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PETTIGREW ON THE MECHANISM OF FLIGHT. 23 



The inference to be deduced from this correspondence between the twisting wave-like 

 action of the pinion and the sinuous movements of the swimming-animals, whether these 

 occur in their bodies, tails, fins, flippers or feet, is this — that the liquid media of the 

 water and air respond to similar impulses, whatever the direction of the impulse (the 

 flying wing strikes forwards, and the diving and swimming wing and foot backwards), 

 and that the gliding spiral movements which are indispensable to the successful navi- 

 gation of the air and water differ in no essential particular from those required in 

 land transit as witnessed in the ordinary movements of walking and running. Compare 

 the spiral track described by the wing in action (Diagram 6 of text) with the movements 

 of the fish (fig. 76, Plate XV.), with those of the feet of swimming-birds (tig. 77, Plate 

 XV.), and with those described by our own extremities in walking (fig. 78, Plate XV.). 



The down and up strokes of the icing consist of four stages. — The action of the wing 

 may be conveniently divided into four stages, — the first being devoted to preparing the 

 wing for making the down stroke, i. e. in elevating it above the body and in distending 

 it when necessary; the second, to the down or effective stroke, as it is usually termed; 

 the third, to preparation for raising and flexing the wing (when this is required) prior to 

 the up or back stroke ; the fourth, to the so-called non-effective, up or back stroke. These 

 movements, however, be it understood, are continuous and run into each other (Dia- 

 gram 6, p. 233). In the first stage the wing supports or sustains, and partially elevates ; in 

 the second it elevates, propels, and sustains to an extent which is quite remarkable; in 

 the third it sustains and partially elevates, as in the first ; and in the fourth and last it 

 acts chiefly as a sustainer, although it also acts as an elevator by being carried against 



the wind by the body which is in motion. 



In the first stage the angle which the wing makes with the horizon may be stated at 

 from 15° to 20°; in the second it increases to something like one of 30°; in the third it 

 gradually diminishes again to 15° or 20°; and in the fourth it probably does not exceed 

 5° or 10°. The angle, as has been stated, consequently gradually increases, and attains 

 a maximum towards the middle of the down or effective stroke (Diagram 0.r), whereas 



it decreases and attains a minimum towards the middle of the non-effective or up stroke 

 (Diagram 6 d). The terms effective and non-effective are here used in a comparative and 



not an absolute sense. 



The wings may or may not present the proper angle for flight when the insect is re- 

 posing; and when they do not, a certain degree of preparation is necessary before the 

 insect can take wing. In the Butterfly the wings during repose are raised vertically 

 above the body (Plate XIII. fig. 26), so that they require to be lowered and slewed round 

 until they make the requisite angle before flight can be effected. In the Cicada, on the 

 other hand, where the wings droop (Plate XIII. fig. 25), they require to be raised and 

 rotated till the desired angle is obtained ; so that it is only in such insects as the Drago 

 flies (Plate XIII. fig. 24), where the angle during repose and in flight approaches to 30 , 

 that flight is instantaneous. It matters not, therefore, whether the wings in leaving 

 their position of rest roll from above downwards, or from below upwards, if when fully 

 extended, they make the angle of 30° with the horizon, as specified. The Beetles are 

 illustrative in these respects. In them the wings, when the insect is at rest, are arranged 



