S\0 THE MECHANICAL FUNCTH)NS. 



The shape of the wings is more or less triangular. 

 They are moved by numerous muscles, which oc- 

 cupy a large space in the interior of the trunk, and 

 consist of various kinds of flexors, extensors, re- 

 tractors, levators, and depressors; the whole forming 

 a very complicated assemblage of moving powers. 

 The largest, and consequently most powerful 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 contractions, which are capable of being 

 renewed in very rapid succession ; for, indeed, un- 

 less they had this power, even so light a body as 

 that of an insect could not have been sustained for 

 a moment in so rare a medium as the atmosphere ; 

 far less raised to any height by its resistance. The 

 membranous wings of insects, as appears from the 

 observations of Straus, are weaker at the parts far- 

 thest from the trunk ; and consequently they bend 

 on being raised ; and presenting, when thus bent, 

 a smaller surface, the resistance which the air op- 

 poses to their ascent is less than that which it op- 

 poses to their descent, in which latter action they 

 are expanded to the utmost. 



The simple ascent and descent of the wings would 

 be sufficient, 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 created by 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 



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