DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 339 



At g 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 forwards, 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 ghijkl of figures 4 and 6 ; 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 en- 

 ables the wing during the back stroke still to afford a considerable amount of sup- 

 port. This arrangement permits the wing to travel backwards when 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 downwards and for- 

 wards to a slight extent at this peculiar juncture, the fall of the body, as will be 

 more fully explained hereafter, contributing to the elevation of the wing. 



If figure 5, representing the down or forward stroke, be placed upon figure 

 6, representing the up or backward stroke, it will be seen that the wing crosses 

 its own track more or less completely at every stage of the down and up strokes. 

 As, moreover, the wing draws a current after it, and is pursued in its passage 

 from above downwards by a stream of air which it meets in its passage from 

 below upwards, it follows that the pinion, during the down or forward stroke, 

 creates a current on which it operates during the up or backward stroke, and 

 vice versa; hence the reciprocating action of the wing. 



The wing reciprocates most perfectly, and the figure of 8 is most dis- 

 tinct when the insect is fixed artificially, or when it is hovering of its own accord 

 in a given spot, as is well shown at a b c d efghijklm n op of fig. 8, 

 p. 340, where the wing is represented as screwing steadily downwards. 



Points wherein the Wing differs from the Scull of the Boatman. — The down- 

 ward screwing movement of the wing somewhat resembles the action of an oar 

 in sculling, as represented at a b, c d, x s, of fig. 7, the 

 cross movement occasioned by the rotation of the 

 wing on its long axis as it darts to and fro being 

 shown at m n, o p, q r. There is, however, this 

 marked difference. It is the upper surface of the oar 

 which is effective in sculling, whereas it is the under 

 surface of the wing which is effective in flying." This 

 is accounted for by the fact that the oar simply propels Fi g 7. 



— the boat being buoyant, the wing propelling and 



* A precisely similar difference is found to exist between the aerial or flying wing and the subaquatic 

 VOL. XXVI. TART II. 4 T 



