Cave Isopod Ecology 
111 
It is worth pointing out that C. cannulus is, overall, larger than C. 
holsingeri. The basic question is what determines body size, both in the 
ultimate and proximate sense. It is unlikely that size differences are due 
to differences in age structure, since age (or at least size) structure is 
skewed toward older (or larger) individuals (Culver 1971). However, it is 
ISOPODS 
GRAVELS 
10 - 
s- 
HARMAN 
Fig. 1. Gravel and isopod sizes for Harman, Linwood, Bowden, and Glady caves. 
See Table 5 for description of the distribution. 
impossible, with available data, to determine whether size differences are 
genetically or environmentally determined. Dickson’s (1977) finding that 
drip pool populations of the amphipod Crangonyx antennatus had con- 
sistently larger size individuals than stream populations, but showed no 
