346 DWIGHT E. MINNICH 



These investigations have covered between fifty and sixty spe- 

 cies, including the four chief classes of arthropods. Among the 

 insects, where most of the work has been done, representatives 

 of most of the larger orders have been experimented upon. These 

 embrace Orthoptera, Blattoidea, Hymenoptera, Coleoptera, 

 Odonata, Lepidoptera, Diptera, Homoptera, and Hemiptera. 

 The phenomenon of circus movements — or perhaps better, asym- 

 metrical response — must, therefore, be regarded as general rather 

 than exceptional for the members of this phylum. 



The form of response naturally varies with the peculiarities of 

 locomotion in a given species. It is not the same for a sidewise 

 moving crab, such as Carcinus, as it is for an insect which moves 

 forward. With the usual type of forward locomotion, however, 

 arthropods with one eye blackened generally circle toward the 

 functional eye, if they are positively phototropic ; toward the non- 

 functional eye, if they are negatively phototropic. 



It is true there are cases which, on first examination, do not 

 appear to conform to this generalization. Thus Holmes ('05, pp. 

 332-336) has demonstrated clearly that an animal with one eye 

 blackened may at first perform circus movements in creeping 

 toward a light, only to modify its behavior after a time and creep 

 in a straight path. Such was true of both Ranatra and Noto- 

 necta. Axenfeld ('99, p. 375) had previously made similar 

 observations, and more recently Brundin ('13, pp. 337, 346-348) 

 and Dolley ('16, pp. 371-382) have demonstrated the same phe- 

 nomenon in the species with which they worked. 



There can be no doubt, therefore, that many arthropods with 

 one eye blackened are able in time to modify their behavior to 

 light. This, however, in nowise lessens the significance of the 

 initial tendency of the animal to perform circus movements. In 

 fact, this initial tendency is the all-important one as far as the 

 question of orientation in the normal animal is concerned. I do 

 not believe, therefore, that the presence of modifiability in an 

 animal warrants considering its behavior as an exception to the 

 general occurrence of circus movements. 



A second difficulty in the way of any generalization concerning 

 circus movements has been encountered in the behavior of cer- 



