168 



ANIMAL LOCOMOTION. 



forwards. To this there is no exception. A sheet of paper 

 or a card will float along if its anterior margin is slightly 

 raised, and if it be projected with sufficient velocity. The 

 wings of all flying creatures when made to vibrate, twist and 

 untwist, the posterior thin margin of each wing twisting 

 round the anterior thick one, like the blade of a screw. The 

 artificial wing represented at fig. 53 (p. 107) does the same, cd 

 twisting round a b, and g h round e f.' The natural and arti- 

 ficial wings, when elevated and depressed, describe a figure-of-8 

 track in space when the bodies to which they are attached 

 are stationary. When the bodies advance, the figure-of-8 is 

 opened out to form first a looped and then a waved track. I 

 have shown how those insects, bats, and birds which flap 

 ,their wings in a more or less vertical direction evolve trattile 

 or propelling power, ^ and how this, operating on properly 

 constructed inclined surfaces, results in flight. I wish now 

 to show that flight may also be produced by a very oblique 

 and almost horizontal stroke of the wing, as in some insects, 

 e.g. the wasp, blue-bottle, and other flies. In those insects 

 the wing is made to vibrate with a figure-of-8 sculling 



motion in a very oblique direction, and with immense energy. 

 This form of flight difl'ers in no respect from the other, unless 

 in the direction of the stroke, and can be readily imitated, as 

 a reference to fig. 54 will show. 



i 



Fig. 54. 



