No. 459.) REACTIONS OF POMACE FLY. 165 
portion of its excursion and alight on the vertical side of the 
cylinder. When this occurred the fly came to rest with its head 
uppermost, no matter whether its flight had been directed 
upward or downward. This orientation of the body on alighting 
may have been due to negative geotropism. 
Effect of Increased Intensities of Light — In order to subject 
the flies to light of different intensities use was made of an arc 
light suspended at one end of a dark room with dead black 
walls. The glass cylinder containing a single fly was placed 
horizontally on a small movable table, one end of the cylinder 
being directed toward the light. The movements of the fly were 
observed while the end of the cylinder through which the rays 
fell was at four different distances from the arc light, vis., 800 
cm., 300 cm., 80 cm., 40 cm. The candle power of the arc light 
used was approximately 64. Between the cylinder and the light 
a rectangular glass vessel containing 3500 cc. of water was inter- 
posed as a heat screen. The front and back walls of this vessel 
were 7 cm. apart. 
At each of the four positions the responses of the fly were 
timed as follows. A small opaque screen was set up between 
the arc light and the cylinder, and at the opposite end of the 
cylinder an incandescent lamp was turned on. The fly, attracted 
by the light of the lamp, crept toward it. At the moment the 
insect reached the middle of the cylinder the screen obscuring 
the arc light was removed, the incandescent light extinguished, 
and the stop watch started. The fly, headed away from the arc 
light, usually stopped momentarily when suddenly exposed in 
this manner to the light from behind, and then resuming its 
course moved on until the opposite (dark) end of the cylinder 
was reached. Here it turned and hastened toward the illumi- 
nated end, either by creeping, or by flying, or by both creeping 
and flying. With the cylinder close to the arc light the fly 
sometimes turned about immediately upon the removal of the 
screen. When the fly crossed the boundary line of the section 
nearest the arc light the watch was stopped and the time 
recorded. At distances of 800 and 300 cm. a large majority of 
the responses were of the creeping kind. At positions nearer 
the light the insect often flew through a part of the distance — 
