42 ANIMAL LIFE 



that its tip describes a figure of eight between every 

 two up or down strokes ; and in so doing uses the 

 effective stroke for forward movement, just as a 

 boat's sail, set at any angle with the wind, plays off 

 the greater part of its breeze into a propelling force 

 brought to bear on the front edge of the sail. For 

 turning, the two sides of the bod}' require to act inde- 

 pendently, and to lower the wing on that side towards 

 which it is desired to turn. The intensity and rapidity 

 of muscular vibration during flight are extreme ; 

 indeed, the efficiency of insect muscle is quite extra- 

 ordinary, both for strength, mobility, and persistence, 

 and the wing-muscles are the most perfectly qualified 

 of any in the body for sustained control and strong 

 action. 



We have only to watch a hovering fly in still air, or 

 attempt to see* a humming-bird hawk-moth flash, hover, 

 and dart as it passes over flowers like a sunbeam, to 

 realise that such insects have brought flight to a 

 degree of perfection that seems astounding, unnatural, 

 and unnecessary. So far are we behind the meanest 

 fly in agility that we marvel, if at all, very much as 

 a savage at machinery. 



The problem of effectively using two pairs of 

 wings simultaneously is, however, too difficult for 

 most insects, and we find a tendency in many groups 

 for the reduction of one of them. Like a boat sailing 

 just astern another, the back-wash and eddies in the 

 air caused by the front wings render the effective 

 working of the hind wings a great problem. Never- 



