'PLIGHT OF INSECTS. 



183 



is to be studied, is held by the lower part of the abdomen, in 

 a delicate pair of forceps ; it is placed in such a manner that 

 one of its wings brushes against the blackened paper at every 

 movement. Each of these contacts removes a portion of 

 the black substance which covers the paper, and, as the 

 cylinder revolves, new points continually present themselves 

 to the wing of the insect. We thus obtain a perfectly regular 

 figure, if the insect be held in a steadily fixed position. These 

 figures, of which we give some examples, differ according as 

 the contact of the wing with the paper has been more or 

 less extended. If the contact be very slight, we obtain a 

 series of points or short cross-lines, as in fig. 70. 



Fig. 70. — Showing the frequency of the strokes of the wing of a drone-fly 

 (the three upper lines), and of a bee (the lower dotted line). The fourth 

 line is produced by the vibrations of a chronographic tuning-fork, fur- 

 nished with a style which registers 250 double vibrations per second. 



Knowing that the cylinder revolves once in a second and 

 a-half, it is easy to see how many revolutions of the wing 

 are thus marked on the whole circumference of the cylinder. 

 But it is still more convenient and accurate to make use of a 

 chronographic tuning-fork, and to register, near the figure 

 traced by the insect, the vibrations of the style with which 

 the tuning-fork is furnished. 



Fig. 70 shows, by the side of the tracing made by the 

 wing of a drone-fly, that of the vibrations of a tuning-fork, 

 which executes a double oscillation 250 times in a second. 

 This instrument, enabling us to give a definite value to any 

 portion of the tracing, shows that the wing of the drone per- 

 formed from 240 to 250 complete revolutions per second. 



