THE REACTIONS OF DROSOPHILA 79 



muscles distinct from that caused by creeping. When a fly is 

 creeping against centrifugal force a similar tension of the leg 

 muscles is produced. Furthermore, creeping against an air 

 current causes the same kind of tension. Very probably, then, 

 tl;e stimuli in all three cases are due to this tension and are 

 received by the sensory nerves of the leg muscles, the response 

 being an attempt to preserve the equilibrium of the body. Nega- 

 tive geotropism in Drosophila, then, is concerned with the muscle 

 sense. Radl ('05) expressed the view that the insect muscles are 

 capable of acting as special sense organs when he wrote ' ' das 

 Gehor der Insekten ist ein verfeinertes Muskelgefuhl." 



The flying response does not fit into this explanation and it 

 may be that it is not influenced at all by gravity. It is a matter 

 of common observation that the house flies on a brightly illu- 

 minated window usually creep upward but fly in all directions. 

 The flying is much more indefinite than the creeping. In my 

 observations on geotropism only a very few cases of flying 

 (about 3 per cent) were seen. Cylinders with a diameter as 

 large as 12 cm. were used so as to allow flying, but no greater 

 proportion of cases was seen than in the smaller cylinders. When 

 disturbed the animals flew about indifferently for a short time 

 and then, after alighting, continued their upward creeping. In 

 the centrifuging experiments no flying at all was seen. The 

 air currents often caused flying, and in the large percentage of 

 cases the animals flew with the current, although they were able 

 to withstand it. It seems therefore that the response to gravity 

 is much less marked in flying than in creeping, where it is very 

 definite. 



CONCLUSIONS 



1. Drosophila ampelophila Loew, when creeping, reacts nega- 

 tively to gravity, to a centrifugal force which is equal to or 

 slightly greater than gravity, and to air currents without regard 

 to other stimuli. Gravity is, then, a kinetic as well as a direc- 

 tive stimulus. 



2. The stimuli causing these reactions are probably received 

 by the sensory nerves of the leg muscles. 



3. It is probable that flying reactions of Drosophila are not 

 influenced by gravity. 



