160 



AKIMAL LOCOMOTION. 



and as a consequence likewise of the speed at which the insect 

 is travelling, the wings during the back stroke are for the 

 most part actually travelling forwards. This is accounted for 

 by the fact, that the body falls downwards and forwards in a 

 curve during the up or return stroke of the wings, and be- 

 cause the horizontal speed attained by the body is as a rule 

 so much greater than that attained by the wings, that the 

 latter are never allowed time to travel backward, the lesser 

 movement being as it were swallowed up by the greater. For 

 a similar reason, the passenger of a steam -ship may travel 

 rapidly in the direction of the stern of the vessel, and yet be 

 carried forward in space, — the ship sailing much quicker than 

 he can walk. While the wing is descending, it is rotating 

 upon its root as a centre (short axis). It is also, and this is 

 a most important point, rotating upon its anterior margin 

 (long axis), in such a manner as to cause the several parts 

 of the wing to assume various angles of inclination with the 

 horizon. 



Figs. 84 and 85 supply the necessary illustration. 



Fig. 84. 



Fig. 85. 



In flexion, as a rule, the under surface of the wing (fig. 84 

 a) is arranged in the same plane Mnth the body, both being in 

 a line with or making a slight angle with the horizon {x x)} 



^ It happens occasionally in insects that the posterior margin of the wing 

 is on a higher level than the anterior one towards the termination of the up 

 stroke. In such cases the posterior margin is suddenly rotated in a downward 



