56 
Psyche 
[March 
tremes ranged from 3.3 to 23.3 C in January and July respectively. 
The mean hydrogen ion concentration (pH) was 7.1 with a range 
of 6.7 to 7.5 in August and December respectively. The mean dis¬ 
solved oxygen concentration was 8.9 mg/1 and ranged from 6.3 to 
11.0 mg/1, August and January respectively. The oxygen saturation 
mean was 83.8 percent and ranged from 73.0 percent in August to 
95.0 percent in April. The mean bicarbonate alkalinity concentra¬ 
tion was 47.5 mg/1 with a range of 30.0 to 65.0 mg/1, February and 
September respectively. The mean total hardness was 59.6 mg/1 
with a range of 50.0 to 70.0 mg/1, January and October respectively. 
Nymphal Length-Frequencies. Length-frequency histograms of 
the B. bajkovi nymphs are shown in Figure 1. The smallest and 
earliest nymph was collected in July 1976 and measured 0.76 mm in 
length. The largest nymph, a female with a length of 10.4 mm, was 
collected in May 1977. The monthly length distributions, the 
absence of nymphs in June, and the total absence of mature nymphs 
in the late summer, fall, and winter months indicates a one year 
(univoltine) life cycle. Similar findings on the life cycle from other 
studies in the genus Baetisca have been reported by Traver (1931), 
Berner (1955), Pescador and Peters (1971, 1974), and Lehmkuhl 
(1972). 
Nymphal Growth. Head width measurements of B. bajkovi 
nymphs showing the monthly progression of growth are illustrated 
with a population range diagram (Figure 2.) Head widths ranged 
from the smallest nymph in July (0.31 mm) to the largest male 
(2.67 mm) and the largest female (2.81 mm) in May. Figure 2 also il¬ 
lustrates the univoltine life cycle and shows a definite female size 
superiority. 
The greatest growth rate exhibited at any time during the life 
cycle was 80.0 percent which occurred from July to August when the 
mean monthly head width increased from 0.45 to 0.81 mm. The 
lowest growth rate (1.3%) occurred from April to May when the 
mean monthly head width increased from 2.32 to 2.35 mm. Nym¬ 
phal growth rate was also very low (3.5%) from January to Febru¬ 
ary when the lowest water temperatures (3.3 and 4.4 C) were 
recorded for the study. 
Nymphal Food Habits. The relative abundance of foregut con¬ 
tents of B. bajkovi is given in Table 1. The nymphs are detritivoroiis 
with almost two-thirds (65.3%) of the diet composed of plant and 
leaf detritus. Diatoms were the next most abundant item (17.1%) in 
