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M. Butler etal. 



the larvae. When fourth instar animals were sorted into the nine phases of 

 Wiilker and Gotz, three cohorts became apparent. This analysis was 

 checked by following changes in phase and size class of stronger cohorts at 

 3- or 4-week intervals. It has not been possible to study each species with 

 the same intensity, but we can say at least that Tanytarsus species have 2- 

 year cycles and that no species of chironomid in the pond centers matures 

 in less than 2 years. Analysis of this type is complicated by the presence of 

 pairs of closely related species, so far indistinguishable as larvae, in 

 important genera. 



The 7-year generation time of Chironomus pilicornis in Pond J is 

 portrayed in Figure 7-6. Larvae hatched from eggs within 2 weeks of 

 oviposition. Egg masses appear to have been concentrated near the pond 

 edges either by oviposition or wind, as we found few newly hatched first 

 instars in the center. Early in the second summer the larvae molted to the 

 second instar and apparently completed their move to the center of the 

 pond. During the third summer, at about 2 years of age, the larvae molted 

 to the third instar and in 2 more years, to the fourth. Growth and 

 development in this last larval instar continued through the next 3 years. 

 When the pond thaws in the eighth summer the larvae have reached a 

 length of 16 to 18 mm (females are generally larger than males), and are in 

 developmental phase 8 or 9. Pupation occurs soon after thawing, and it is 



Egg I 



V, VI 



V, VI 



Age.yrs. 



FIGURE 7-6. Life cycle o/ Chironomus pilicornis. Ar- 

 rows indicate time of molting to the next instar. Growth 

 and molting schedules were determined from data taken 

 in Pond J from 1975 to 1977. 



