398 DP. PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



occupied by it ; i.e., the wing (such is its speed) is in every portion of the space 

 at nearly the same instant, the space representing what is practically a solid 

 basis of support. As, moreover, the wing is jointed to the upper part of the 

 body (thorax) by a universal joint, which admits of every variety of motion, 

 the insect is always suspended (very much as a compass set upon gimbals is 

 suspended), the wings, when on a level with the body, vibrating in such a manner 

 as to occupy a circular area, in the centre of which the body is placed (vide 

 r dbf of fig. 51, page 399). The wings, when vibrating above and beneath the 

 body occupy a conical area, the apex of the cone being directed upwards when 

 the wings are below the body, and downwards when beneath it. Those points 

 are well seen in the bird at figures 18 and 19, Plate XIV. In figure 18 

 the inverted cone formed by the wings when above the body is represented, 

 and in figure 19 that formed by the wings when below the body is given. In 

 these figures it will be observed that the body, from the insertion of the roots of 

 the wings into its upper portion, is always suspended, and this, of course, is 

 equivalent to suspending the centre of gravity. In the bird and bat, where the 

 stroke is delivered more vertically than in the insect, the basis of support is 

 increased by the tip of the wing folding inwards and backwards in a more or 

 less horizontal direction at the end of the clown stroke ; and outwards and for- 

 wards at the end of the up stroke. This is accompanied by the rotation of the 

 outer portion of the wing upon the wrist as a centre (vide t of figures 9 and 10, 

 Plate XII.), the tip of the wing, because of the ever varying position of the 

 wrist, describing an ellipse. In insects whose wings are broad and large 

 (butterfly), and which are driven at a comparatively low speed, the balancing 

 power is diminished. In insects whose wings, on the contrary, are long and 

 narrow (blow-fly), and which are driven at a high speed, the balancing power 

 is increased. It is the same with short and long winged birds, so that the 

 function of balancing is in some measure due to the form of the wing, and the 

 speed with which it is driven, the long wing and the wing vibrated with great 

 energy increasing the capacity for balancing. When the body is light and 

 the wings very ample (butterfly and heron), the descent of the wing and the 

 reaction of the air during the up stroke displaces the body to a marked 

 extent. When, on the other hand, the wings are small and the body large, the 

 reaction produced on the trunk by the vibration of the wing is scarcely per- 

 ceptible. Apart, however, from the shape and dimensions of the wing, and the 

 rapidity with which it is urged, it must never be overlooked that all wings (as 

 has been pointed out) are attached to the bodies of the animals bearing them 

 by some form of universal joint, and in such a manner that the bodies, whatever 

 the position of the wings, are accurately balanced, and swim about precisely 

 after the fashion of a compass set upon gimbals. To such an extent is this true, 

 that the position of the wing is a matter of indifference. Thus the pinion may be 



