DE PETTTGREW ON THE PHYSIOLOGY OF WINGS. 



349 



hinged and geared to the body so loosely that the body is free to oscillate in a 

 forward or backward direction, or in an up, down, or oblique direction. As a 

 consequence of this freedom of movement, and as a consequence likewise of the 

 speed at which the insect is travelling, the wings during the back stroke are for 

 the most part actually travelling forwards. This is accounted for by the fact 

 that the body falls downwards and forwards in a curve during the up or return 

 stroke of the wings, and because the horizontal speed attained by the body is 

 as a rule so much greater than that attained by the wings, that the latter are 

 never allowed time to travel backward, the lesser movement being as it were 

 swallowed up by the greater. For a similar reason the passenger of a steam- 

 ship may travel rapidly in the direction of the stern of the vessel, and yet be 

 carried forward in space, — the ship sailing much quicker than he can walk. 

 While the wing is descending, it is rotating upon its root as a centre (short axis). 

 It is also, and this is a most important point, rotating upon its anterior margin 

 (long axis), in such a manner as to cause the several parts of the wing to 

 assume various angles of inclination with the horizon. 



Figures 16 and 17 will supply the necessary illustration. 



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Fig. 16. 



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If, for example, we take the common blow-fly when reposing we will find 

 that the plane of the wing (fig. 16 a) is arranged in the same plane with the 

 body, and that both are in a line with the horizon (x #').* When, however, the 



It happens occasionally in insects that the posterior margin of the wing is on a higher level than 

 the anterior one towards the termination of the up stroke as shown at a (dotted line) of fig. 16. In 

 such cases the posterior margin is suddenly rotated in a downward and forward direction at the 



