198 DWIGHT E. MINNICH 



alone, however, may also be the correct explanation of many- 

 such instances, and, in view of the sensitivity of the tarsi, it is 

 not difficult to understand why the butterflies remain for long 

 periods of time with proboscis extended, either probing the 

 ground or sucking up the water. 



A very natural question suggests itself in connection with the 

 effect of distilled water on the tarsi. Does water stimulate the 

 same receptors that are affected by the various solutions em- 

 ployed, or does it, as in man, merely afford tactile and tempera- 

 ture stimuli to which the butterfly responds. I have no data 

 with w^hich to answer this question. The simpler hypothesis, 

 in the absence of evidence, is that a single type of sense organ 

 is here operative, and it is my belief that this is correct. 



As stated above, the difference in the number of responses 

 to the various substances employed indicates that the butter- 

 flies clearly discriminate between at least some of these sub- 

 stances. Under the conditions of the experiment, a 1 M solu- 

 tion of saccharose w^as 15 per cent more effective -than distilled 

 water, while solutions of 1 M hydrochloric acid, M/600 quinine 

 sulphate, and 1 ^M sodium chloride were from 14 to 33 per cent 

 less effective than distilled water. There can be no doubt, there- 

 fore, that the tarsal chemoreceptors are able to distinguish a 

 1 M solution of cane-sugar from distilled water or a 1 M solu- 

 tion of sodium chloride from either of these. It is interesting 

 also to note that the order, 1 M saccharose > distilled water > 

 1 M hydrochloric acid > M/600 quinine sulphate > 1 M sodium 

 chloride, is not the order of osmotic effectiveness for these 

 substances, so that the tarsal receptors cannot be osmotic organs, 

 but must be considered as true chemoreceptors. 



We have already seen that the removal of antennae, labial 

 palps, and rudimentary fore legs does not materially affect the 

 response evoked by contact chemical stimulation of the tarsi. 

 It has also been demonstrated that Pyrameis is able to distin- 

 guish between various substances through the tarsal organs. 

 Therefore, the removal of the antennae, labial palps, and rudi- 

 mentary fore legs should not exert any important effect on the 

 degree of responsiveness to difTerent chemical stimuli. This 



