312 



Relations within the Species 



Active Locomotion 



Groups of the same species brought about by the active locomotion 

 of the individuals may arise (1) from the guidance of the organisms 

 towards the same area by oriented responses to inanimate features 

 of the environment or (2) from the attraction of the organisms to 

 others of their own kind. The formation of groups by active locomo- 

 tion is, of course, found most frequently in the animal kingdom, but 

 certain motile algae and the swarm spores of aquatic plants also exhibit 

 this behavior. 



Common Orientation. If the individuals of a species react in the 

 same way to some physical stimulus in the environment, their locomo- 

 tion will bring them to the same locality with the result that an aggre- 

 gation will be formed. A familiar example is the clustering of insects 

 about a source of light at night as a result of their positive phototaxis, 

 or the attraction of fish and invertebrate animals to a torch held over 

 the water (Maeda, 1951). Since land isopods, such as the wood 

 louse, move more slowly, or stop creeping entirely, under moist con- 

 ditions they tend to collect in damp places. A dead fish on the shore 

 or a dead deer in the forest acts as a lodestone to which a large num- 



FiG. 9.1. Aggregation of mud snails (Nassa obsoleta) exposed at low tide in 

 Barnstable Harbor, Mass. 



