CHEMICAL SENSITIVITY OF TARSI OF CERTAIN FLIES. 169 



plete extension were weighted at .5, while extensions judged to 

 be one half complete or more were weighted at I. As a matter 

 of fact the great majority of responses were complete extensions. 

 The figures which will be chiefly discussed are the percents of 

 weighted responses, called the per cent, of response, computed 

 by dividing the total weighted response obtained in a series of 

 trials by the maximum weighted response possible in the trials. 



EXPERIMENTS. 



The questions to be answered by these experiments were two: 



1. Do flies possess contact chemoreceptors in the tarsi similar 

 to those present in butterflies? 



2. If present, what are some of the substances discriminated 

 by these sense organs? 



It is easy to demonstrate that mounted flies when placed with 

 their tarsi in contact with absorbent cotton saturated with dis- 

 tilled water or a iM saccharose solution will usually extend the 

 proboscis. Such responses may conceivably be due chiefly or 

 solely to: (i) tactile stimulation of the tarsi; (2) distance chem- 

 ical stimulation of the antennae or other receptors by water vapor; 

 (3) contact chemical stimulation of the tarsi. Experiments were 

 accordingly devised to test out these possibilities. 



Two general types of experiments were devised, the first to 

 test the ability of the tarsi to discriminate between water and 

 paraffin oil; the second, to test the ability of the tarsi to dis- 

 criminate between water and iM saccharose solution. 



Three sets of experiments of the first type were carried out, 

 alike as to object and general procedure but differing slightly 

 in apparatus and methods. Under the general conditions of 

 these experiments it was found that the great majority of flies 

 had become 100 per cent, responsive to distilled water by the time 

 trials were begun, viz., two to four hours after anesthetization. 

 A few individuals failed to become responsive in time to complete 

 the experiment the same day, and still a few others failed to 

 respond at all. Occasionally such animals were carried over to 

 the second day and experimented upon, but, in general, they 

 were discarded. The flies which did become responsive were 

 given three trials with water, and alternating with these, three 



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