244 THE MECHANICAL FUNCTIONS. 



the several pieces of which its dorsal portion is composed, 

 are loosely connected together by ligaments.* 



The shape of the wings is more or less triangular. They 

 are moved by numerous muscles, which occupy a large 

 space in the interior of the trunk, and consist of various 

 kinds of flexors, extensors, retractors, levators, and depress- 

 ors; the whole forming a very complicated assemblage of 

 moving powers. The largest, and consequently most pow- 

 erful of these muscles, are those which depress, or bring 

 down the wings. They form a large mass, marked a, in 

 Fig. 144. All these muscles exert great force in their con- 

 tractions, which are capable of being renewed in very rapid 

 succession: for, indeed, unless they had this power, even so 

 lio-ht a body as that of an insect could not have been sus- 

 tained for a moment in so rare a medium as the atmosphere, 

 far less raised to any height by its resistance. 



The simple ascent and descent of the wings would be suf- 

 ficient, without any other movement being imparted to them, 

 to carry forwards the body of the insect in the air. The 

 action in which the muscles exert the greatest force is in 

 striking the air during the descent of the wing; an impulse 

 in the opposite direction being the result of the reaction of 

 the air. The axis of motion of the wings is a line inclined 

 at a small angle to the axis of the body, and directed, from 

 before, backwards, outwards, and downwards; and they move 

 in a plane which is not vertical, but inclined forwards. The 

 angle which the plane of the wing forms with the horizon 

 varies continually in the different positions of the wing; but 

 the general resultant of all these successive impulses is a 

 force directed forwards and upwards; the first part of this 

 force produces the horizontal progression of the insect, while 

 the second operates in counteracting the force of gravity; 

 and during the advance of the insect, either maintains it at 

 the same height, or enables it to ascend. 



When the insect wishes to turn, or to pursue an oblique 



• See Chabrier's "Essai sur le Vol des Insectes," Memoires du Museum 

 d'Histoire Naturelle? vi. 410, vii. 297, and viii. 47 and 349. See, also, Zoolo- 

 gical Journal, i. 391. 



