No. 459] REACTIONS OF POMACE FLY. 169 
continued locomotor reflexes bring it into other regions. It may 
by accident reach the vertical surfaces of the vessel intermediate 
between the surface nearest and the one farthest from the win- 
dow. Here the kinetic influence of the light, owing to decreased 
intensity, is least. The fatigued fly may still, however, be stim- 
ulated sufficiently to cause it to creep or at least to turn about. 
If it chances to place itself with the head directed away from the 
window, the reduced light stimulus received by the eyes may be 
inadequate to call forth further muscle reflexes, and the fly re- 
mains quiet in this position. Jarring the insect, or turning the 
vessel about so that more light enters the eyes, may increase the 
kinetic stimulation to such an extent that renewed movements 
are induced. 
In Drosophila just as light causes locomotion, so mechanical 
stimulation seems to bring about progressive movements. From 
the nature of the case this stimulus was very roughly applied in 
the foregoing experiments, there being no effort to limit its 
application to one side of the body with a view to detecting its 
directive effect, if any, under such conditions. On the other 
hand, an allied form of stimulus, the pullof gravity, directs the 
movements of the organism, although it does not appear to 
induce them. A condition of symmetrical stimulation is un- 
doubtedly brought about when, on a vertical surface, the fly places 
its long axis parallel to the direction of the lines of the earth's 
attraction. This pull of gravity on the materials which make up 
the animal's body differs more in degree than in kind from the 
impact of surrounding objects to which the animal is subjected 
when mechanical stimulation is applied. If the attraction of 
gravity and mechanical stimulation are regarded, then, as closely 
allied forms of stimuli, their two effects upon the organism 
admit of an almost exact comparison with the two effects of 
light. When left quietly in darkness, Drosophila gives little 
evidence of a tendency either to orient itself, or to move 
about. Thus gravity alone seems insufficient to induce either 
of these responses. When, however, the insect is mechan- 
ically stimulated, locomotor movements do ocour, and in con- 
nection with them the directive influence of gravity becomes 
effective. The animal orients itself with its head away from 
