1978] Lilly, Ashley, & Tarter — Population of Sialis itasca 211 
Fifteen larvae were collected in April and returned to the 
laboratory for rearing in vials containing pond water. These vials 
were connected to vials containing sand by a short section of rubber 
tubing (Pritchard and Leischner, 1973). Wire mesh and strips of 
foam rubber were placed in the vials and rubber tubing to enable the 
larvae (one per vial) to move freely between the vials. The vials were 
kept at room temperature which was approximately 70 F. 
Fecundity was determined by a direct count of ovarian eggs under 
a compound microscope. The ovaries of 3 adults were removed and 
a total of 1616 eggs were counted. The diameters of 90 eggs were 
measured to the nearest 0.01 mm with an ocular micrometer using a 
Bausch and Lomb compound microscope. 
Results and Discussion 
Pond Environment 
Temperature. — The average annual temperature for the pond 
was 18.1 C. The extreme monthly temperatures were 0.5 C in 
February and 35.0 C in August and September. 
Water Chemistry. — The average annual pH was 8.4 (7.5 to 9.0). 
The average annual dissolved oxygen was 8.5 mg/ 1 (6.5 to 10.0). 
The average annual total hardness was 139.6 mg/ 1 (102.6 to 153.9). 
The average annual carbon dioxide concentration was 9.6 mg/ 1 (5.0 
to 25.0). The average annual alkalinity was 110.5 mg/ 1 (68.4 to 
145.4). The average annual total acidity was 12.4 mg / 1 (5.7 to 28.5). 
Larval Stage 
Development. — Length-frequency histograms indicated that the 
population of S. itasca contained one size class (Fig. 1). Hatching 
occurred at the end of May. There were 18 egg masses located in the 
field at the end of May. The egg masses were found on the leaves of 
hornbeam and buckeye trees. Three egg masses which were returned 
to the laboratory hatched within 2 days. In July all the egg masses 
were empty but no larvae were located. The earliest and smallest 
larvae were collected on August 2, 1975. Their average length was 
6.45 mm, and their average head width was 1.02 mm. The last and 
largest larvae were collected on April 6, 1976. Their average length 
was 13.67 mm, and average head width was 1.59 mm. 
Head width was used to show the monthly variation in growth 
rate and the percent increase in growth rate (Fig. 2). Due to the 
