360 



DR PETTIGKEW ON THE PHYSIOLOGY OF WINGS. 



in the overlapped arched condition, and towards the end of the up stroke they 

 are gradually separated to increase the area and prepare them for making the 

 down stroke in a manner precisely analogous to what happens in bats and birds. 

 They are then made to descend in their flattened condition, the first wing passing- 

 over the second towards the termination of the down stroke as just stated. 

 Nor is this all. While the wings are being depressed and made to overlap 

 more or less completely, and while they are being elevated and spread out, 

 double and opposite curves are being developed along their anterior, posterior, 

 and outer margins. This is a somewhat remarkable circumstance, as the butter- 

 fly is perhaps the most awkward flying creature that exists. It seems to prove 

 that the presence of double or figure of 8 curves, is indispensable to flight. These 

 points are illustrated at figs. 29, 30, 31, 32, 33, and 34. At a, of fig. 29, the 



Fig. 29. 



Fig. 30. 



^^^□XQ^ 



Fig. 32. 



Fig. 33. 



Fig. 34. 



concavity of the first wing is directed downwards, the concavity of the second 

 wing being directed slightly upwards as at b. The two curves taken together 

 give a double or wave curve. In this figure the two wings are separated or 

 spread out and ready to give the down stroke. At fig. 30 the two wings are 

 separated to the utmost, and in the act of making the clown stroke. Here the 

 concavity of both wings is directed downwards as at a, a very small portion of 

 the second wing only curving upwards (b). At fig. 31 the down stroke is com- 

 pleted, the first wing overlapping the second, and both being deeply concave on 

 their under surfaces, as shown at a. They are now in a condition to make the 

 up stroke, which is the reverse of the down one, and need not be described. 

 The curves produced along the anterior and posterior margins of the wings of 

 the butterfly during the up and down strokes are seen at figs. 32, 33, and 34. 

 At fig. 32, the curves formed along the anterior (c d) and posterior (ef) margins 

 of the first wing at the beginning of the down stroke, are represented. At fig. 

 33 the wing is represented, as seen at the middle of the down stroke, and the 



