FORMATION OF ANIMAL AGGREGATIONS 51 



described above. The reaction will be repeated whenever the posteri- 

 or end is stimulated, and only ceases when the abdomen curls over 

 the edge of the leaf. In this way, and as a result of these reactions, 

 the colony takes on its well-organized appearance, which depends on 

 the interaction of the following factors: (i) the crowding of many 

 individuals into a small space; (2) the tropic reactions of the larvae; 

 (3) the character of the anterior and posterior end reflexes; and (4) 

 the manner of locomotion and of feeding. 



Here, as in Szymanski's analysis of the group formation in Hypo- 

 nomenta, primary reactions play the principal role in the colony 

 formation ; but there are some elements of the behavior of the colony 

 that Szymanski thinks may be due to secondary or social behavior. 

 Thus, when the leaf is shaken, the posterior end of each larva is 

 raised simultaneously. When we remember the great individual dif- 

 ferences usual in behavior, the synchrony of this response suggests 

 that there may be a social factor at work. However, it is possible 

 that this, too, is merely an expression of primary or individual reac- 

 tion, with the synchrony either more apparent than real or due to 

 the proximity of the responding larvae. 



These investigations of Szymanski's lead to the same conclusion 

 as my own, formed independently, concerning the method of forma- 

 tion of aggregations of land isopods and of Ophiodcrma. In these 

 cases it is the primary, individual reactions that produce the group- 

 ings, not the expression of a community spirit or of a social appetite. 

 The only social trait necessarily present is that of toleration for the 

 presence of numerous other similar animals within the same region. 

 If this analysis be sound, as it appears to be, then one of the early 

 stages of mutual interdependence is the appearance of toleration for 

 the presence of other animals in a limited space, where they have 

 collected as a result of tropistic reactions to environmental stimuli. 

 Once formed, aggregations may persist for a considerable time, mere- 

 ly because of the lack of disruptive stimuli. 



The conclusions of Szymanski are supported by Krizenecky (1923) 

 in his work on the transitory aggregations of the enchytraeids al- 

 ready mentioned in the chapter on classification. He thinks that 

 individual reactions are important in the formation of these aggre- 



