THE ORIENTATION OF PORCELLIO TO LIGHT 115 



of our experiments with unequivocal, definite, tropic reactions. 

 So the operation was discontinued as useless. In later experi- 

 ments on blow fly larvae and Allolobophora, the same definite 

 tropic reactions were observed. 



6 



For the sake of clearness it should be pointed out not only 

 that " random movements ' and ' trial movements ' are ex- 

 pressions not always used in the same sense, but that appar- 

 ently spontaneous random movememts may be controlled to 

 some extent by the environment. The exploring movements 

 of the second antennae of Porcellio are largely initiated and 

 regulated by internal conditions; this is evident especially when 

 environmental conditions remain constant. A slight change in 

 the texture of the substratum, however, may produce marked 

 changes in its behavior; in the absence of the antennae, contact 

 differences may make themselves effective upon the path of 

 locomotion through the legs or body. It is a truism that the 

 behavior of an organism is a resultant of the responses to all 

 simultaneously acting stimuli. A movement initiated from 

 within, when the organism is exposed to various contact stimuli, 

 may frequently be modified if not entirely inhibited by them. 

 The same may be said of movements initiated from without. 



It happens, therefore, that so-called " trial " movements in 

 Porcellio and blow fly larvae and earthworms vary their char- 

 acter and intensity with circumstances. They may be so aug- 

 mented by external stimuli as largely to obscure the tropic 

 reactions which under other conditions are readily perceived. 

 The source of the external stimulation may, however, be very 

 inconspicuous. This was especially true in the case of a blow 

 fly larvae that had been traveling away from the light in a 

 direct course with very slight lateral movements of the anterior 

 end. Suddenly the anterior half of the body was lifted and 

 swung from side to side, up and down, in irregular movements 

 of large amplitude that continued for several seconds. The 

 cause of this change in behavior was finally discovered in a bit 

 of filament from the paper substrate that had been picked up 

 and was adhering to the anterior end. For the time, these 

 vigorous " trial " movements, initiated probably by internal con- 

 ditions but owing their conspicuous characters to contact stim- 



