1980] 
Morris, Chaffee, & Tarter — Baetisca 
43 
NYMPHAL STAGE 
Size-Class Distribution. —Length-frequency distributions indicate 
a univoltine life cycle (Figure 1). The smallest nymph (2.1 mm) was 
collected in July and the largest nymph (11.2 mm) was taken in May. 
The increasing monthly progression of the size class and the total 
disappearance of large nymphs in the July sample indicated a one 
year life cycle. Nymphs hatched as early as July and progressed in size 
until late May or June when they emerged. Numerous nymphal skins 
were found on stones less than 5 cm from the waters edge or water lies 
in mid-June. The population range diagram, based on nymphal head 
widths, is presented in Figure 2. The smallest head width occurred in 
July (0.7 mm) and the largest head width occurred in May (3.1 mm). 
Traver (1931) collected Baetisca Carolina in North Carolina in 
March and emergence occurred in April and May in the laboratory. 
Small nymphs of B. callosa in West Virginia were found as early as 
August (Traver, 1931). Traver (1937) found skins of B. thomsenae (= 
Carolina) in Florida in May and June. Berner (1955) reported 
laboratory emergence of B. obesa from early March to early April in 
D JF'MUMjJISON 
MONTH 
Figure 2. Population range diagram of nymphs of B. berneri based on head 
widths. Vertical lines = range, horizontal lines = mean, open rectangles = one 
standard deviation, closed rectangles = one standard error of the mean, m = male, and 
f = female. 
