How insects move 



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Fig. 53. A flesh-fly. Order Diptera 



twice, this does not compare with the controlled, purposeful flight of 

 the other three groups. 



The earhest insects were without wings, and some of the existing 

 groups evidently branched off from the main evolutionary stem in this 

 early, wingless period. These are the Sub-class Apterygota (see Chapter 

 I). All other insects have winged ancestors, even though some of their 

 modern representatives may have lost their wings again. 



The construction of the wings has been discussed in Chapter II : here 

 we are concerned with how they are used in flight. The wings beat with 

 an up-and-down movement, combined with a twisting at the base, 

 which serves both to drive the insect through the air, and to steer it. 

 What the wing-beat does, in eff"ect, is to. bring about a reduaion of air- 

 pressure above the insect — which balances its weight — and to one side 

 or the other — which causes the insea to move bodily in that direaion. 



In most insects the up-and-down beat of the wings is produced by 

 the indirect wing-muscles. The wing is pivoted on the side-plate 

 {pleuron) of the thorax, and its base attached to the dorsal plate (tergum). 

 When the horizontal muscles cause the tergum to bulge upwards this 

 forces the wing-tip down; when the vertical muscles pull the tergum 

 back again, the wings are raised. 



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