THE TARANTULAS 127 



failed to alter its form to cope with altered environment. More 

 advanced offshoots from this same primitive stock are the sheet- 

 web atypical tarantulas (family Mecicobothrndae), the folding- 

 door tarantulas and relatives (family Accatymidae), and the 

 above-mentioned purse-web spiders. The atypical tarantulas have 

 paralleled in their development the other principal branch of the 

 suborder, the tarantulas and trap-door spiders, and have matched 

 rather closely their handiwork in silk. 



The outstanding characteristic of this whole series is the clear- 

 cut visual evidence of segmentation on the dorsum of the abdomen. 

 In the past, the abdominal tergites of the Liphistiidae have been 

 hailed as evidence, along with the spinnerets and some other fea- 

 tures, to set the family apart by a very wide margin from all other 

 spiders, and place it in a separate suborder. This early evaluation 

 of the liphistiids has become so fixed in the minds of most spider 

 students that they have denied that any other living spiders are 

 segmented in the adult stages. One has only to look at the abdomens 

 of Antrodiaetus, Hexura, or At y pus to see tergites that differ little 

 or not at all from those of Liphistius. And a study of the other 

 features of these genera demonstrates with little question that the 

 relationship between the more generalized Liphistius and its modern 

 cousins is a real one. 



The atypical tarantulas are of moderate size, few of them ex- 

 ceeding an inch in length, and in general form and appearance they 

 resemble the typical trap-door spiders. Most of them are accom- 

 plished burrowers, but only the folding-door tarantulas and close 

 relatives have the chelicerae fitted with a rake of coarse teeth for 

 digging. The unpaired claw is present on the tarsi, but no claw 

 tufts or tarsal brushes have been developed. The full complement of 

 eight spinnerets is present in the liphistiids, and the pudgy lateral 

 pairs bear some resemblance to those of ancient spiders. The other 

 atypical tarantulas long ago lost the anterior median pair. The per- 

 sistence of the anterior lateral pair is noteworthy, since it is present 

 elsewhere among the mygalomorph spiders only in one or two 

 primitive members of the family Dipluridae. The anterior lateral 

 spinnerets are two-segmented and functional in Aliatypus, uniseg- 

 mented and small in Atypoides and most other genera, and com- 

 pletely missing in Antrodiaetus. 



In some respects the atypical tarantulas have outdistanced the 

 typical tarantulas, even though the physical heritage of the former 

 includes more generalized features. The male palpus is provided 



