Flight I: Structure & Function of Wings - Biology Courses Server

Flight I: Structure & Function of Wings• Wings in living insects serve a number of functions,including active flying, gliding, parachuting, altitudestability while jumping, thermoregulation, and soundproduction. Today’s lecture covers the structure andfunction of wings in modern insects.• Understanding the evolution of wings requires anunderstanding of the adaptive value of the intermediate ortransitional stages in their development. The next lecturecovers the evolution of wings and problems associatedwith its study.

Structure of wings• Cross section through the wing.Membrane is two layers ofintegument. Veins including nerve,blood space and tracheae. Wingsdo not contain muscle.• Venation. Irregular network ofveins found in primitive insects.Longitudinal veins with limitedcross-veins common in manypterygote groups. Extremereduction of all veins common insmall insects. Longitudinal veinsconcentrated and thickened towardthe anterior margin of the wing.This gives increased efficiency andsupport.

Structure of wings• Pterostigma. Darkened area onforewing in Hymenoptera,Pscoptera, Megaloptera andMecoptera and on both wings inOdonata. Functions as inertialmass in flight. Reduces wing flutterduring gliding in odonates, therebyincreasing flight efficiency.Provides passive control of angle ofattack in small insects, whichenhances efficiency during flappingflight.• Wing folding. Flexion lines reducepassive deformation and enhanceswing as an aerofoil. Fold lines usedin folding of wings over back.

Mechanisms of wing movement• Wing coupling. Orthoptera andOdonata wings are not anatomicallycoupled. Coordination of forewings andhindwings in flight is accomplished bypattern-generator neurons in the centralnervous system. Details of anatomicalwing-coupling varies among taxonomicgroups, suggesting that it evolvedindependently several times.• Halteres in Diptera. Derived from thehindwings. Functions to maintain stabilityin flight.

Two general mechanisms of wingmovement• Direct mechanism. Downwardmovement of the wing is the result of thecontraction of muscles attached directlyto the wing. This flight mechanism isunder control of synchronous flightmuscle. Because each wingbeat iscontrol by a nervous impulse, the directmechanism of insect flight is said to beneurogenic in origin.• Indirect mechanism. Downwardmovement of the wing is the indirectresult of the contraction of musclesattached to the thorax. This flightmechanism is under the control ofasynchronous flight muscle. Becauseseveral to many wingbeats occur forevery nervous impulse, the indirectmechanism of insect flight is said to bemyogenic in origin.

Two general mechanisms of wingmovement• Taxonomic distribution of direct andindirect flight mechanism. Direct mechanismof wing movement is found in the Palaeopteraand the Blatteria. Indirect mechanism of wingmovement is found in the Hymenoptera (bees),Diptera, some Coleoptera and someHemiptera. Other groups (some Coleopteraand Orthoptera) use a combination of directand indirect mechanisms to move wings.• Efficiency of flight production. Muscles usedin flight arise in the coxa in many insects andalso function in leg movement during terrestriallocomotion. Elastic properties of wing hinges,wing muscles and thorax greatly enhance flightefficiency. Elasticity of these structures is dueto the presence of the protein resilin. In locust,86% of the energy used in the upstroke can byrecovered during the downstroke. Elasticity ofthe thorax means that wings are in stableposition only at the top of the upstroke or at thebottom of the downstroke.

Movement of the wings• Stoke plane is the plane in whichwings move relative to the long axis ofthe body. Stroke plane determines therate of forward movement during flight.Insects control turning movements bychanging the stroke plane on one sideof the body relative to that on the otherside of the body. Stroke plane in locustaverages about 30 o . Hovering requiresan average stroke plane of 0 o .• Amplitude of wingbeat is the distancein degrees travelled by the wing tipfrom the top of the upstroke to thebottom of the downstroke. Greateramplitude produces greater poweroutput. Insect control turningmovements by varying the amplitude ofthe wingbeat on both sides of the body.

Movement of the wings• Wingbeat frequency is the number ofwingbeats per second (Hz). Insects withsynchronous flight muscles have lowwingbeat frequencies (≤ 50 Hz) relative toinsects with asynchronous flight muscle(100-1000 Hz). Wingbeat frequency isalso negatively correlated with body size.The greater the wingbeat frequency thegreat the power output and the greater thelift production.• Wing twisting occurs when the relativeposition of the leading and tailing edges ofthe wing changes during the wingbeat.Both passive and active (=muscular)forces are responsible for changing wingtwisting. Wing twisting controls the angleof attack which control lift and forwardmovement of the insect in space.

• Relative wind is the movement of airrelative to the wing. Its two componentsare due to 1) the airspeed of the insectand 2) the velocity of the wing in the strokeplane.• Angle of attack is the angle at which therelative wind strikes the chord of the wing.Insects control the angle of attack byactive and passive twisting of the wing.Changes in the angle of attack are used tocontrol the force of relative wind.• Force of relative wind has twocomponents. Lift is the vertical forceproduced by relative wind. This force iswhat gets insects into the air and keepsthem there. Thrust is the horizontal forceproduced by relative wind. This forcemoves insects forward through the air.Forward thrust is resisted by profile drag(the cross-sectional area the insectpresents to the air) and mostly by induceddrag (development of vortices at the wingtips.)Aerodynamics

Variations in forward flight• Hovering in flight isaccomplished by changingthe stroke plane to nearlyhorizontal and maintaining apositive angle of attackthroughout the wingbeat.• Gliding requires a high lift-todragratio. This isaccomplished mostly bychanging the angle of attackto maximize thrust andminimize drag and negativelift.

Control of flight