



226 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



in flexion. The upward and downward screwing of the anterior margin of the wing, and 



the increase and diminution of its area in extension and flexion, in the bird, is represented 



at fig. 29 of Plate XIV. In the figure referred to, the wing to the left of the observer is 



fully extended and ready to give the effective or down stroke, the anterior border (d' e f) 



and the under surface of the pinion being inclined obliquely upwards and backwards (see 



arrow). The wing to the right of the observer is, on the contrary, fully flexed and ready 



to make the return or up stroke, the anterior border (e,f) and under surface of the pinion 



being inclined obliquely downwards and forwards {vide arrow). The wing consequently 



rotates on its long axis in opposite directions during extension and flexion, i. e. during 



its descent and ascent ; and if the rotation be conducted sufficiently far, it acts as an 



elevator in both directions, propulsion being performed almost exclusively by the more 



vigorous play of the pinion during its descent. The wing may act as an elevator 



when rotation is performed in a minor degree in quite another manner, as explained 

 at p. 257. 



The position of the wings during repose, as well as their condition, varies, the wings 

 in one case being folded transversely and arranged on the back in a more or less longi- 

 tudinal direction (Plate XIII. fig. 16 e) ; in another they are folded both longitudinally 

 and transversely, as in the May-fly ; whilst in a third they are crushed together like a 

 fan (Plate XIII. fig. 23 a, b). When not folded, they may be raised vertically above 

 the body (Plate XIII. fig. 26 a, a% or slightly lowered or extended horizontally 



(Plate XIII. figs. 20 and 24), or inclined downwards, as in a roof of a house (Plate XIII. 

 fig. 25). 



Direction of the Down and Tip Strokes in Flight, Centre of Gravity, fyc— From the fore- 

 going account it is evident that the wings of insects vary as regards their number, size, 

 and shape. They also differ as regards their surfaces, margins, venation, degree of con- 

 sistence, and position ; so that it might naturally be asked, Do the several orders of wings 

 act according to a common principle, or does each wing act according to a principle of 

 its own ? There can be, I think, but one answer to this question. All wings obtain 

 their leverage by presenting oblique surfaces to the air, the degree of obliquity gra- 

 dually increasing during extension, when the sudden or effective stroke is being given, 

 and gradually decreasing in an opposite direction during flexion, or when the ..~ 



being more slowly recovered preparatory to making a second stroke. As the win? of the 



wing 



insect does not bend in a direction from before backwards, and from without inwards, dm 

 ing flexion, as happens in the bird and bat, the terms extension and flexion, when applied 

 to it, are used m an arbitrary sense-extension signifying the carrying of the wing in a 

 forward direction away from the body, flexion meaning the reverse, or the drawing of the 

 wing from before backwards towards the body. The effective stroke in insects (and this 



noicls true also of bats and birds, vide nn 9** *^a ok«\ • a i- it , ^ r 



j , ' 06ae PP- Zo ° and 256) is delivered downwards and for- 



v.ards and not, as is commonly believed, vertically, or even slightly backwards *. This 

 arises from the curious circumstance that insects, bats, and birds, when flying, actually fall 

 through the medium which elevates them, their course being indicated bv the resultant 

 of two forces, viz. that of gravity pulling vertically dowmvardSj and that / f ^ wing act . 



For prevailing „pi ni „„ s „ t0 the direction „ f ^ efl . ^ ^ ^ ^ ^ ^ ^ ^^ ^ ^ 















