February, ’24] 
PETERSON: CHEMOTROPISM OF HYLEMYIA 
93 
time, 24 to 48 hours, to become attractive but after fermentation started 
the baits made with dry yeasts-often remained attractive over a longer 
period of time than those made with the wet yeast. In a number of 
cases the baits with dry yeasts attracted many flies 14 to 21 days atter 
they were placed in the field provided there was sufficient moisture 
present in the dishes. The wet yeast in some instances proved to be 
attractive almost from the start when mixed with some of the media. 
Baits containing yeasts usually attracted many more flies over a 
period of seven days or longer than any of the alcohols mentioned 
above. In some cases the alcohol baits caught as many and more 
flies the first 48 hours as the yeast baits but after the alcohol evaporated 
they were no longer attractive. 
It is the opinion of the author that the baits containing yeast were 
attractive because of the alcohols and ethereal odors produced in fermen¬ 
tation. It is possible that carbon dioxide given off in fermentation 
might play some part as an attractive agent. This is a point which 
needs further investigation. After discovering the attractiveness of 
several alcohols and fermentation odors produced by yeasts a study was 
made of the effect of adding stomach poisons to the baits containing the 
attractive agents. Sodium arsenite was most extensively used. When 
sodium arsenite in moderate amounts ounce to 1 quart of media) 
was added to the baits containing the alcohols there was little or no 
perceptible change in the results while the addition of sodium arsenite 
ounce to quart of media) to baits containing yeast organisms 
there was a marked diminution in the attractiveness of the bait. This 
was undoubtedly due to the influence of the arsenical on the yeast 
organism. Sodium arsenite seemed to have the greatest effect on the 
results obtained where dry yeast was employed to bring about fermenta¬ 
tion. The wet yeast organism withstood and seemed to function better 
than the. dry yeast organism in the presence of sodium arsenite. In a 
number of instances where sodium arsenite was used at the rate of y 4 
ounce to 5 quarts of media with wet yeast the attractiveness of the bait 
was not materially reduced. Further study is needed to determine the 
maximum amount of sodium arsenite which may be used without re¬ 
ducing the fermentation process. 
Other stomach poisons were given a few trials; such as mercuric 
chloride, formaldehyde, sodium fluoride, sodium cyanide, nicotine sul¬ 
fate, hellebore and extracts of derris. Mercuric chloride and formalde¬ 
hyde killed the yeast organisms as expected. The other stomach poisons 
did not necessarily kill the yeast organisms but in most of the tests 
