SUBTERRANEAN AMPHIPOD STYGONECTES 131 



and spines, are characterized by having well-defined patterns of insular 

 speciation and little or no appreciable sexual dimorphism. As already 

 implied, this lineage is perhaps best regarded as representitive of one 

 of the oldest fresh-water invasions of ancestral stygonectid stock. The 

 reduction and more often the complete loss of certain setae and spines 

 on the gnathopodal propods and the tendency for gnathopodal propod 

 2 to equal gnathopodal propod 1 in overall size are characters that 

 readily distinguish species in this lineage from the majority of others 

 in the genus. With the exception of only one species (S. stellmacki) 

 there has been a complete loss of rastellate setae on both gnathopods, 

 loss of posterior marginal setae on the propod of gnathopod 1, and 

 loss of all but one spine on the outside of the posterior angle of 

 the propod of gnathopod 2. Besides the near equality in size of the 

 gnathopodal propods, in most species pereopod 6 approaches and 

 often equals pereopod 7 in total length. Although admittedly still 

 poorly known and even rather aberrant, S. stellmacki possesses a 

 number of characters highly diagnostic for the emarginatus group, but 

 at the same time retains several structures that are interpreted as 

 being intermediate between the more highly specialized lineage 

 discussed here and the more primitive tenuis-pizzinii lineage described 

 below. 



2. The tenuis and pizzinii species groups compose a second major 

 lineage characterized in general by rather poorly differentiated and 

 commonly vagile species, which are further distinguished by lack of 

 well-defined patterns of insular speciation, although a few of the poorly 

 kno^vn species of the tenuis group appear to have greatly delimited 

 ranges. In the Edwards Plateau of central Texas, representatives of 

 this lineage are almost exclusively cavernicolous, but elsewhere species 

 occur more often in interstitial habitats than in caves. Morpho- 

 logically, species in this lineage normally have gnathopodal propod 1 

 larger than 2, setae on the posterior margin of the first gnathopodal 

 propod, more than one spine on the posterior angle of the second 

 gnathopodal propod, and rastellate setae. Pereopod 7 is also usually 

 longer than pereopod 6, and pereopod 5 is, with few exceptions, pro- 

 portionately shorter than in species of the first or third lineages. 

 Sexual dimorphism is typically more pronounced, although species 

 that occur in more inland areas are an exception to this rule. Finally, 

 this lineage is represerited by species that occur throughout most of 

 the generic range (excepting the central, folded Appalachians of 

 Virginia and West Virginia) and are interpreted as being more primi- 

 tive morphologically and generally less specialized ecologically, 

 although the latter trend might be debatable. This lineage is further 

 believed to occupy an evolutionary position closer to ancestral 



