Gammarus Variability 
135 
were scored as broad lobe up, equal, or down. This character is not ob- 
viously related to habitat but is simply an easily scored trait that may 
show interdemic variation. 
(3) Lobe angle. The reniform eye has a shallow, slightly variable 
angle opening anteriorly. This angle was not measured but was scored 
relatively on an ordinal scale from 1 to 3 indicating, respectively, more 
acute, intermediate, and more obtuse. Like lobe orientation this is not an 
ecotype differentiating character. 
(4) Eye length. The length of the eye is successively less in Form II 
and Form I amphipods. It was measured (mm) parallel to the long dor- 
soventral axis of the eye. 
(5) Length of antenna 1. Elongate first antennae are particularly 
striking in cave ecotypes. The first antenna was measured from pedun- 
cular base to flagellar tip. 
(6) Length of pereopod 7. The seventh pereopod is relatively longer 
in hypogean habitats. It was measured extended from coxal base to tip of 
dactyl. 
(7) Length of uropod 3. This is a highly variable biramous structure 
in G. minus (Holsinger 1976) and is usually longer in cave forms. The long 
ramus (exopod) was measured. 
Measurements were taken on population samples from sites listed 
below in inferred rank order, from most nearly hypogean to most open 
eipigean. 
(i) Emma Spring, Huntingdon Co., PA: strongly shaded spring dis- 
charging from a subterranean conduit in a limestone rockface. 
(ii) Smoke Hole Spring, Giles Co., VA: partially shaded spring pool 
formed from outflow of Smoke Hole Cave. 
(iii) Greenland Gap, Grant Co., WV: partly shaded runoff 4 m 
downstream from a spring. 
(iv) Cunninghams, Huntingdon Co., PA: unshaded runoff 7 m 
downstream from spring. 
(v) Marklesburg, Huntingdon Co., PA: shaded, heavily vegetated 
first order stream collected 12 m downstream from small spring. 
(vi) Church Camp, Centre Co., PA: shaded runoff 15 m downstream 
from series of large ground seeps. 
(vii-viii) Petersburg I and II, Huntingdon Co., PA: large, partly 
shaded springfed stream; site I is a large, open, impounded pool, site II is 
runoff about 20 m downstream from the pool. 
(ix) James Creek, Huntingdon Co., PA: unshaded site in a second- 
order stream about 3 km downstream from spring sources. 
This rank order is subjectively based on proximity to subterranean 
water source, likelihood that discharge is from a sizeable cave or conduit, 
and amount of cover or shade. Emma Spring is very secluded. Smoke 
Hole Spring receives the immediate discharge of Smoke Hole Cave, 
which probably contains Form II amphipods as does nearby Tawneys 
Cave, a site of Holsinger and Culver. These localities would be expected 
to harbor populations bordering on Form IT The James Creek site is 
