FLIGHT OF INSECTS. 31 1 



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 gene- 

 ral 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 counter- 

 acting 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 course, it effects its purpose very easily by 

 striking the air with more force on one side than 

 on the other; or, by employing certain muscles 

 which bend the body to one side, it shifts the situa- 

 tion of the centre of gravity, so that the reaction of 

 the air on the wings is exerted in a difierent direc- 

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

 body is modified accordingly. 



By exerting with the wings a force just sufficient 

 to balance that of gravity, insects can poise them- 

 selves in the air, and hover for a length of time 

 over the same spot, without rising or falling, ad- 

 vancing or retreating; and the body may, ail 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 with- 

 out urging it to a greater speed. Libellulce, Sphinges, 

 and a great number of Diptera, exhibit this kind of 

 action : among the latter the Eristalis, or common 

 drone-fly, is most remarkable for its power of re- 

 maining long in the same fixed position. 



The number, form, and structure of the wings of 



