1875.] Aérial Locomotion. 243 
the wing made to vibrate, it will fly in an upward direction 
with an undulating motion; that if the anterior or thick 
margin of the wing be directed downwards, the wing will 
describe a waved track and fly downwards ; and if the under 
surface of the wing makes no angle, or a very small angle 
with the horizon, it will dart forward i in a series of curves i 
a horizontal direction. 
Similarly, Prof. Marey says (p. 207) that if the anterior 
margins of the main ribs of his artificial inse¢t be inclined 
upwards, the insect rises vertically, and that if the anterior 
margins of the main ribs be turned downwards a descending 
vertical force 1s developed ; and that if the main ribs be turned 
upwards, and slightly forward, it developes the force which 
sustains it in the aiv, and directs its course in space. 
We may point out many other parallel passages. Dr. 
Pettigrew states (op. cit., p. 335)— 
“The direction of the stroke varies slightly, according to circumstances ; 
but it will be quite proper to assume that the wing of the insect is made to 
vibrate in a more or less horizontal direction, and that of the bird and bat ina 
more or less vertical direction. By a slight alteration in the position of the 
body or by a rotation of the wing in the direction of its length, the vertical 
direGtion of the stroke is converted into a horizontal direction, and vice versa. 
‘¢ The facility with which the direction of the stroke is changed is greatest 
in insects ; it is not uncommon to see them elevate themselves by a figure-of-8 
horizontal screwing movement, and then suddenly changing the horizontal 
screwing into a more vertical one, to dart rapidly forward in a curved line.” 
Compare with the foregoing the following from Professor 
Marey’s new work (p. 207) :— 
“When an insect hovers over a flower, and we see it illuminated obliquely 
by the setting sun, we may satisfy ourselves that the plane of oscillation of 
its wings is nearly horizontal. This inclination must evidently be modified as 
soon as the inseét wishes to dart off rapidly in any direction ; but then the eye 
can scarcely follow it and detect the change of plane, the existence of which 
we are compelled to admit by the theory and the experiments already 
detailed.” 
When speaking of the wing of the bird, Dr. Pettigrew 
points out (Trans. Linn. Soc., vol. xxvi., p. 242) that— 
‘The anterior or thick margin of the wing and the posterior or thin margin 
present different degrees of curvature, so that under certain conditions the 
two margins cross each other, and form a true helix. The anterior margin 
presents two well-marked curves, a corresponding number being found on the 
posterior margin. 
“These curves may, for the sake of clearness, be divided into axillary and 
distal curves; the former occurring towards the root of the wing, the latter 
towards the extremity. 
‘‘The anterior, axillary, and distal curves completely reverse themselves 
during the acts of extension and flexion, and so of the posterior, axillary, and 
distal curves.” 
In like manner Prof. Marey, in his first chapter on the 
flight of birds (at p. 210), says that— 
“Tf we take a dead bird and spread out its wings . . . we see that, at dif- 
ferent points in its length, the wing presents very remarkable changes of plane. 
