FLIGHT OF INSECTS. 245 



course, it effects its purpose very easily by strikinir the air 

 with more force on one side than on the other; or, by em- 

 ploying certain muscles which bend the body to one side, 

 it shifts the situation of the centre of gravity, so that the re- 

 action of the air on the wings is exerted in a different direc- 

 tion to what it was before; and the motion of the body is 

 modified accordingly. 



By exerting a force with the wings just sufficient to ba- 

 lance that of gravity, insects can poise themselves in the air, 

 and hover for a length of time over the same spot, without 

 rising or falling, advancing or retreating; and the body may, 

 all the while, be kept either in the horizontal, or in the erect 

 position. In the latter case, the motions are similar to those 

 which take place in ordinary flying, only they are more 

 feebly exerted, since all that is required is to sustain the 

 weight of the body without urging it to a greater speed. 

 LibeUulde, Sphinxes, and a great number of Diptera, exhibit 

 this kind of action: among the latter, the St7^aiiomys is most 

 remarkable for its power of remaining long in the same fixed 

 position. 



The number, form, and structure of the wings have fur- 

 nished entomologists with very convenient characters for 

 their classification: on these are founded the orders of the 

 Coleopiera, Orthoptei^a, Rhipiptera, Hemiptera, Neurop- 

 tera, Hymenoptera, Diptera, and Lepidoptera. To enter 

 into any detail in a field of such vast extent as is presented 

 by the infinitely diversified mechanism of the insect crea- 

 tion, would, it is obvious, far exceed the proper limits of 

 this treatise. I must, therefore, confine myself to a few lead- 

 ing points in their structure and modes of progression. 



In the Coleopiera, an order which comprehends by far the 

 largest number of genera of insects, the lower pair of wino-s 

 (w, Fig. 150, p. 228) are light and membranous, and of a 

 texture exceedingly fine and delicate. They are of great 

 extent, compared with the size of the body, when fully ex- 

 panded: and are curiously folded when not in use. For the 

 protection of these delicate organs, the parts which cor- 

 respond to the upper pair of wings of other insects, are here 



