254 MOVEMENT 



Fig. 179 shows a bee in various phases of flight. The 

 insect sometimes assumes almost a horizontal position, 

 in which case the lower part of its body is much nearer 

 the object-glass than is its head, and yet both ex- 

 tremities are equally well defined in the photograph. 



The successive images are separated by an interval 

 of ^0 of a second (a long time when compared to the 

 total time occupied by a complete wing movement, i.e. 

 j^q of a second). And hence it is useless to attempt 

 to gain a knowledge of the successive phases of move- 

 ment, by examining the successive photographs of a 

 consecutive series representing an insect in flight. 

 Nevertheless, an examination of isolated images affords 

 information of extreme interest with regard to the 

 mechanism of flight. 



We have seen that owing to the resistance of the 

 air the expanse of wing is distorted in various direc- 

 tions by atmospheric resistance. Now, as the oscilla- 

 tions during flight are executed in a horizontal plane 

 the obliquity of the wing surface ought to diminish 

 the apparent breadth of the wing. This appearance 

 can be seen in Fig. 180. There is here a comparison 

 between two tipulse : the one in the act of flight, the 

 other perfectly motionless and resting against the 

 glass window. 



The motionless insect maintains its wings in a 

 position of vertical extension, the plane is therefore 

 at right angles to the axis of the object-glass. The 

 breadth of the wing can be seen in its entirety ; the 

 nervures can be counted, and the rounding off of 

 the extremities of the wings is perfectly obvious. On 

 the other hand, the flying insect moves its wings in 

 a horizontal direction, and owing to the resistance of 

 the air the expanse of the wings is obliquely disposed, 

 and only the projection of its surface can be seen 

 in the photograph. This is why the extremity of the 



