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respectively. At 5°C, larvae remained in the first instar throughout the 40- 

 day experiment, an observation consistent with the evidence for threshold 

 temperatures from Johnson and Brinkhurst (1971a) and Danks and Oliver 

 (1972). At 10°C, all larvae reached the second instar. These responses were 

 also reflected in field observations that larvae reach the second instar at 

 about 50 to 60 thaw-days in their second summer. The next temperature 

 level (15°C) allowed much more rapid growth with some larvae reaching 

 the middle of the fourth instar in 40 days after hatching. Growth varied 

 from chamber to chamber (Figure 7-8) but there was a strong negative 

 correlation between size achieved and final larval density in the chamber. 

 At 25°C larval growth (as length) was the same as at 15°C, but mean 

 weight of a larva was less. The last temperature was much warmer than 

 field populations would encounter; temperatures above 15°C were brief 

 and infrequent in the ponds. This experiment illustrates the controlling 

 effect which normal pond temperatures may have on larval growth and 

 production. 



Similar results were obtained in a pond experiment in 1972. 

 Enclosures 1 m^ in area containing sediments and water were established 

 in one of the ponds and larval populations were counted (28 July). One 



FIGURE 7-8. Mean lengths 

 and weights o/Chironomus 

 larvae grown in culture 

 from single egg masses at 

 four temperatures for 40 

 days. Bars indicate 95% 

 confidence limits. 



