FLIGHT OF ANIMALS — GRAY 301 



can resume flight about 30 seconds after being given a drink of sugar 

 solution. Sustained flight is possible as long as the fly is provided 

 with 3-10 milligrams of sugar per minute. 



For an insect to fly along a level path, its wings must move in such 

 a way as to exert a downward and backward thrust against the air; 

 as the insect moves levelly forward, the air is driven downward and 

 backward. We can see the air moving in this way in plate 2, left. 

 The fly is photographed from one side against a dark background. 

 The wings are vibrating, and catch the light at the top and bottom of 

 their beat, and while the wings are beating, a shower of very fine 

 iycopodium particles is allowed to fall on the fly from above and a 

 photograph of 14 5 -second exposure is taken. The path traveled by 

 a particle in this short time is shown as a white streak in the photo- 

 graph, and the length of these streaks gives the speed at which the 

 particle is moving. You will see in the photograph that they enter 

 from above the zone of the wings at a relatively slow rate, and are 

 forced backward and downward out of this zone at a much higher 

 speed (in this particular case about 6 ft. per sec). An insect's wings, 

 as a free-flying bird's wings, do the work, at one and the same time, 

 of propeller and wings of an airplane. 



An ordinary house fly weighs about 14 milligrams or about i/4ooo 

 ounce, and a force equal but opposite to this weight represents the 

 "lift" derived from the wings during level flight, while the backward 

 component exerted by the wings against the air is equal but opposite 

 to the drag encountered by the fly as it moves forward through the air. 

 Some of the ways in which these forces are kept in balance with the 

 weight of the body are being investigated by Dr. Hollick at Cambridge 

 by means of a very ingenious and very delicate balance. This con- 

 sists essentially of a very thin glass rod fixed firmly at one end so that 

 it extends horizontally. The fly is mounted at the unfixed end of this 

 rod, and any horizontal backward thrust developed by the fly causes 

 the rod to bend to one side, and any lift force causes the rod to bend up. 

 The magnitude of the forces can be determined by measuring the 

 amount to which the rod bends sideways and upward. Any move- 

 ment of the rod is detected by the reflection of light from two small 

 mirrors attached to the rod, which reflect light onto a graduated scale. 



As with a bird, the flight of a fly must be stable in the sense that 

 accidental disturbances in the balance of lift, drag, and weight must 

 be quickly or automatically corrected. Precisely how this is done is 

 not known, but it is quite clear that insects possess special sense 

 organs which inform the brain of changes in direction of motion 

 of its body. Grasshoppers have special hairs on their heads, and it 

 is the feel of the air as it flows past these hairs that enables the 

 insect to keep its head facing into the direction of its flight. Dr. J. 

 W. S. Pringle has investigated a particularly interesting organ of 



