September, 1910. 



KNOWLEDGE. 



345 



by the insect itself. As soon as this has been done, 

 the operator actuates the sighting mirror above 



end of their course the wings, which describe a more 



or less wide loop, will invert the direction of their 



motion, returning to their start- 



~"^ ing point. The hind wings 



describe a curve similar to that 



of the front ones, but for their 



motion 



Ip cro*m c 



Figure 



Lenses of quartz, glass and calcite 

 comparati\-e transparency. 



Figure 6. M. Lucien Bull's complete apparatus for photographing the fliglit of insects. 



referred to, and thus closes the electric contacts backward motions 

 which are necessary for working the shutter described 



In connection with 

 the experiments so far 

 made, M. Bull has found 

 the flight of all insects 

 to be practicall\- alike 

 in so far as the motion 

 of wings is concerned. 

 The best experiments 

 were made on the 

 dragon -flv ^^■hich. in 



addition to being very enduring, possesses especially 

 long wings which are well seen on the photographic 

 records (see Figures 1-4). 

 Their period of vibration also 

 is relativelv long (one thirty- 

 fifth of a second), which allows 

 of a considerable number of 

 pictures being obtained during 

 one full motion. 



The dragon -fl\- wlien at rest 

 keeps its wings in a vertical 

 plane above its back with their 

 axes in the extension of the 

 transverse axis of the thorax. 

 At the moment of flying up the 

 insect first lowers its front 

 wings, and afterwards its hind 

 wings, bringing them at the 

 same time in front of its head. 

 The ends of the wings describe 

 a curve turned ujjwards and 

 forwards, the general direction 

 of which cuts the fl\"ing axis at 

 an angle of about forty-five 

 degrees. When arriving at the 



from one-eighth 

 to one-quarter behind that of 

 the former. 



The back curve of the wing, 

 so far from coinciding with the 

 one described in the first part 

 of the motion, cuts it towards 

 the centre so as to form an 

 eight. The same peculiar feature 

 is observed in connection with 

 all other insects tested, excejit 

 that the width of the loop is 

 \ariable. 



The speed of the end of the 

 wings during the middle part 

 ot each reciprocating motion is 

 about two and a half metres 

 per second in the case of the 



dragon-fly. 



The forward and 

 in opposition to the flight of 

 birds, occur at about the same speed. 



The variable inclina- 

 tion of the wings is 

 quite interesting to 

 observe. Throughout 

 nearly the whole time 

 the wing is lowered its 

 plane is practically 

 horizontal: when arriv- 

 ing at the lower end of 



o 



howing their 



the 



the 



undergoes a sudden rotation through ninety degrees 

 round the front edge, returning to its starting point 



Figure S. The details of the screen, and tube for retaining the insects temporarily. 



