Macrobenlhos 309 



Danks and Oliver (1972). Actual dates of first emergence of a species 

 varied between ponds by as much as 1 week, but "early" ponds with 

 respect to emergence were also the first to thaw in early June. Differences 

 in thaw time appear to be related to degree of snow cover; ponds swept free 

 of snow thawed earliest while those situated in areas of snow accumulation 

 thawed later. Several weeks after the thaw when emergence began, there 

 were no detectable differences in pond temperatures; this suggests that 

 thermal cues at or soon after the time of thaw determine emergence 

 phenology. 



Emergence synchrony is an important requirement for mating success 

 in species such as midges with short adult lives. This is especially true in 

 the Arctic where there are highly unpredictable conditions for adult 

 survival and mate location. Consistent synchrony of species' emergence 

 periods may serve two functions. The potential for between-pond mating is 

 increased, and the temporal isolation afforded these species may serve as a 

 reproductive isolating mechanism. While two or more species were usually 

 emerging simultaneously at any time during the season, the emergence 

 periods of congeneric species overlapped little if at all. 



Two genera of Orthocladiinae, Psectrocladius and Corynoneura, 

 provided the only cases of prolonged emergence throughout several weeks. 

 Since the taxonomy of these genera is not well established, it is possible 

 that these collections represent a sequence of sympatric species with 

 synchronously emerging populations. 



Life Cycles 



At first, cohort distinction and the determination of lifespans of 

 chironomid species seemed straightforward, but it proved to be a complex 

 task as more data accumulated. Our early definitions of cohorts of 

 Chironomus indicated a 4-year cycle with 1 year spent in the egg and first 

 2 larval instars, 1 year as a third instar and 2 years as fourth instar, then a 

 brief period as pupa and adult. Both Bierle's data and ours seemed to show 

 two cohorts, a small and a large one, in the fourth instar. However, when 

 examined in detail our later samples showed an almost continuous 

 variation in size through the last larval instar, and moreover indicated that 

 extremely high mortality ( --83%) would have to occur in the older of the 

 presumed fourth instar cohorts in the last weeks before they emerged as 

 adults to account for the numerical discrepancy. 



A reexamination of fourth instar larval growth at more frequent 

 intervals during two summers showed that at least 7 years were required to 

 complete growth in Chironomus of the pond centers. For distinction of 

 cohorts, it was necessary not only to measure lengths of larvae precisely, 

 but also to determine their relative maturity by use of the Wiilker and 

 Gotz (1968) definitions of developmental phases (of adult primordia) in 



