No. 459.] REACTIONS OF POMACE FLY. 167 
of light produces very rapid locomotor movements, but while the 
kinetic effect of the light is thus increased, its orienting effect 
may be strikingly diminished. Throughout the experiments 
with high intensities there was no evidence of a change from 
positive to negative phototropism even under the influence of the 
highest intensity employed, that of a 250 c. p. arc light at a 
distance of 40 cm. 
Discussion of Results.— In the preceding pages it has been 
found convenient to distinguish between two factors in the effect 
of light on Drosophila, a kinetic factor and a directive one. 
These two factors undoubtedly give rise to different kinds of 
responses, yet they are so closely related that it may be well to 
lay some stress on their interdependence. The directive effect 
of light manifests itself only in connection with a kinetic influ- 
ence sufficient to induce locomotion. The insects while at rest 
do not regularly lie with their heads directed toward the. source 
of illumination ; in fact, those flies of the stock culture already 
referred to, which, after continued exposure to direct sunlight, 
came to rest in the least brightly illuminated portions of the 
vessel, were found with their heads turned away from the win- 
dow. In the experiments with incandescent lights the turning 
on of one of the lamps was rarely if ever followed by mere ori- 
entation on the part of the fly, z. e., simply an adjustment of the 
body in such a manner that its long axis became parallel with 
the light rays and that both eyes received an equal amount of 
light, — a condition of symmetrical stimulation which Loeb (97) 
has maintained to be the essential factor of orientation. While 
the muscle reflexes necessary to put the insect in this position 
were usually forthcoming under such stimulation, these move- 
ments proved to be initiatory locomotor movements, being con- 
tinuous with the series of locomotor reflexes which followed. 
When, on account of light-fatigue or other causes, the insect 
ceased moving about, no satisfactory evidence of orientation 
either to light or to gravity could be adduced from the position 
the animal assumed. 
In the case of one fly subjected to the influence of a very high 
intensity of light, a reaction was obtained which involved only 
the kinetic factor. This fly while stimulated to great activity 
