1978] Lilly, Ashley, & Tarter — Population of Sialis itasca 213 
small sample size (4) in August, no statement can be made 
concerning the growth rate from August to September. An eighteen 
percent increase in growth rate was recorded from October to 
November. There was a decrease in growth rate in December. Small 
sample sizes in January (2) and February (6) prevented any growth 
rate information. There was a six percent growth rate from March 
to April. The largest mean head width was measured in April at 1.59 
mm and ranged between 0.95 and 1.86 mm September and Novem- 
ber, respectively. No larvae were located in May, June and July. 
Woodrum and Tarter (1973) also found a decrease in growth 
during the winter months in S. aequalis. Azam and Anderson (1969) 
found a decrease in the growth rate for S. rotunda and S. californica 
during the winter months. 
0 
1 
M 
5 
Figure 2. Monthly variation of the head width in S. itasca larvae. Vertical lines = 
ranges, horizontal lines = means, open rectangle = one standard deviation, numbers 
= sample sizes, and dotted lines = temperature (C). 
