PROGRESSION IN OR THROUGH THE AIR. 



141 



the anterior or thick margin of the wing is directed down- 

 wards, and the posterior or thin one upwards. In the act of 

 extension, the margins, in virtue of the wing rotating upon its 

 long axis, reverse their positions, the anterior or thick margins 

 describing a spiral course from below upwards, the posterior 

 or thin margin describing a similar but opposite course from 

 above downwards. These conditions, I need scarcely observe, 

 are reversed during flexion. The movements of the margins 

 during flexion and extension may be represented with a con- 

 siderable degree of accuracy by a figure-of-8 laid horizontally. 



In the bat and bird the wing, when it ascends and de- 

 scends, describes a nearly vertical figure-of-8. In the insect, 

 the wing, from the more oblique direction of the stroke, 



Fig. 69. Fig. 70. 



FiG8. 6T, 68, 69, and 70, show the area mapped out by the left wing of the 

 wasp when the insect is fixed and the wing made to vibrate. These figures 

 Illustrate the various angles made by the wing as it hastens to and fro, how 

 the wing reverses and reciprocates, and how it twists upon itself and de- 

 scribes a figure-of-8 track in space. Figs. 67 and 69 represent the forward or 

 down stroke ; Figs. 68 and 70 the backward or up stroke. The terms for- 

 ward and back stroke are here employed with reference to the head of the 

 insect— Original, 



describes a nearly horizontal figure-of-8. In either case the 

 wing reciprocates, and, as a rule, reverses its planes. The 

 down and up strokes, as will be seen from this account, cross 

 each, other, as shown more particularly at fi^s. 67, 68, 69, 

 and 70. 



In the wasp the wing commences the down or forward 

 stroke at a of figs. 67 and 69, and makes an angle of some- 



