Insects and their world 



Fig. 54. An acalyptrate fly, Order Diptera 



The twisting of the wing is caused by direct muscles, which are 

 attached to one or other of the axillary sclerites, Httle hard pieces fixed 

 in the base of the wing. The principal veins, or stiffening ribs of the 

 wing arise from these sclerites, and the action of the direct flight 

 muscles twists the wing by acting on these veins. 



It will be noted that none of the flight muscles is aaually inside the 

 structure of the wing, which is entirely operated by forces acting on the 

 extreme base of the veins; a remarkable mechanical feat. 



There is nearly always a tendency for the veins of the wing to be 

 thicker and closer together towards the leading edge of the wing, giving 

 this additional stiffness, as in an aircraft wing. The trailing edge of an 

 aircraft wing is thin, and mainly composed of hinged portions, flaps and 

 ailerons, which are used to control speed and stabihty in flight. The 

 trailing edge of an insect's wing is similarly divided into lobes, the shape 

 and flexibility of which varies enormously in different insects, (cf. Figs. 

 26-28, 30-32). The remarkable thing about the insect's wing is that these 

 lobes of the flexible trailing edge are not under muscular control, but 

 bend automatically as the wing moves, and their efficiency in action is a 

 matter of design, not of operation. 



The direct and indirect muscles are not independent of each other, 

 because the side- wall of the thorax is not a rigid fulcrum. Although 

 strengthened internally, the pleura have some elasticity, and con- 



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