174 D WIGHT E. MINNICH 



The nymphalid butterflies, Pyrameis atalanta Linn, and 

 Vanessa antiopa Linn., meet these experimental requirements 

 in an admirable way. They may often be observed, in company 

 with one or two other species of nymphalids, hovering about a 

 tree trunk, where they frequently alight and remain, wings 

 closed, for considerable intervals of time. Investigation will 

 show that the portion of the trunk visited has been injured in 

 some way and that the butterflies alight there to feed upon the 

 exuding sap. Similar feeding activities may be observed about 

 orchards in the autumn, when fallen and decaying fruit, such as 

 apples and pears, attract the animals. Pyrameis and Vanessa 

 thus respond to food substances by uncoiling the proboscis. 

 This response — the proboscis response, as I shall term it — is an 

 absolutely clear one, for in the unstimulated animal the organ 

 remains compactly coiled against the head. Consequently, a 

 partial extension of even slight magnitude is readily observed, 

 while a complete extension with the subsequent probing of the 

 substrate is unmistakable. Moreover, fruit juices, which evoke 

 this response in the natural environment, are easily obtainable. 



Chemoreceptors may be divided into two classes: first, those 

 affected in general by volatile materials, the source of which 

 may be more or less remote from the receptive surface; second, 

 those affected in general by non-volatile materials, the source 

 of which must be in intimate contact with the receptive surface. 

 The former serve as distance chemoreceptors; the latter, as 

 contact chemoreceptors. In the last analysis both are stimu- 

 lated by a solution of the exciting material, the solvent consisting, 

 at least in part, of the secretion present on the sensitive surface. 

 The above distinction, therefore, far from being an absolute one, 

 is merely useful as the best single condition by which the two 

 groups of sense organs may be conveniently differentiated. Fol- 

 lowing the objective nomenclature, the appropriate stimuli for 

 these two classes of sense organs may be designated, respectively, 

 as distance chemical stimuli and contact chemical stimuli. For 

 brevity, however, it will frequently be useful to omit the quali- 

 fying term, chemo or chemical, understanding that in the present 

 paper the discussion is limited to organs of chemical sense. 



