42 THORAX, LEGS, AND WINGS. 
resistance of the air acts upon it from above down- 
wards. 
The wing which descends has at the same time a 
forward motion, and the inclination taken by the 
plane under the influence of resistance causes the 
oblique descent. An inclined plane which strikes the 
air has a tendency to move in the direction of its own 
inclination, therefore the insect is propelled forwards. 
In an ascending wing the plane is reversed, by which 
means the forward motion is still maintained. Marey 
found that owing to the slight bending of the nervure 
the tips of the wings really described a figure 8 . 
He illustrated these movements mechanically, by 
means of an instrument carrying a rod, to which was 
fixed a membrane similar to an insect’s wing, and by 
moving this rapidly in a vertical plane between two 
candles, he demonstrated that, by the pushing aside 
of the membrane, at each alternate movement the air, 
acted upon by the wing, received an impulse in an 
opposite direction. In the candle placed by the thin 
edge of the wing, the flame is strongly blown away by 
the air produced. In the front of the wing, on the con- 
trary, the flame is strongly drawn towards it, showing 
that the current of air is in the same direction both in 
the upward and downward movement of the wings. 
The wings, in part, act, Marey says, as a propeller 
does in water, or as an oar, when used in the stern of 
a boat, propels it forward. 
Bees, as we have said, and as every bee-keeper 
will have noticed, can fly backwards, and can stop 
when they like, almost suddenly. This is accom- 
plished by changing the inclination of the plane of 
