Invertebrate Cave Fauna 
133 
allied morphologically, the latter has a much wider geographic dis- 
tribution and contains many more species (Holsinger 1977). 
Bactrurus and Stygobromus are exclusively of subterranean facies 
and occupy virtually every conceivable type of groundwater habitat. 
Moreover, Stygobromus occurs throughout a large part of North 
America north of Mexico and is represented by numerous species (more 
than 160 counting undescribed forms). These facts strongly imply that 
these amphipods are very old stygobionts that have inhabited ground- 
waters for a long period of time. The invasion of cave waters, especially 
small drip and seep pools, appears to be a dynamic, ongoing process 
that has occurred in the past and is continuing at present (Holsinger 
1978, Culver 1982). The original colonization of subterranean ground- 
waters by various crangonyctid amphipods probably occurred long 
before the present generation of cave habitats was available (Culver 
1982). Our observations (Holsinger 1978, Culver 1982) indicate that 
cave drip-pools are not usually the primary habitats of many small 
species of Stygobromus. These cave habitats are only secondary or 
marginal biotopes that are periodically populated by animals from 
interstitial groundwaters outside caves per se. Some good examples in 
the study area of small species recorded only from drip/seep pools 
include S. cumberlandus, S. ephemerus, S. estesi, S. finleyi, S. hoffmani , 
S. leensis, and S. pseudospinosus. Other species in the study area appear 
to be permanent members of the cavernicole fauna and include S. 
baroodyi, S. conradi, S. gracilipes, S. mackini, S. morrisoni, and 
Bactrurus sp. These species are usually comparatively large in size and 
inhabit small streams, although some, especially V. mackini, are 
commonly found in drip pools as well. One cavernicolous species in the 
study area, S', stegerorum, is unique, however. It is known only from 
deep lakes of phreatic water in two caves in Cave Hill in Augusta 
County, where it is associated with the cirolanid isopod Antrolana lira. 
Another amphipod, Gammarus minus (Gammaridae), although not 
considered a troglobite, is of considerable interest. In the Appalachian 
Valley this species is apparently represented by three morphological 
forms (see Holsinger and Culver 1970). Form III has relatively short 
antennae, well-developed eyes, and dark pigmentation; it inhabits surface 
springs throughout the range of the species. Form II has relatively long 
antennae, slightly reduced eyes, and weak pigmentation (variable); it 
inhabits caves over a broad geographic area. Form I has relatively long 
antennae, greatly reduced eyes, and weak pigmentation; it is known 
only from caves in the Ward Cove karst of Tazewell County, Va., and 
the Great Savannah karst of Greenbrier County, W.Va. These two areas 
are situated in different drainage basins and are separated 
geographically by a distance of 122 km and several prominent mountains. 
Form II occurs in many caves in the Virginias but is rarely as common 
