DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



233 



wings during repose. In these, in extreme flexion, the anterior or thick margin of the 

 wing is directed downwards, and the posterior or thin one upwards. In the act of ex- 

 tension, however, the margins, in virtue of the wing rotating upon its axis, reverse their 

 positions, the anterior or thick margin descrihing a spiral course from helow upwards, the 

 posterior or thin margin describing a similar but opposite course from above dotcuirards. 

 The movements of the margins during flexion and extension may be represented with a 



considerable degree of accuracy by a figure of 8 laid horizontally, as in Diagram 5. 



Posterior or thin margin 



Anterior or thick margin 



Diagram 5 



In this diagram (5) the course pursued by the anterior or thick margin of the pinion 



during extension is indicated by 



thicker portion of the figure, that pursued by 



posterior or thin margin by the thinner portion. These conditions, I need scarcely ob- 

 serve, are reversed during flexion. 



Erom these remarks it will appear that not only the margins, but also the direction of 

 the planes of the wing are more or less com pletely reversed at each complete flexion and 

 extension; and it is this reversing or screwing and unscrewing which enables the wing 



to lay hold of the air with such avidity during extension and to disentangle itself 



such facility during flexion — to present 



fact, a mor 



less concave, oblique, and 



instant, and a comparatively narrow, non-resistin 



strongly resisting surface the one 



cutting edge the next. 



The figure-of-8 action of the wing explains how an insect or bird may fix itself in the 

 air, the backward-and-forward reciprocating action of the pinion affording support, but 



propulsion 



In these instances the backward and forward strokes are made to 



counterbalance each other 



The Wing, when advancing with the Body, desci 



a Wane-track 



figure of 8 



^presents 



with considerable fidelity the twisting of tl 



-Although the 



wing upon it^ 



axis during extension and flexion, when the insect is playing its wings before an 



object 



or 



still better when it is artificially fixed, it is othe 



hen the down 



In this 



stroke is added and the insect is fairly on the wing and progressing rapidly 



the win**, in virtue of its being carried forward by the body in motion, describes 



undulating or spiral course, as shown in the accompany 



dia 



o 



(6) 



here the 



Diagram 6 



# 



thick portions {a, b) of the spiral, represent the effective or down strokes, the thin por- 

 tions (c, d, e) the up or back strokes. 



Prom this diagram (6) it will be evident that the resistance offered by the wing to 



