348 THE MECHANICAL FUNCTIONS. 



By exerting a force with the wings just suffi- 

 cient to balance 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 horizon- 

 tal, 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. Libellula, Sphinxes, and a great 

 number of Diptera, exhibit this kind of action : 

 among the latter the Stratiomys is most remark- 

 able for its power of remaining long in the same 

 fixed position. 



The number, form, and structure of the wings 

 have furnished entomologists with very conve- 

 nient characters for their classification : on these 

 are founded the orders of the Coleoptera, Orthop- 

 tera, Rhipiptera, Hemiptera, Neuroptera, Hyme- 

 noptera, 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 creation, would, it is obvious, far 

 exceed the proper limits of this treatise. I must 

 therefore confine myself to a few leading points 

 in their structure and modes of progression. 



In the Coleoptera, an order which comprehends 



