202 
Psyche 
[December 
water vapor, and those not so responding were considered ne- 
gative. This served as a check on the contact responses of the 
insects. The contact water tests served as a check on the res- 
ponses to sugar solution. 
Flies received no water until the first test. For flies tested 
the day after emergence this time interval was twenty-four hours; 
otherwise it was about four hours after emergence. Experience 
soon developed the fact that flies kept for a few hours without 
water, almost without exception extended the proboscis as soon 
as their tarsi encountered moisture. Such flies were said to be 
water sensitized. The only difficulty encountered was the general 
restlessness of the insects; this made it necessary to extend the 
period of water inanition, in many cases, to the following day. 
The only effect this had was to quiet the flies. There was no 
essential difference in their responses, from those kept for shorter 
periods without water. 
The same flies were then given an abundance of water and 
tested again, usually about half an hour later. Such flies did not 
usually respond by proboscis extension. They are hence des- 
cribed as water non-sensitized. None of these flies were given 
food until tested with sugar solution. They were tested im- 
mediately after becoming water non-sensitized , that is, imme- 
diately after the tests just described. This interval probably 
never exceeded thirty minutes. Flies that were water non-sen- 
sitized were therefore sugar sensitized. They responded to sugar 
solution but not to pure water. All flies were carried through the 
three tests. 
All the water used in the tests was distilled. The sugar 
solution consisted of' 10 gms. of sucrose in 100 c. c. of water. 
Tests were also made with a solution of urea (5 gms. in 100 c. c. 
of water). No preliminary tests were made for vapor reactions 
with urea, but the flies were all water non-sensitized. A nearly 
equal number of male and female flies were used. Hypersen- 
sitive specimens were discarded. 
The following is a summary of the response data with water 
and with sugar solution: 
