DR. PETTIGREW ON THE MECHANISM OF FLKH1T. 225 



o> " iv o 



connexion with the small body of the insect, fully explains the falterin 

 course described by the order in their flight. 



The threefold Action of the Wing in Insects 8fC. — The wing in the inject, as has been 

 stated, is more flattened than in the bird; and advantage is taken on some occasions of 

 this circumstance, particularly in heavy-bodied, small-winded, quick-flying insects, to 

 reverse the pinion completely during the down and uj> strokes — the wing, during its 

 descent, having its anterior or thick margin inclined upwards and backwards, when is, 

 during its ascent, the anterior or thick margin is inclined downwards and forwards. 



The object of this arrangement is to increase the elevafing-jtowor, while it docs not 



. . 



necessarily impair the propelling-ipoweY. This is eifec ted in the following manner 

 The posterior margin of the wing is made to rotate, during the down stroke, in a direction 

 from above downwards andfrom behind forwards, — the anterior margin travelling in 

 an opposite direction and reciprocating. The wing may thus be said to attack the air 

 by a screwing movement from above. During the up or return stroke, on the otln*r 

 hand, the posterior margin rotates in a direction from below upwards and from before 

 backwards, so that by a similar but reverse screwing motion, the pinion attacks the air 

 from beneath. It may likewise happen, though more rarely, that the anterior or thick 

 margin of the pinion maybe directed upwards and forwards during the return or up 

 stroke. I infer this from having observed that the anterior margin of the wing of the 

 Wasp (when the insect is fixed and the wings are being driven briskly) is not unfre- 

 quently directed upwards and backwards at the beginning of the down stroke and up- 

 wards and forwards at the commencement of the up or return stroke. A figure of 8, 

 compressed laterally and placed obliquely with its long axis running from left to right 

 of the spectator, represents the movements in question. The down and up strokes, as 

 will be seen from this account, cross each other, the wing smiting the air during 

 its descent, from above, as in the bird and bat, and during its ascent, from below, 

 as in the Elyingfish and boy's kite. The pinion thus acts as a helix or screw in a 

 more or less horizontal direction, from behind forwards and from before backwards; 

 but it has a third function, and it is to this I would more especially direct attention. It 

 likewise acts as a screw in a nearly vertical direction from below upwards. It is to the 

 upward screwing, or third function, that the wing of the insect owes its great elevating- 

 power ; and to this, in all probability, the almost perpendicular ascent of the lark is like- 

 wise due. The threefold action of the insect's wing is well seen in the Humble-Bee and 

 Hornet ; and if it were possible to make a longitudinal section of the blur made by the 

 rapidly oscillating wings of the latter insect (examine blur as represented at a a' of figures 

 58 and 59, Plate XV.), it would be found to present the form of a truncated cone— the 



or narrow portion of the cone displaying a concave surface, the lower or broad 

 omwev one The threefold action of the wing is more perceptible in the wing 



upper 

 part 



of the insect than in those of the bird and bat ; and this is most probably due to the fact 

 that the wing of the insect, with few exceptions, is in one piece, the maximum and mi- 

 nimum of surface being secured by a simple rotation of the pinion on its long axis 

 whereas in the bird and bat it is in several pieces, the pinion, in addition to bein 

 rotated on its Ion"" axis, having its area more or less increased in extension and decrease 



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