Muscles And Locomotion

FIGURE 14.11. (A) Changes in the angle at which a wing is held in flight relative to direction of movement. Arrows indicate the angle at which air strikes the wing. Numbers indicate chronological sequence of wing positions during a stroke; and (B,C) Magnitude of lift and thrust approximately midway through downstroke and upstroke, respectively. [A, after M. Jensen, 1956, Biology and physics of locust flight. III. The aerodynamics of locust flight, Philos. Trans. R. Soc. Lond. Ser. B 239:511-552. By permission of The Royal Society, London, and the author. B, C, after R. F. Chapman, 1971, The Insects: Structure and Function. By permission of Elsevier/North-Holland, Inc., and the author.]

air flows unequally over the top and bottom of the ball causing it to swerve. Dickinson et al. (1999) suggested that the transient increase in lift which occurs as a wing rotates just before the end of a stroke has a similar explanation. The key to rotation-generated lift is the timing of wing rotation. If rotation precedes stroke reversal, which is analogous to imparting backspin to the ball, positive lift is generated; if it follows stroke reversal (akin to topspin), the lift is negative. Rotation-generated lift is probably of particular importance in the fine control of flight maneuvres. The remaining contribution is derived from wake

FIGURE 14.12. Air flow around the wing and the resulting forces at points during a wing stroke. Delayed stall (1) results from formation of a leading edge vortex on the wing. Rotation-generated lift (2,3) occurs when the wing rapidly rotates at the end of the stroke. Wake capture (4,5) results from collision of the wing with the wake shed during the previous stroke. [Reprinted from Encyclopedia of Insects, M. Dickinson and R. Dudley, Flight, pages 416-426, @ 2003, with permission from Elsevier.]

FIGURE 14.12. Air flow around the wing and the resulting forces at points during a wing stroke. Delayed stall (1) results from formation of a leading edge vortex on the wing. Rotation-generated lift (2,3) occurs when the wing rapidly rotates at the end of the stroke. Wake capture (4,5) results from collision of the wing with the wake shed during the previous stroke. [Reprinted from Encyclopedia of Insects, M. Dickinson and R. Dudley, Flight, pages 416-426, @ 2003, with permission from Elsevier.]

capture; that is, after reversing direction the wing does not move through undisturbed air but re-encounters the vortices shed from the wing tips during the previous stroke to produce a pulse of lift. Collectively, delayed stall, rotational circulation and wake capture enable insects to generate lift equivalent to several times their body weight.

A few, especially very small, insects such as thrips, whitefly, and parasitoid wasps (whose wingspans are about 1 mm or less) use a particular form of rotation-generated lift known as the "clap and fling" mechanism (Weis-Fogh, 1973) (Figure 14.13). In this system lift is generated by rotation of the wings as they separate after they have clapped together at the end of the upstroke and are flung apart as the downstroke begins. As the wings rotate, a starting vortex is formed above each wing, which serves to generate a bound vortex around each wing. As outlined above for conventionally flapping wings, the vortex causes the velocity of air passing over the top of each wing to increase, while that passing beneath the wing decreases. Thus, by Bernoulli's principle, lift is generated.

3.3.3. Mechanics of Wing Movements

All insects use the indirect tergosternal or tergocoxal muscles to raise the wing. Contraction of these muscles pulls the tergum down so that its points of articulation with the wing fall below the articulation of the wing with the pleural wing process which serves as a fulcrum (Figure 14.14A). In most insects indirect muscles are also used to lower the wing. Shortening of the dorsal longitudinal muscles causes the tergum to bow upward, raising the anterior and posterior notal processes above the tip of the pleural wing process

FIGURE 14.13. Clap and fling mechanism for generating lift. The wings clap together at the end of the upstroke (A), then are flung apart as the downstroke begins (B,C), creating a bound vortex (D) (greater air speed over the upper wing surface compared to the lower wing surface, thereby creating lift). [From T. Weis-Fogh, 1975, Unusual mechanisms for the generation of lift, Sci. Amer. 233 (November):80-87. Original drawn by Tom Prentiss. By permission of Nelson H. Prentiss.]

FIGURE 14.13. Clap and fling mechanism for generating lift. The wings clap together at the end of the upstroke (A), then are flung apart as the downstroke begins (B,C), creating a bound vortex (D) (greater air speed over the upper wing surface compared to the lower wing surface, thereby creating lift). [From T. Weis-Fogh, 1975, Unusual mechanisms for the generation of lift, Sci. Amer. 233 (November):80-87. Original drawn by Tom Prentiss. By permission of Nelson H. Prentiss.]

FIGURE 14.14. Diagrammatic transverse sections of thorax to show muscles used in upstroke and downstroke. (A) Use of indirect muscles to raise wing; (B) use of indirect muscles to lower wing; and (C) use of direct muscles to lower wing. [After R. F. Chapman, 1971, The Insects: Structure and Function. By permission of Elsevier/North-Holland, Inc., and the author.]

FIGURE 14.14. Diagrammatic transverse sections of thorax to show muscles used in upstroke and downstroke. (A) Use of indirect muscles to raise wing; (B) use of indirect muscles to lower wing; and (C) use of direct muscles to lower wing. [After R. F. Chapman, 1971, The Insects: Structure and Function. By permission of Elsevier/North-Holland, Inc., and the author.]

Beekeeping for Beginners

Beekeeping for Beginners

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