186 



ANIMAL LOCOMOTION. 



rotation of the bones of the wing on their axes. The spiral 

 conformation of the pinions, to which allusion has been so 

 frequently made, is best seen in the heavy -bodied birds, as the 

 turkey, capercailzie, pheasant, and partridge; and here also 

 the concavo-convex form of the wing is most perceptible. In 

 the light-bodied, ample-winged birds, the amount of twisting 

 is diminished, and, as a result, the wing is more or less flat- 

 tened, as in the sea-gull (fig. 103). 



Fig. 103. — Shows the twisted levers or screws formed by the wings of the gulL 

 Compare with fig. 53, p. 107 ; with figs. 76, 77, and 78, p. 147, and with figs. 

 82 and 83, p. 158. — Original. 



Consideration of the Forces which propel the Wings of Insects. 

 — In the thorax of insects the muscles are arranged in two 

 principal sets in the form of a cross — i.e. there is a powerful 

 vertical set which runs from above downwards, and a powerful 

 antero-posterior set which runs from before backwards. There 

 are likewise a few slender muscles which proceed in a more 

 or less oblique direction. The antero-posterior and vertical 

 sets of muscles are quite distinct, as are likewise the oblique 

 muscles. Portions, however, of the vertical and oblique 

 muscles terminate at the root of the wing in jelly-looking 

 points which greatl}^ resemble rudimentary tendons, so that I 

 am inclined to believe that the vertical and oblique muscles 

 exercise a direct influence on the movements of the wing. 

 The shortening of the antero-posterior set of muscles (indi- 

 rectly assisted by the oblique ones) elevates the dorsum of the 

 thorax by causing its anterior extremity to approach its 

 posterior extremity, and by causing the thorax to bulge out 

 or expand laterally. This change in the thorax necessitates 

 the descent of the wing. The shortening of the vertical set 

 (aided by the oblique ones) has a precisely opposite eff'ect. 



