An Introduction to the Invertebrates, Second Edition - tiera.ru

198 INSECTAmeet the airstream over the upper surface. A vortex is set up atthe back (the ‘trailing edge’), pulling towards it the air over theupper surface so that it moves faster than the air over the lowersurface, providing lift. A large bird can glide: a large wing providesa great difference between the rates of air flow over the upper andlower wing surfaces. In large flying animals, flapping is not essentialto obtain lift.The small size of insects demands a rather different mechanism,as detailed below.Flapping: an insect, like a small bird, must flap to obtainsufficient lift, by increasing the rate of air flow over wingsmoving much faster than the main body (lift is proportional tothe square of the velocity). The small size of insects is favourablefor flapping, because the power-to-weight ratio is high.The boundary layer of inert air sticking to the wing surfacesconstitutes the greatest problem for a very small flier. Owing tothe viscosity of air any wing has a boundary layer, of the samewidth for any size of wing: a boundary layer of 1 mm isnegligible in birds, but relatively enormous in insects (Figure15.2d). In consequence of this large ‘viscous drag’, the liftto-dragratio (on which the speed and efficiency of flightdepends) is at best only about a tenth of that in birds. Anyincrease in surface area increases viscous drag, and accordinglyinsect wings are flat plates, not aerofoil sections. However, anaerofoil section is produced when a flat plate with an adheringboundary layer is moved through the air, and this shape enableslift to be produced.Obtaining sufficient lift remains difficult for a very small flier,as there is very little difference between air flow rates over thetwo wing surfaces, and to obtain sufficient lift small insectsmust flap their wings very fast indeed. Many insects turn theirwings to get lift on the upstroke as well as the downstroke. Anyincrease in the rate of air movement over the upper surfacecompared to the lower surface will increase lift: for examplesmall insects may bend the wing along its length and suddenlyrelease it, or clap the wings together dorsally and then fling theleading edges apart (Figure 15.2e). Hovering, when there is noair movement provided by the whole body moving forward, isthe most exacting form of flight. Lift is acquired only afterseveral strokes and even then it is only obtained near thebottom of the downstroke and lost again as the wing turns forthe upstroke.Recent analysis emphasises that the forces acting on a wing thatstops and starts repeatedly as it beats up and down are different fromthose on a wing moving at constant speed. Air over the leading edgeforms a vortex bound to the upper surface of the wing and spirallingout towards the wing tip (Figure 5.2f). This is the main aerodynamicmechanism enabling small insects to fly.