326 DP PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



manner that the wing as a whole presents the same curves as those displayed 

 by the insect's wing, or by the wing of the eagle where the bones, muscles, and 

 feathers have attained a maximum development. The conformation of the 

 wing is such that it presents a waved appearance in every direction — the waves 

 running longitudinally, transversely, and obliquely. The greater portion of the 

 pinion may consequently be removed without essentially altering either its form 

 or its functions. This is proved by making sections in various directions, and 

 by finding that in some instances as much as two-thirds of the wing may be 

 lopped off without materially impairing the power of flight. Thus, in the summer 

 of 1866,"" I removed the posterior two-thirds from either wing of a blow-fly, 

 and still the insect flew, and flew well. The only difference I could perceive 

 amounted to this, that the fly, while it could elevate itself perfectly, flew in 

 circles, and had less of a forward motion than before the mutilation. It had 

 in fact lost propelling or driving power, the elevating or buoying power remain- 

 ing the same. I took another blow-fly and removed the tip or outer-third of 

 either wing, and found that the driving-power was the same as before the muti- 

 lation, while the elevating or buoying power was slightly diminished. These 

 experiments prove that the posterior or thin elastic margin of the wing is more 

 especially engaged in propelling, the tip in elevating.! " The spiral nature of 

 the pinion is most readily recognised when the wing is seen from behind and 

 from beneath,^ and when it is foreshortened. § It is also well marked in some 

 of the long winged oceanic birds when viewed from before, || and cannot escape 

 detection under any circumstances, if sought for, — the wing being essentially 

 composed of a congeries of curves, remarkable alike for their apparent sim- 

 plicity and the subtlety of their detail." 



The Wing during its action Reverses its Planes, and describes a Figure of 8 

 track in space. — " The twisting or rotating of the wing on its long axis is parti- 

 cularly observable during extension and flexion in the bat and bird, and like- 

 wise in the insect, especially the beetles, cockroaches, and others which fold 

 their wings during repose. In these in extreme flexion the anterior or thick 

 margin of the wing is directed downwards, and the posterior or thin one up- 

 wards. In the act of extension, however, the margins, in virtue of the wing 

 rotating upon its long axis, reverse their positions, the anterior or thick mar- 

 gins 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 



* Op. cit, pages 219, 220, 221, 222. 



t Eor further experiments in this direction, see footnote to pages 3G1 and 362. 



% Op. cit, Plate XV. figs. 68, 69, 70, 73, and 74. 



§ Op. cit., Plate XV. figs 61 and 62. 



I| Op. cit, page 253 ; Diagram 18 A, a'b', d'e'f.' 



