ANATOMY AND PHYSIOLOGY. 115 
at ducks and drakes with flat bits of tile, and the 
Australian boomerang is essentially a flying-machine 
of the same humble order. So too, according to 
most observers, is the flying-fish. In all these, how- 
ever, an initial impulse is necessary. Kites supply a 
nearer illustration of what the insect world displays 
in such infinite variety, successful flying-machines, 
though they have been oftener recommended than 
applied to practical uses. In kites the traction on 
the string may be taken to represent the weight of 
the body of the insect, which pushes it forwards and 
downwards, just as the string draws the kite in the 
same direction. In this instance the difficulty of 
supporting a heavy body in the air by mechanical 
means is fairly surmounted; but this is only half 
the problem of flight, for the string limits the range 
within which it can fly. And even this amount of 
success, and within these narrow limits, is dependent 
on the direction of the wind which supports the 
weight by its upward pressure. 
It might seem that the proper way to investigate 
this question would be to study the sustaining and 
the propelling principles separately. This may be done 
to a certain extent; and the beetle, deed, supplies 
a living instance where the instruments of support and 
propulsion are distinct, bemg represented by the 
wing-covers and the wings respectively. In wasps, 
however, which are the example which we have to keep 
constantly before us, the two functions are inseparably 
united; and, generally speaking, the difference between 
support and propulsion is a mere matter of direction. 
Still, we may find an available distinction accordign 
as the current of air, and the influence of gravity 
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