FORMATION OF ANIMAL AGGREGATIONS 6i 



where the animals may be resting quietly or where they may exhibit 

 what appears to be a perfect frenzy of erratic activity. As stated 

 above, Deegener regarded these as forming play societies, while 

 Schulz thought of them as having protective values. 



From the analysis of Brown and Hatch (1929) it appears that the 

 aggregating behavior of these beetles is largely due to visual stimuli, 

 since the aggregations break up in the dark. Further, the position 

 they occupy in the laboratory tanks, though not necessarily in na- 

 ture, may be determined by the lighting. These authors believe that 

 the gyrinids are exhibiting a more complex type of behavior than 

 that which is usually called "tropistic," and refer it rather to some 

 sort of configurationist behavior, in which orientation behavior con- 

 sists of movements so co-ordinated that an invariant relationship is 

 maintained between movements and variations of the visual field. 



They find evidence of two sorts of orientation : one in which the 

 body axis is maintained in a relatively fixed position with respect to 

 the base of orientation, and another in which the body is maintained 

 in a relatively fixed region but without body orientation. The former 

 is like the orientation called for by the tropistic theory. The latter 

 bears at least a superficial resemblance to those cases where organ- 

 isms move along a physical or chemical gradient in one direction 

 without reaction to it but execute negative "avoiding" reactions 

 when moving in the opposite direction, like the trapping of Parame- 

 cia in weak acids, as described by Jennings. 



They believe that the location of an aggregation in nature is due 

 to habituation to certain visual patterns, possibly of light and shade, 

 to which the animals respond; these patterns are not significant in 

 themselves but are a sign of the location of general environmental 

 conditions which are of vital importance to the beetle and the spe- 

 cies. If the patterns are slowly changed, the beetles may remain in a 

 given position; and collections have been observed not to shift their 

 position as much as a meter during a whole day, although the pat- 

 tern of the field of vision changed radically in that time. If, however, 

 the patterns are rapidly changed by a sudden increase in the com- 

 plexity of the visual pattern, marked stimulation to activity results, 

 which may cause a breaking-up of the aggregation. 



