56 THE HOUSE FLY—DISEASE CARRIER 
were liberated in fine weather, and out of the 300 five 
were recovered in fly traps at distances of from thirty 
to 190 yards from the place of liberation, and all within 
five days. He further states that he had found them 
at an altitude of eighty feet above the ground, -and 
calls attention to the fact that such a height would fa¬ 
cilitate their carriage by the wind. 
An experiment made under the direction of Prof. 
S. A. Forbes, of which he has sent me a written ac¬ 
count, indicates that house flies may spread naturally 
for at least a quarter of a mile, going, in one significant 
instance, from the tuberculosis hospital to the general 
hospital of Cook County, Illinois. House flies trapped 
at one point were sprayed with a chemical solution and 
liberated. Then flies caught on fly paper elsewhere 
were sprayed with another solution, the result being 
that those which had previously been sprayed were 
turned dark blue in color by the second solution. 
Marking Flies for Experiment 
Professor Hine found that it was a very difficult 
matter to mark flies so that they might be recognized 
from others, since they are very sensitive to anything 
unusual, and any foreign substance on their bodies or 
wings causes them to act abnormally. They contin¬ 
ually try to remove the foreign substance and seem to 
tire themselves out. He found that many specimens 
marked with the greatest care would hardly fly after 
they were marked, so that it was easy in many cases to 
approach them and pick them up with the fingers. He 
