"REVERSAL OF INHIBITION" BY ATROPINE. 245 



spines, shows normally a rather violent, jerky response to even 

 mild stimulation. This is obviously a point of some significance, 

 ethologically, since it increases the effectiveness of the "hairs" in 

 penetrating and stinging. But under appropriate drug injection 

 the quick, "snappy" type of behavior rather peculiar to lo can 

 be easily brought about, for example, in a Sauna caterpillar. 

 Pilocarpine gave especially instructive comparisons of this sort. 

 The sensory thresholds are so lowered by this substance that a 

 Protoparce or Sainia twists sharply in response to a single light 

 touch, contracting at the place of stimulation, just as lo normally 

 does. Biting reactions are likewise enhanced, so that Saniia snaps 

 savagely at its own spines encountered in writhing, or at forceps 

 touching the head, much as lo normally does. The production of 

 behavior normal to one species in another form characteristically 

 more sluggish shows that the drug produces no new forms of 

 response, but merely accentuates types of reaction for which the 

 structural pathways already exist. Some light is thus given on 

 structural basis of behavior differences in related forms. 



The action of atropine shows clearly that reciprocal innervation 

 exists in these insects, at least so far as concerns the action of the 

 antagonistic muscle groups of the prolegs and walls of the seg- 

 ments. That strychnine, even in high concentration, fails to react 

 in its characteristic manner with the synapses necessarily involved 

 in these reciprocally acting nervous elements, shows that strychnine 

 may fail to reveal the presence of synapses ; and also that synapses 

 even in types so closely related as annelids and arthropods may 

 differ from one another ; 15 or else that the " reversal " phenomena 

 induced by atropine are brought about through action of the latter 

 upon a locus distinct from that usually reacting with strychnine. 



is Cf. Cushney, A. R., Science, N.S., Vol. XLIV., p. 482, 1916, It may 

 be suggested that the slight acidity of caterpillar hsemolymph (p H 6.8-6.6; 

 Jameson and Atkins, Biochem. Jour., Vol. XV., p. 209, 1921) might influence 

 the action of the drugs ; but I doubt that the low toxicity, at least, of strych- 

 nine can be explained in this way. 



