THE TARANTULAS 109 



physical equipment (if we measure this in terms of the degree of 

 change from ancestors), and has come down as a reminder of what 

 most of the mygalomorph types were like during the Mesozoic. 

 These latter we refer to as the atypical tarantulas. 



THE TYPICAL TARANTULAS 



In this series, which includes the tarantulas, the sheet-web ta- 

 rantulas, and the true trap-door spiders, there is no visual evidence 

 of dorsal segmentation of the abdomen. The maxillary lobes are not 

 at all developed in the American species, but in some exotic forms 

 a small angled spur or a well-developed process may be present. 

 Nearly all have but four spinnerets, the hind lateral and median 

 pairs; these are located close in front of the anal tubercle. The 

 commonly associated characteristics of tarantulas large size and 

 hairy covering should not mislead one in identifying members of 

 this group. Many are relatively small in stature. Only the wander- 

 ing hunters, the true tarantulas, are thickly clothed with velvety 

 wool and long silken hairs; others appear quite naked by compar- 

 ison. 



A few of the mygalomorphs have become vagrant, but none 

 has attained the degree of freedom enjoyed by certain true spiders. 

 Failure to improve vision has resulted in the development of very 

 few accomplished runners, jumpers, or climbers, and none of these 

 tarantulas has become dependent on silk as have the aerial true 

 spiders. Their reliance on touch is perhaps even stronger than in 

 the araneomorphs; the hairy covering of the vagrants, for example, 

 serves admirably to make them aware of the presence of their prey. 



The typical tarantulas have been most successful in living a 

 secretive life hidden in the ground, with the consequence that many 

 have become specialists in subterranean existence. Their general 

 makeup fits them eminently for a successful life in tropical regions, 

 where competition is not so keen. Few Americans realize that in 

 the southern portion of the United States exists a rich and varied 

 fauna of mygalomorph spiders, eighty or ninety species including 

 many with curious habits. 



Trap-Door Spiders. Many spiders tunnel into the soil, but the 

 true trap-door spiders of the family Ctenizidae are the most accom- 

 plished burrowers and the most gifted artisans. They and their 

 relatives can claim to be the inventors of that superb mechanism to 



