328 DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



strongly resisting surface the one instant, and a comparatively narrow, non- 

 resisting cutting edge the next. The figure of 8 action of the wing explains 

 how an insect or bird may fix itself in the air, the backward and forward 

 reciprocating action of the pinion affording support, but no propulsion. In 

 these instances, the backward and forward strokes are made to counterbalance 

 each other." 



The Wing, when advancing ivith the body, Describes a Waved Track. — 

 " Although the figure of 8 represents with considerable fidelity the twisting of 

 the wing upon its axis during extension and flexion, when the insect is playing 

 its wings before an object, or still better, when it is artificially fixed, it is other- 

 wise when the down-stroke is added, and the insect is fairly on the wing, and 

 progressing rapidly. In this case the wing, in virtue of its being carried for- 

 wards by the body in motion, describes an undulating or spiral course. * .... 

 The down and up strokes are compound movements, — the termination of the 

 down-stroke embracing the beginning of the up-stroke, the termination of the 

 up-stroke, on the other hand, including the beginning of the down-stroke. 

 This is necessary in order that the down and up strokes may glide into each 

 other in such a manner as to prevent jerking and unnecessary retardation, — 

 the angle made by the under surface of the wing with the horizon during the 

 first part of the down-stroke being increased to support and propel the insect, 

 and decreased during the second part to prepare it for making the up-stroke, 

 and to diminish the friction caused by the wing itself, while it does not inter- 

 fere with its sustaining power." .... 



The Margins of the Wings thrown into Opposite Curves diwing Extension 

 and Flexion. — " The anterior or thick margin of the wing and the posterior or 

 thin margin present different degrees of curvature, so that under certain con- 

 ditions the two margins cross each other, and form a true helix (page 361, 

 fig. 37). t The anterior margin (r, s) presents two well-marked curves, a corre- 

 sponding number being found on the posterior margin (t, u). These curves ma) 7- , 

 for the sake of clearness, be divided into axillary curves and distal curves, the 

 former occurring towards the root of the wing, the latter towards its extremity. 

 The curves (axillary and distal) found on the anterior margin of the wing are 

 always the reverse of those met with on the posterior margin, i.e., if the con- 

 vexity of the anterior axillary curve be directed downwards (r),\ that of the 

 posterior axillary curve (t) is directed upwards,§ and so of the anterior and 

 posterior distal curves (s, u). The two curves, axillary and distal, occurring on 

 the anterior margin of the wing, are likewise antagonistic, the convexity of the 

 axillary curve (r) being always directed downwards,|] when the convexity of the 



* Op. cit, page 233, Diagram. 6. f Op. cit.. Plate XV. figs. 70, 73, and 74. 



| Op. cit, Plate XV. fig. 73, e. § Op. cit, Plate XV. fig. 73, a, c. 



\\ Op. cit., Plate XV. fig. 73, c. 



