258 AMERICAN SPIDERS 



been transported and distributed largely by man in his ships. All 

 are known to balloon, and have further distributed themselves in 

 this manner. Transportation of animals by man in ships and air- 

 planes is going on constantly, but relatively few kinds are able to 

 establish themselves in new climatic situations. Their distribution 

 by means of ballooning threads is restricted only by prevailing 

 winds. There is little reason to believe that aerial spiders cannot 

 cover great distances at high altitudes, and live through the ordeal 

 in sufficient numbers to establish themselves. Only one successful 

 flight is necessary; nature provides time for innumerable ventures. 

 The fact that relatively few kinds become established demonstrates 

 that mere access to a new region is not enough. The immigrant 

 must be able to cope with a complex climate possessing numerous 

 characteristics, any single factor of which may be capable of ex- 

 cluding it from survival. 



Some immigrants into the New World are thought to have 

 arrived recently. One is a handsome jumping spider, Sitticus pubes- 

 cens, which has become established in Allston, Massachusetts, and 

 may well expand its range in all directions from this center, since 

 it seems to have habits similar to the well-known zebra spider, 

 Salticus scenicus. Nesticus cellulanus and Steatoda bipunctata are 

 also of relatively recent introduction; these spiders are gradually 

 moving westward from their original centers in Nova Scotia and 

 along the northeastern coast of the United States. Both live in 

 buildings or in adjacent trash heaps, and perhaps will not find com- 

 petition with our own spiders of similar habitat too strenuous. The 

 only Steatoda the author found near several houses in Toronto, 

 Ontario,, was the European bipunctata. Inasmuch as this is the 

 normal territory for our American Steatoda bor calls (which was 

 observed to be abundant two hundred miles north at Lake Tema- 

 gami), it is possible that the European invader will reduce or crowd 

 the native population out of particular regions as time goes on. 



The spiders known to occur both in the Palearctic and Nearctic 

 regions are listed below. As has been indicated, most of the species 

 are old residents and represent a panboreal or Holarctic fauna. 

 Those that are cosmopolitan are identified by the letter "C," and 

 similarly those that are tropicopolitan are labeled with a "T." 



