132 U.S. NATIONAL MUSEUM BULLETIN 2 59 



stygonectid stock than either the first lineage discussed above or the 

 third one described below. 



3. The flagellatus group of central Texas constitutes what has 

 been inteipreted as a third, but somewhat more obscure, lineage, 

 which appears to be morphologically intermediate between the two 

 described above. Species in the flagellatus group, like those in the 

 emarginatus group, are almost exclusively cavernicolous, greatly 

 delimited geographically, Avithout appreciable sexual dimorphism, and 

 exemplified by patterns of insular speciation. This lineage also 

 parallels the first lineage in the tendency for species to be characterized 

 by the near equality in the size of the gnathopodal propods, by the 

 approximately equal length of pereopods 6 and 7, and by the propor- 

 tionately long fifth pereopods. It differs from the first lineage, 

 however, and, therefore, more closely resembles the second lineage by 

 presence of setae on the posterior margin of the first gnathopodal 

 propod, by presence of more than one spine on the outside of the 

 posterior angle of the second gnathopodal propod, and by presence of 

 rasteUate setae on one or both gnathopods. Certain similarities 

 between species in the flagellatus group and emarginatus group might 

 have resulted from parallel evolution, and it is suggested that these 

 similarities have developed independently in each group in response to 

 similar selection pressures brought about by what was probably an 

 extended existence of both groups in the cave environment. 



The evolutionary position of the hadenoecus group (known only 

 from one species) is still rather vague. Whether or not this group 

 composes a fom"th, distinct evolutionary lineage or should merely be 

 regarded as an aberrant m.ember of the third lineage is problematic. 

 Its isolated range (known only from a single cave) and its presence 

 further inland than any other stygonectid species in the Edwards 

 Plateau poses, however, some interesting zoogeographic questions. 

 Perhaps further collecting in Texas caves will help clarify the present 

 problems associated with this group. 



Speciation and Dispersal 



The emarginatus group. — Present patterns of species distribution 

 in the emarginatus group are insular and appear to have been greatly 

 influenced by extrinsic barriers to dispersal. Specific ranges are 

 restricted to valleys floored with cavernous limestone and separated 

 by ridges of insoluble, noncalcareous bedrock. Geographic distribu- 

 tion of species and their proposed phylogenies are shown on the map 

 in figure 31 and by the dendogram in figure 32, respectively. 



The overall distribution of species in the emarginatus group is to 

 some extent a reflection of the distribution of ancestral species. 



