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3.3.4. Control of Wing Movements

CHAPTER 14 Like leg movements, wing flapping is a centrally generated, rhythmically repeated process; that is, each wing has its own muscles and motor neurons whose activity is initiated by specific interneurons in each segmental ganglion. In Locusta migratoria Robertson and Pearson (1982) identified 25 such interneurons whose spike activity or membrane potential changed rhythmically during fictive flight.* Flight is not, of course, simply the flapping of the wings. It is a complex process and at any point in time an insect is concerned with monitoring, and varying if necessary, many parameters. During flight, an insect must be able to control the frequency of wing beat, the amount of lift and thrust developed, and the direction (stability) of flight. It must also have control mechanisms for the initiation and termination of flight. The sense organs that provide the central nervous system with information on what changes are occurring in relation to flight are the compound eyes and proprioceptors strategically distributed over the body, especially on the head, wings, and legs. Responses to stimuli received by these organs are usually mediated via changes in the nature of the wing movements (particularly the degree of twisting and wing-beat frequency).

Wing-beat frequency varies widely among different insects as has been noted already in the introduction to this discussion of flight. In fliers whose wing-beat frequency is low (e.g., the desert locust, about 15-20 beats/sec) neurogenic (synchronous) control occurs. That is, there is a 1:1 ratio between wing-beat frequency and nervous input. In such fibers, therefore, wing-beat frequency can be varied by altering the rate at which nerve impulses arrive at the muscles. However, there are limits to this arrangement (maximum frequency about 100 beats/sec) because of the refractory period required to return the muscle to its resting state after each contraction. Insects whose wing-beat frequency is high, for example, the bee (190 beats/sec) and midge Forcipomyia (up to 1000 beats/sec), use a myogenic (asynchronous) system where the frequency of muscle contraction is much greater than that of nervous input (ratios of between 5:1 and 40:1). Such a system is possible only where there are antagonistic muscles that contract regularly and alternately. These muscles have the property of contracting autonomously when the tension developed in them as a result of stretching reaches a critical value. In the case of the flight musculature, the alternating contractions are, in a sense, self-perpetuating, though their initiation and cessation are under nervous control. There is also evidence to suggest that changes in the frequency at which nerve impulses arrive at these muscles can modify their frequency of contraction. However, as contractions are tension-dependent, their frequency can also be altered by modifying the elastic resistance in the exoskeleton. In Diptera, for example, this is achieved by contraction or relaxation of the pleurosternal muscles, which serves to move the pleural wing process closer to or farther from the tergal hinge (Figure 14.8).

As noted earlier, during a beat a wing does not make simple up and down movements, but rather twists about the vertical axis so that its tip describes an ellipse (Schistocerca) or figure eight (Apis, Musca).'The size and direction of the twisting force (torque), which effectively measure the lift and thrust developed, are monitored by campaniform sensilla at the base of each wing (halteres in Diptera—see below). Input from the sensilla initiates reflex excitation of the basalar and subalar muscles, which regulate the extent to which the wing twists. This mechanism is known as the lift control reflex. Schistocerca employs such a reflex to control the value of lift and, in flight, holds its body at a fairly steady angle to the horizontal. Other insects vary the angle at which the body is held in flight in order to alter the lift and thrust components.

* Fictive flight: The locust preparation is actually a wingless, legless insect pinned in a dish! It is stimulated to "fly" by gently blowing on the frons.

FIGURE 14.15. Diagram showing axes about which a flying insect may rotate.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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