400 DR PETTTGREW ON THE PHYSIOLOGY OF WINGS. 



hinge. Thus if a b of figure 51 (p. 399) be made to represent the rod hinged at x, 

 it travels through the space d b /in the same time that it travels through j k I; 

 and through the space j k I in the same time that it travels through the space 

 g h i ; and through the space g him the same time it travels through eac, which 

 is the area occupied by the thorax of the insect. If, however, the rod a b travels 

 through the space d bf in the same time that it travels through the space eac, 

 it follows of necessity that the portion of the rod marked a moves very much 

 slower than that marked b. The muscles of the insect are applied at the point 

 a, as short levers (the point referred to corresponding to the thorax of the 

 insect), so that a comparatively slow and limited movement at the root of the 

 wing produces the marvellous speed observed at the tip, the tip and body of 

 the wing being those portions which occasion the blur or impression produced on 

 the eye by the rapidly oscillating pinion. But for this mode of augmenting the 

 speed originally inaugurated by the muscular system, it is difficult to comprehend 

 how the wings could be driven at the velocity attributed to them. The wing of 

 the blow-fly is said to make 300 strokes per second, i.e., 18,000 strokes per minute. 

 Now it appears to me that muscles to contract at the rate of 18,000 times in the 

 minute would be exhausted in a very few seconds, a state of matters which 

 would render the continuous flight of insects impossible. (The heart contracts 

 only between 60 and 70 times in a minute.) I am therefore disposed to 

 believe that the number of contractions made by the thoracic muscles of insects 

 has been greatly overstated, the high speed at which the wing is made to vibrate 

 being due less to the separate and sudden contractions of the muscles at its 

 roots than to the fact that the speed of the different parts of the wing is increased 

 in a direct ratio as the portions in question are removed from the driving point, 

 as already explained. Speed is certainly a matter of great importance in wing 

 movements, as the elevating and propelling power of the pinion depends to a 

 great extent upon this condition. Speed, however, may be produced in two ways 

 — either by a series of separate and opposite movements, such as is witnessed in 

 the action of a piston, or by a series of separate and opposite movements, acting 

 upon an instrument so designed that a movement applied at one part increases in 

 rapidity as the point of contact is receded from, as happens in the wing. In the 

 piston movement the motion is uniform, or nearly so, all parts of the piston 

 travelling at very much the same speed. In the wing movements, on the con- 

 trary, the motion is gradually accelerated towards the tip of the pinion, where 

 the pinion is most effective as an elevator, and decreased towards the root, 

 where it is least effective ; an arrangement calculated to reduce the number 

 of muscular contractions, while it contributes to the actual power of the wing. 

 This hypothesis, it will be observed, guarantees to the wing a very high speed, 

 with comparatively few reversals and comparatively few muscular contractions. 

 In the bat and bird the wings do not vibrate with the same rapidity as in 



