1928 ] The Tarsal Chemical Sense of the Screw Worm Fly 203 
Water sensitized 
Number (of flies) 
Percent 
Vapor 
Liquid 
Positive 
Negative 
Positive 
Negative 
7 
3.5 
190 
96.5 
173 
97.9 
24 
12.1 
Water non-sensitized 
Number 
Percent 
10 
4.8 
198 
95.2 
32 
10.5 
176 
98.5 
Sugar sentized 
Number 
Percent 
7 
3.2 
209 
96.8 
198 
91.7 
18 
8.3 
All flies tested while water sensitized were used in the other 
tests. The experiments with urea gave the following results: 
Positive 
Negative 
Number 
5 
44 
Percent 
3.2 
89.1 
The fact that the sexes were almost equal in number, and 
that they responded equally, indicates that there can be no great 
difference between them as regards this sense. The data given 
indicates that, water vapor was not an important factor in in- 
itiating responses. The feeding reactions of the flies depended 
upon physiological states; those kept from four to twenty-four 
hours without water gave more reponses than those which had 
taken in water within thirty minutes. A comparison of the 
contact responses demonstrates that the flies easily distinguish 
between pure water and the sucrose solution. 
The results of the experiments with urea were unexpected. 
Not only did the specimens fail to give a feeding response; they 
apparently made violent efforts to escape continued contact with 
the solution. Urea is very bitter to human end organs, and it 
