456 DWIGHT E. MINNICH 



dilute solution would have produced a response. But this ob- 

 jection is at once ruled out by the fact that a IM solution of 

 saccharose always yielded a 100 per cent response. The tarsal 

 sense organs, therefore, appear to be quite unaffected by osmotic 

 pressure and must be regarded as specific chemoreceptors. 



The tarsal sensitivity of the red admiral to saccharose is quite 

 remarkable when compared with the sensitivity of other animals 

 to the same substance. Among the marine invertebrates sac- 

 charose appears to have little or no stimulating power apart from 

 the osmotic effect of high concentrations. The same holds true 

 for the lower aquatic vertebrates. According to the table given 

 by Parker ('12, p. 228), AmphiOxus, Ammocoetes, Mustelus, 

 and Amiurus are entirely unresponsive to saccharose. Moreover, 

 the weakest solution which the human tongue can detect is but 

 M/50. But as we have seen in two of the seven butterflies tested, 

 nos. 11 and 24, the tarsi were able to discriminate an M/12,800 

 solution. In other words, the tarsi of these two butterflies were 

 256 times more sensitive to saccharose than the human tongue. 



The sensitivity of Pyrameis to saccharose is thus a highly 

 specialized one. The reason for this is not far to seek. Pyrameis 

 feeds on nectar, exuding sap, and juices of ripe and decaying fruit. 

 In all of these substances sugars are found. In the laboratory 

 I have kept the butterfly alive and in good condition for thirty 

 days on IM saccharose solution. Sugars thus appear to be the 

 most important food of this insect, and the highly developed 

 sensitivity to saccharose is doubtless directly correlated with this 

 fact. Like the organs of olfaction, therefore, the organs of taste 

 may be very highly developed in certain lepidopterous forms. 



CONCLUSIONS 



1. In Pyrameis the threshold of response to saccharose solutions 

 varies directly with the nutritional condition of the individual. 

 During periods of total inanition followed by periods of water 

 diet, i.e., during saccharose inanition, it gradually falls. But 

 with the initiation of a period of saccharose diet, it rises abruptly 

 to a level which remains approximately constant throughout the 

 remainder of the period. 



