358 DR PETTIGREW ON THE PHYSIOLOGY OE WINGS. 



endeavoured to show are indispensable in flight. In short, he confides to the air 

 the delicate task of arranging the details of flight — those details constituting in 

 reality the most difficult part of the problem. 



Objections to the Mechanical Theory of Wing Movements. — There are many 

 facts which militate against Chabrier's mechanical theory of the movements of 

 the insect's wing. I find, for example, that if the wing of the wasp, fly, humble bee, 

 or butterfly be depressed by a delicate rod, its posterior margin is made to curve 

 downwards, and to make various angles with the horizon (fig. 18, ab cd efg, 

 page 351) ; the wing, the instant the rod is removed, being flexed and elevated 

 by the action of elastic ligaments which obliterate the angles formed during the 

 depression (fig. 19, ghijklm, page 351). This implies the existence of a 

 muscular system for depressing the wing, and a fibro-elastic system for elevating 

 it, similar to what is found in the bat and bird, to be described presently. It 

 also proves that the wing is jointed to the body in such a manner that it cannot 

 either descend or ascend without changing the direction of its planes — the air 

 taking no part in the change of plane referred to. 



I find, secondly, that insects have the power of vibrating either wing by 

 itself in any part of a radius not exceeding a half circle, and that the wing may 

 be played above the body or on a level with or beneath it, as circumstances 

 demand. These facts argue a much more intimate relation between the muscular 

 system and the wings than Chabrier is inclined to admit. 



Thirdly, The wing in most insects is composed of two distinct portions at its 

 root (figure 25, a b, p. 359), those portions being endowed with independent move- 

 ments, which enable the insect to incline the anterior or thick margin (a cfe)oi 

 the wing in one direction, and the posterior or thin margin in another — to twist, 

 in fact, the wing upon its long axis. This twisting of the wing upon its long axis 

 exerts upon the organ precisely the same influence which the extending and 

 flexing of the pinion does upon the wing of the bird and bat (figures 39, 40, 41, 42, 

 and 43, p. 362). It in short developes double figure oj "8 curves along the anterior 

 and posterior margins, and converts the iving into a, screiv capable of change of 

 form. 



Fourthly, In the humble bee and other insects supplied with two pairs of 

 wings geared to each other by hooklets, the posterior or thin margin of the first 

 wing glides along the anterior or thick margin of the alula or second wing, 

 which latter, acting as a long lever, has the power of adjusting the posterior 

 or thin margin of the first wing. 



Fifthly, In the wasp the first wing can be distinctly folded upon itself in the 

 direction of its length, the alula or second wing folding upon the first wing previ- 

 ously folded, so that the area of the two wings is reduced to about one-third of what 

 it was before the folding took place. When the wing is so folded it is very compact, 

 and presents a well-defined cutting edge, which points in a backward direction. 



