GENERAL FACTORS CONDITIONING AGGREGATIONS 73 



will collect in a limited area because it provides an oasis of moisture 

 or of dryness in an otherwise overdry or overwet environment. Thus 

 land isopods can be made to collect at will in a given spot by making 

 it moist. Selous (1907) gives a striking picture of the congregating of 

 large ungulates about an African drinking-hole in the dry season. 

 The common fruit fly, Drosophila, struggling to escape too great 

 moisture, aggregates in shifting masses at the top of a projection; 

 these masses continually fall apart and re-form as the flies move up 

 again. Under optimal conditions all of these move out of contact 

 with their fellows. 



LACK OF NORMAL ENVIRONMENT 



The snake starfish, Ophioderma, lives in eelgrass in certain loca- 

 tions along our eastern coast. Repeated attempts have failed to 

 find this animal in contact with others of its own kind in nature 

 during the summer (Aflee, 1927). They are often found near together 

 but never aggregated. 



Ten of these starfish were introduced into a laboratory aquarium 

 made to approach normal living conditions by the introduction of 

 eelgrass. Nineteen hours later 7 of the 10 animals were sighted after 

 a search lasting half an hour. One was found on the bottom at the 

 side away from the strongest light ; 6 animals were in the densest part 

 of the vegetation in the same region; and, although not in immediate 

 contact, all of them could probably have been inclosed in a 5-inch 

 cube. The exact location of the other 3 animals could not be ob- 

 served without disturbing them. These animals in the field may 

 also be close together without actually touching. Only such loose 

 collections were ever seen in this eelgrass aquarium. Extended ex- 

 perience with these animals in the laboratory leads me to conclude 

 that the tendency to bunch is greatly reduced in proportion as 

 favorable natural conditions are approximated, and that the animals 

 so congregated are usually found in regions to which they have been 

 directed by their tropistic reactions. 



When, however, Ophioderma are placed as they are collected in a 

 glass or similar container, they form dense mats of bunched animals 

 with arms closely interwoven. The aggregations form in the shadiest 



