312 
Psyche 
[September-December 
was searched for adults on the wing or exuviae in the months 
when no naiads were found (May through July). The rocks and 
vegetation were checked for the shed skins of adults. 
Fecundity in the adult stonefly was determined by direct counts 
of the ovarian eggs using a binocular dissecting microscope 
(Bausch and Lomb). Both ovaries were excised from 15 females 
(only eight contained eggs) and 241 eggs were counted. The di- 
ameters of 20 percent of the eggs from eight females were meas- 
ured with an ocular micrometer (nearest 0.1 mm) under a Bausch 
and Lomb dissecting microscope. 
The chi-square test was applied to 23 laboratory reared adults 
to determine any significant departure from the 1:1 ratio at the 
0.05 confidence level. 
Results and Discussion 
Stream Environment. — The average annual temperature in the 
tributary for the study period was 13.9 C. The monthly extremes 
were 5 and 22.7 C in November and August, respectively. The 
hydrogen ion concentration (pH) ranged from 7.5 to 8, with an 
average of 7.8. Dissolved oxygen concentration ranged from 4 
to 15 mg/1 in August and January, respectively, with a mean of 
8.5 mg/1. Carbon dioxide values ranged from 5 mg/1 in April, 
May, June, January and February to 25 mg/1 in October. The 
mean was 18.7 mg/1. Total hardness ranged from 88.5 to 205.2 
mg/1 in April and November, respectively, with a mean of 135.6 
mg/1. Carbonate alkalinity values ranged from 34.2 to 136.8 mg/1 
in April and October, respectively: the mean value was 72.7 mg/1. 
Naiadal Development. — Length-frequency histograms indi- 
cated that one size class was present in the naiadal population 
of D. robusta (Figure 1). The earliest naiad was collected 30 
August 1975 and measured 2.8 mm in body length. The largest 
naiad was found 26 April 1976 and measured 16.9 mm in body 
length. No naiads were found in the stream from May through 
July. 
Frison (1935) stated that Isoperla duplicata (= D. robusta) is 
univoltine in Turkey Run State Park (Newby Gulch), Indiana. 
Minshall and Minshall (1966) did a study on two other perlodid 
stoneflies, Isoperla clio (Newman) and Isogenus decisus Walker 
and found both to be univoltine. 
