Holmes, The Reactions of Ranatra to Light. 307 



veers continually toward the weaker side. A small difference 

 in the muscular tension of the two sides of an insect body which 

 would be sufficient to cause the creature to orient itself to the 

 rays of light might not be patent to direct observation, espe- 

 cially if the movements are rapid or irregular, as they frequent- 

 ly are. There are several forms, however, in which the effect 

 of light upon the muscular tone is quite clearly manifested, but 

 none more so than in the common water scorpion, Ranatra f us - 

 ca. In many ways this species is admirably adapted for the 

 study of phototaxis ; it is of large size, its long slender legs 

 move in a slow and deliberate manner so that one can observe 

 just how each action is performed ; it may be readily kept for a 

 long time in the laboratory, shows no signs of fear when being 

 experimented with, and reacts to light with a remarkable degree 

 of precision. For an investigation of the modus operandi of the 

 phototactic response Ranatra is probably not equalled by any 

 other known form. 



It is especially advantageous to study phototaxis in some 

 such organism if we wish to ascertain how far the reflex theory 

 of orientation will carry us. If orientation is the result of com- 

 paratively direct reflexes we are better able to determine their 

 precise mode of action. If a more involved type of reaction 

 occurs there is a better opportunity afforded for proving its ex- 

 istence, and, perhaps, ascertaining something of its nature. It 

 does not follow that because we can construct a theory to 

 account for orientation by means of direct reflexes that the pro- 

 cess necessarily takes place in so simple a manner. Between 

 the stimulus and the reaction there may be processes of a com- 

 plicated nature whose existence is not ordinarily betrayed by 

 any outward and visible sign. No one would consider a dog's 

 following the scent of a rabbit a matter of simple chemotaxis. 

 While it is not a process requiring conscious ratiocination, it is 

 doubtless one involving psychic operation of considerable com- 

 plexity. The possibility should be borne in mind that many 

 of the tropisms of insects may be less simple and direct reac- 

 tions than is commonly supposed. If a bee finds its way to its 

 hive over miles of woods and fields guided by its memories of 



