SUBTERRANEAN AMPHIPOD STYGONECTES 149 



Variation in meristic characters (e.g., number of spines and setae) 

 varied with the size of specimens. For example, sexually mature 

 animals of smaller size (generally from populations with smaller size 

 means) had fewer marginal setae on the coxal plates than sexually 

 mature animals of larger size (generally from populations with larger 

 size means). Relative to the number of pertinent spines and setae, 

 the female from Mineshoe Island was nearly identical with comparable- 

 sized females from Refton Cave. Animals from the seep in Gables 

 Woods had a few less setae and spines on diagnostic structures than 

 larger animals in Refton Cave and a few more setae and spines on 

 these structures than smaller animals in Wetzels Spring and Chester 

 County. Sexually mature amphipods from the last two populations 

 were generally similar in meristic characters. 



The foregoing observations indicate that variation in size means of 

 sexually mature animals from different populations is not influenced 

 by genetic factors. If differences in size and meristic characters were 

 affected by genetic differences in populations, those popidations 

 inhabiting areas closer together geographically and, therefore, more 

 likely to carry on gene exchange would be more homogeneous than 

 those populations situated further apart. Available data do not 

 indicate this to be true. Reasons for differences in size means of 

 individuals in different populations appear to be caused by ecological 

 factors as will be seen from the following discussion. 



Populations with the smallest sexually mature animals, viz., Chester 

 County and Wetzels Spring, are known from small springs which 

 resurge from crystalline bedrock or a combination of bedrock and 

 overlying mantle. Although the exact physical structure of these 

 biotopes is not known, they are assumed, by the natm'e of their 

 surrounding rock, to be greatly restricted in size and extent. In 

 contrast, the population of S. pizzinii with the largest sexually mature 

 animals is from Refton Cave, where direct observations show this 

 habitat to be a large underground lake occupying more than one-half 

 of a room which measures approximately 60 feet wide and 125 feet 

 long. 



Observations are not available on the habitat in Pequea Silver Mine, 

 but on the basis of observations in other mines, this habitat is believed 

 to be in the form of pools fed by ground-water seepage. Finally, 

 subterranean waters that feed the seep in Gables Woods come from 

 limestone bedrock and probably flow through solution channels or 

 small caves. While the exact nature of the last two biotopes is hypo- 

 thetical, they can almost certainly be regarded as larger in overall 

 dimension than those associated mth the smaU springs in Washington, 

 D.C., and Chester County. Accordingly, individuals in popidations 



