Muscles And Locomotion

in Figure 3.18. The essential features are as follows. Each segment contains two large 453

intersegmental invaginations, the prephragma and postphragma, between which the dorsal longitudinal muscles stretch. The alinotum bears on each side an anterior and a posterior notal process, to which the wing is attached via the first and third axillary sclerites. The pleuron is largely sclerotized and articulates with the wing by means of the pleural wing process, above which sits the second axillary sclerite. The hinge so formed is important in wing movement because of the resilin that it contains (but see Section 3.3.3). Two other important articulating sclerites, which are usually quite separate from the sclerotized portion of the pleuron, are the anterior basalar and posterior subalar. Internally, the pleuron and sternum are thickened, forming the pleural and sternal apophyses, respectively, which brace the pterothorax. In some insects these apophyses are fused, but generally they are joined by a short but powerful pleurosternal muscle (Figure 14.8B).

The muscles used in flight may be separated into three categories according to their anatomical arrangement (Figure 14.8A, B). The indirect flight muscles include the dorsal longitudinal muscles, dorsoventral muscles, oblique dorsal muscles, and oblique intersegmental muscles. The direct muscles are the basalar and subalar muscles, and the axillary muscles attached to the axillary sclerites (including the wing flexor muscle, which runs from the pleuron to the third axillary sclerite). In the third category are the accessory indirect muscles that comprise the pleurosternal, tergopleural, and intersegmental muscles. Their function is to brace the pterothorax or to change the position of its components relative to each other. In addition, certain extrinsic leg muscles may also be important in wing movements. For example, in Coleoptera, whose coxae are fused to the thorax, the coxotergal muscles can assist the dorsoventral muscles in supplying power to raise the wings. In other species with articulated coxae the upper point of insertion of the extrinsic coxal muscles may change to the basalar or subalar. Thus, the muscles can alter both leg and wing positions, that is, they may have a dual locomotory function. For example, in Schistocerca gregaria the anterior and posterior tergocoxal (indirect) muscles act synergistically during flight (1) to provide power for the upstroke of the wings and (2) to draw the legs up close to the body. This is achieved through polyneuronal innervation, whereby the parts of the tergocoxal muscles that receive slow and inhibitory motor axons are responsible for the drawing up of the legs, and the muscle fibers innervated by the fast axon move the wings. In contrast, when the insect is running, the anterior and posterior tergocoxal muscles function antagonistically, effecting promotion and remotion, respectively, of the legs. In the same species, the (direct) second basalar and subalar muscles, while acting synergistically to aid the indirect muscles in the production of power for the downstroke of the wings, act antagonistically to bring about wing twisting (pronation and supination, respectively) or, when running, promotion and remotion, respectively, of the legs (Figure 14.9) (Wilson, 1962).

3.3.2. Aerodynamic Considerations

Flight occurs when the air pressure is greater on the lower than on the upper surface of a body. The wings, with their large surface area, act as aerofoils, that is, the portion of the body that is largely responsible for lift (the component of net aerodynamic force that acts perpendicular to the direction of movement). In a fixed-wing aircraft flying horizontally, lift on the wings is vertical (equal and opposite to the aircraft's weight). In insects, however, where the wings flap, twist, and deform cyclically, lift on the wings varies through the cycle.

Lift develops when air is accelerated unequally over the upper and lower surfaces of a wing. Aerofoils on fixed-wing aircraft are designed with their upper surface curved,

FIGURE 14.8. (A) Indirect; and (B) accessory indirect muscles and direct muscles of right side of wing-bearing segment, seen from within. [After J. W. S. Pringle, 1957, Insect Flight. By permission of Cambridge University Press, London.]

their lower surface flat (Figure 14.10A), so as to make use of Bernoulli's principle that the pressure exerted by flowing air is inversely related to the square of the velocity. Air flowing over the upper surface of an aerofoil travels farther and therefore has a greater velocity than the air flowing beneath. Hence, the pressure beneath the aerofoil is greater than that above

FIGURE 14.9. Diagram illustrating the synergistic-antagonistic relationships between bifunctional muscles in the thorax of the desert locust. [After D. M. Wilson, 1962, Bifunctional muscles in the thorax of grasshoppers, J. Exp.Biol. 39:669-677. Bypermission of Cambridge University Press, London.]

FIGURE 14.9. Diagram illustrating the synergistic-antagonistic relationships between bifunctional muscles in the thorax of the desert locust. [After D. M. Wilson, 1962, Bifunctional muscles in the thorax of grasshoppers, J. Exp.Biol. 39:669-677. Bypermission of Cambridge University Press, London.]

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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