No. 567] BIOLOGY OF THE THYSANOPTERA 



175 



distribution, is the possibility that they have learned that 

 certain modes of behavior are best suited (for example) 

 to continued safety. 



The reply to such an argument is first, that most of my 

 studies on behavior have been made in an unsettled re- 

 gion, where the enemies of thrips incident to civilization 

 are practically wanting, and where even the natural 

 enemies are not abundant. It could hardly be assumed 

 that every individual would learn to avoid its enemies in 

 the course of its short lifetime, yet certain species seem 

 to be invariable in their response to certain agents. 

 Furthermore, many of the larvae tested in the experi- 

 ments could have been but a few days old. It is incred- 

 ible that their reactions should have been, as in fact they 

 were, as definite and invariable as those of older larvae, 

 if these responses were dependent on experience. 



It seems to me, therefore, that the only satisfactory ex- 

 planation of outdoor behavior and distribution of the 

 Thysanoptera lies in the assumption that they are in 

 large measure the result of responses to simple stimuli, 

 and do not imply any degree of choice. 



Obigin and Adaptiveness of Responses to External 

 Stimuli 



The origin of such responses in Thysanoptera as have 

 been described above is not, I believe, discoverable. Pur- 

 poseful they most probably are not, as I have shown, if 

 by purpose we mean conscious direction of actions to 

 some end. But adaptive they no doubt are in many cases. 

 Perhaps they are all adaptive, but I confess that my 

 powers of analysis are not keen enough to prove such a 

 view correct. That Euthrips tritici is positively photo- 

 tropic when disturbed is no doubt the cause of frequent 

 escapes from danger. One may even believe the negative 

 Phototropism of larvaa of Anthothrips verbasci to be 

 adaptive, because they are much more sluggish than is 

 Euthrips tritici, and could not escape quickly even if they 

 should emerge into the light. They are probably safest, 



