88 ANIMAL AGGREGATIONS 



integrative action, one is dealing with a crowd, a mere collection of 

 individuals within a limited area. Apparently it was this aspect that 

 Szymanski (19 13) had in mind in distinguishing between primary 

 reactions, the reactions of the individual, and secondary reactions, 

 the reactions of the individuals as members of a group. 



TACTILE INTEGRATION 



The simplest form of group organization is found when animals in 

 physical contact respond as a group to touch stimuli passed from one 

 to another. Such organization may be sufHciently refined for the 

 whole group to show definitely synchronous behavior. Collections of 

 Liobumi7?i, the harvestman, have been observed by Newman (191 7), 

 and later by myself, to give such reaction. One group was found by 

 Newman resting on the under side of an overhanging shelf of rock on 

 a steep hillside. The harvestmen were closely packed together within 

 an area of about 5 feet in diameter. When first seen, they were hang- 

 ing from the rock roof perfectly motionless. When the observer came 

 nearer, they began a rhythmic stationary dance practically in uni- 

 son. This died down shortly but could be started again by appro- 

 priate mechanical stimulation. 



When the colony was first seen, the long legs of neighboring in- 

 dividuals were interlocked, which would sufficiently account for the 

 transmission of stimuli through the group. It should be noted, since 

 we are interested in the state of integration of the aggregation, that 

 the rhythm was not perfectly synchronous at the beginning but be- 

 came practically so after a few seconds. 



Such integration, due to tactile transmission, must be present to 

 some degree in all cases of aggregation in close physical contact. It 

 is highly developed in the sleeping groups of bats (Allen, 1921), 

 which may hang in compact clusters, as already mentioned. If one is 

 touched, the whole cluster may drop. Allen caught eighteen by hold- 

 ing an insect net under the group and touching only one of the outer 

 bats. 



The effect of physical contact in establishing synchrony in two 

 reacting systems is illustrated by the observation of Fischer (1924), 

 who found that two pieces of embryonic heart planted out in tissue- 



