upon howdierent kinematic inputs aect the dependentvariables of position and velocity, as illustratedschematically in Fig. 1. The more general question ofhow many degrees of freedom insects can control isone of the central issues of insect ight control andwill form the theme of this review.The black box between the inputs and outputsof Fig. 1 is properly replaced by the equations ofmotion for a ying body together with a system ofcontrol inputse.g. stroke amplitude,wingbeat frequencypositionattitudevelocityangularvelocityangular accelerationpositionattitudevelocityangularvelocityaccelerationFig. 1. Block diagram illustrating the overall approach ofthis review. The black box represents the equations ofmotion for a ying body, together with a system oftransfer functions providing the mathematical relation-ship between control inputs and their eects upon thedependent variables.451 Mechanics of insect ight controltransfer functions providing the mathematical re-lationship between the control inputs and theireects upon the dependent variables. I shall only bediscussing the mechanics of insect ight control inqualitative terms, however, as the current literaturedoes not allow for even a formal static, let alonedynamic, quantitative framework to