28 University of California Publications. [Entomology 



the simplest and most primitive condition, and is to be seen in 

 a butterfly like Pontia. Fig. 9 indicates the nature of the wing 

 motion under these conditions. The angles at which the wings 

 cut the plane EFGH indicate the oblique position given the 

 wing by the air pressure, and explain its efhciency for forward 

 propulsion during both phases of the vibration. 



If one watches the flight of this insect when, during its 

 passage, it is directly opposite him (Fig. 10), or when it is 

 coming directly toward him, he will notice that the bod}^ moves 

 up as the wing moves down, and vice versa. A still more 

 striking example of this bobbing motion may be seen in many 

 Lycsenida?. In the larger Papilionidae and in the higher 

 Nymphalida' the motion is commonly more steady, and in the 



Til 



FIG. 10. Flight of Pontia rapn', showing up and down motion of body. 



Hesperida'. where the wings move very rapidly, the body 

 vibration is difficult to distinguish at all, but is presumably 

 present. A stroke of this kind is not as eflicient as one in 

 which the wing moves more nearly in the plane of body 

 motion, because of the loss of power in the process of lifting 

 the body up and pushing it down again, which must occur 

 with an up and down stroke. 



From this type of stroke to that exhilnted by a fly or bee, or 

 that of a damsel-fly shown in Fig. 5, there are all stages of 

 transition. These may be found in a single family, the 

 hawkmoths. While the stroke of this type is more efficient it 

 is of necessity more rapid, for the wings cease to act as aero- 

 planes and become paddles. A single stroke of a broad "wing 

 may carry an insect many times its own length, whereas, if the 

 stroke is nearly directly backward, the insect can not make much 

 more than its own length, even counting the movement due to 

 momentum, which acts, during the recoil of the wing, in keep- 

 ing up the progress. The l)ackward stroke is the most efficient 

 propelling force in this type, and while this is being made the 

 tip of the wing must, to be effective, move backward consid- 

 erably faster than the body moves forward. The forward stroke 



