II] OF MODES OF FLIGHT 47 



mechanism, and the disadvantage under which the larger machine 

 or larger creature hes. 



A bird may exert a force at each stroke of its wing equal to 

 one-half, let us say for safety one-quarter, of its own weight, more 

 or less ; but a bee or a fly does twice or thrice the equivalent of its 

 own weight, at a low estimate. If stork, gull or pigeon can thus 

 carry only one-fifth, one-third, one- quarter of their weight by the 

 beating of their wings, it follows that all the rest must be borne by 

 sailing-flight between the wing-beats. But an insect's wings lift it 

 easily and with something to spare; hence saihng-flight, and with 

 it the whole principle of necessary speed, does not concern the lesser 

 insects, nor the smallest birds, at all; for a humming-bird can 

 "stand still" in the air, like a hover-fly, and dart backwards as 

 well as forwards, if it please. 



There is a little group of Fairy-flies (Mymaridae), far below the 

 size of any small famiUar insects; their eggs are laid and larvae 

 reared within the tiny eggs of larger insects ; their bodies may be no 

 more than J mm. long, and their outspread wings 2 mm. from tip 

 to tip (Fig. 2). It is a pecuharity of some of these that their Httle 

 wings are made of a few hairs or bristles, instead of the continuous 

 membrane of a wing. How these act on the minute quantity of air 

 involved we can only conjecture. It would seem that that small 

 quantity reacts as a viscous fluid to the beat of the wing ; but there 

 are doubtless other unobserved anomahes in the mechanism and 

 the mode of flight of these pigmy creaturesf . 



The ostrich has apparently reached a magnitude, and the moa 

 certainly did so, at which flight by muscular action, according to 



t It is obvious that in a still smaller order of magnitude tfie Brownian movement 

 would suffice to make^ flight impossible. 



