No. o03] 



THE ACTIVITIES OE ABANEADS 



707 



degree of perfection hardly equalled by any other ter- 

 restrial animals. 



The question then presses, of what use are the eyes to 

 snare-weavers when their sensations are so particularly 

 tactile! The newly hatched spiderlings evidently use 

 their eyes for they are always positively pliototropic while 

 the adults are generally negatively so. This turning to- 

 wards the light benefits the spiderlings and consequently 

 the species by serving to disseminate them from the home 

 area into new feeding grounds. And I believe it is a 

 quite general phenomenon among all animals whose adults 

 are more or less sedentary and tubicolous, negatively plio- 

 totropic, for the young to be at first positively pliototropic, 

 though I do not know whether any one has drawn atten- 

 tion to the comprehensiveness of this principle; in this 

 most wide-spread kind of migration the beneficial result 

 of the change of tropism is to prevent overcrowding. 

 But as the young snare-weaver grows older and begins 

 to avoid the light as does its parent, it does not employ 

 its eyes in the primal acts of feeding and uniting but main- 

 ly determines by them the source of the light in order to 

 avoid it. Despite their complexity, accordingly, the eyes 

 of snare-weavers, when they have passed infancy, seem to 

 be used mostly as direction eyes. This being the case it 

 seems strange that these eyes should have retained the 

 complexity inherited from hunting forefathers, and it is 

 possible that they have come to subserve some other new 

 function, as, e. g., to have become thermic receptors; 

 this might well be determined experimentally. At any 

 rate we shall have to change current views as to the role 



4. On the Avekage Duration of Life in Araneads 

 In the case of all species that I have studied adult 

 males are found during only a short period of the year, 

 for perhaps not longer than a month or six weeks, and 

 in latitudes where there is a marked winter season they 

 do not live over this period of cold. And from observa- 

 tions on TKeridium tepidariorum I estimate from the rate 



