74 ANIMAL AGGREGATIONS 



part of the dish and are to be explained in part by the fact that the 

 lower animals are shaded by the upper ones, and so, having satisfied 

 a negative phototropism and a positive thigmotropism, they remain 

 quiet. 



The position of the arms shows the strong thigmotropic reaction 

 of these animals. In the recently formed bunches there are a larger 

 number of free arms than in older aggregations ; at first the arms tend 

 to extend out and up into the water. They may be entirely free, or 

 they may touch another arm only where the two cross. Even in the 

 early stages of the bunching some of the arms lie nearly parallel with 

 each other. In bunches of longer duration there are practically no 

 free arms. In one case I saw two starfish with four pairs of arms 

 paralleling each other, and only two free. Larger bunches become 

 ropelike masses composed of parallel arms or of arms intertwined 

 like basketry. In these older aggregations the arms of the animals, at 

 first extending freely, are turned back and interwoven with the 

 others so that the outer edge presents a relatively regular line. When 

 these starfish are isolated and left for a week or more in separate 

 dishes exposed to light, frequently the arms are moved into contact 

 until they present a sort of self-bunching. 



Laboratory aggregations occur in a large number of animals. 

 May-fly nymphs, various isopods, earthworms, frogs, and others 

 may readily be observed to form such bunches. The behavior ap- 

 pears due to similar causes to that which results in the collection 

 of foreigners into communities of their own nationality in our large 

 cities; that is, a group of similar animals tend to minimize for each 

 other the disturbing effects of unusual surroundings. 



"sleep" aggregations 



The sleep aggregations of insects have been relatively little written 

 about, even in research journals; so it seems important to bring to- 

 gether a more extended summary concerning such slumber aggrega- 

 tions than is needed for the better-known overnight assemblies of 

 birds. 



Fabre (191 5) found some hundreds of the wasp Ammophila (Sphex) 

 hirsuta assembled under the shelter of a stone on the mountain side, 



