DR PETTIGREW ON THE PHYSIOLOGY OE WINGS. 



359 



I am induced to believe that the wing is folded after this fashion in certain 

 cases during the back or return stroke, although the action of the pinion is so 

 rapid that I have hitherto failed to make it out. The folding of the first wing 

 upon itself in the wasp occurs in the line g s of fig. 25 ; the folding of the first 

 wing upon itself and of the second upon the first, being seen at fig. 26 (h d) ; 

 and the two wings, when folded and ready to make the return stroke, at fig. 

 27 (ds). The course pursued by the folded wings during the back stroke is 

 indicated at ghijklm of fig. 19, page 351. Figure 28 represents the wing of 



Fig. 25. 



Fig. 26. 



I 9 A 



Fig. 27. 



h 9 



Fig. 28. 



the crane-fly, which has, I believe, a similar action, the thin posterior margin, 

 fg h i, being folded during the back or return stroke, and opened out during 

 the forward stroke. 



Sixthly, Many insects, such as the ephemera, beetles, locusts, &c, have 

 assuredly the power of more or less completely crushing their wings together, 

 and of alternately increasing and diminishing the wing area during the down 

 and up strokes. The wings of most insects, moreover, are during the up stroke 

 thrown into rugae, which are flattened or altogether disappear during the down 

 stroke. They further have the power of arching their wings during the up 

 stroke, and of opening them out so as to increase their area during the down 

 one. The butterfly affords an admirable example. 



The Down and Up Stroke of the Wing of the Butterfly ; Increase and 

 Diminution of the Wing Area; Development of Figure of 8 Curves on the Margins 

 of the Wing. — In the butterfly, as I have sufficiently satisfied myself, the first 

 wing is made to pass above or over the second wing towards the termination of 

 the down stroke, the convexity of both wings increasing meanwhile. This 

 reduction in the wing area is necessary to destroy the momentum acquired by 

 the wings during their descent, and to prepare them for making the up or return 

 stroke. In the butterfly the wings strike downwards and forwards, and have 

 a more vertical play than in almost any other insect. The wings are elevated 



VOL. XXVI. PART II. 



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