142 



ANIMAL LOCOMOTION. 



thing like 45° with the horizon (x x'). At b (figs. 67 and 69) 

 the angle is slightly diminished, partly because of a rotation 

 of the wing along its anterior margin (long axis of wing), 

 partly from increased speed, and partly from the posterior 

 margin of the wing yielding to a greater or less extent. 



At c the angle is still more diminished from the same 

 causes. 



At d the wing is slowed slightly, preparatory to reversing, 

 and the angle made with the horizon (x) increased. 



At e the angle, for the same reason, is still more increased; 

 while at / the wing is at right angles to the horizon. It is, 

 in fact, in the act of reversing. 



At ^ the wing is reversed, and the up or back stroke 

 commenced. 



The angle made at g is, consequently, the same as that 

 made at a (45°), with this difference, that the anterior margin 

 and outer portion of the wing, instead of being directed for- 

 wards^ with reference to the head of the insect, are now 

 directed backwards. 



During the up or backward stroke all the phenomena are 

 reversed, as shown at ghij kl of figs. 68 and 70 (p. 141); the 

 only difference being that the angles made by the wing with 

 the horizon are somewhat less than during the down or forward 

 stroke — a circumstance which facilitates the forward travel 

 of the body, while it enables the wing during the back stroke 

 still to afford a considerable amount of support. This 

 arrangement permits the w^ing to travel backwards while the 

 body is travelling forwards ; the diminution of the angles 

 made by the wing in the back stroke giving very much the 

 same result as if the wing were striking in the direction of 

 the travel of the body. The slight upward inclination of the 

 wing during the back stroke permits the body to fall down- 

 wards and forwards to a slight extent at this peculiar junc- 

 ture, the fall of the body, as has been already explained, 

 contributing to the elevation of the wing. 



The pinion acts as a helix or screw in a more or less hori- 

 zontal direction from behind forwards, and from before back- 

 wards ; but it likewise acts as a screw in a nearly vertical 

 direction. If the wing of the larger domestic fly be viewed 



