CHAPTER IV 

 GENERAL FACTORS CONDITIONING AGGREGATIONS 



In many animal species the formation of an aggregation depends 

 on the physiological state of the animal. This may be controlled by 

 internal developments, such as the maturing of the sex products, or 

 by external factors, as when land isopods are made to bunch by con- 

 trolling the moisture of the substratum; but more commonly the in- 

 ternal and external conditioning factors work together closely. Some 

 of the more outstanding of these are discussed here. 



THE BREEDING SEASON 



Water isopods. — My own attention was drawn to the general prob- 

 lem of animal aggregations in 191 1, while studying the factors con- 

 trolling the rheotropic reaction in the common water-isopod Asellus 

 communis. As spring came on, the stream isopods no longer gave 

 highly regular, positive responses to the water current; but, as stated 

 in the preceding chapter, one might strike across a strong current, 

 guided apparently by sight, and seize another isopod, male or fe- 

 male. From such a beginning one might soon have all the isopods 

 under observation gathered into a compact rounded cluster, rolling 

 over and over in the water. 



During the height of the breeding season stream isopods disregard 

 the stimulus of a water current almost completely unless they are 

 relatively isolated. On the other hand, I have repeatedly tried to 

 induce half-grown Aselli to form such a cluster, even placing them 

 in a watch glass with rounded, smooth bottom, where they were con- 

 tinually brought in contact with each other, but no real aggregation 

 resulted. Bunching may be induced in adults out of the breeding 

 season, but many conditions that favor it in April during the height 

 of the breeding season have little or no effect in late May (Allee, 

 1923)- 



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