426 S. F. MacLean, Jr. 



of flight on warm days. This species has the most restricted larval habi- 

 tat, and thus may require more searching for oviposition sites. 



Adults do not feed. Copulation may occur almost immediately after 

 emergence, and egg-laying commences soon thereafter. Clement (1975) 

 hypothesized the release of a sex pheromone by nearly emerged (or 

 emerging) females to attract males. Given favorable conditions, males 

 are quite active. The weight-specific respiration rate of adult male T. car- 

 inifrons is 3 to 4 times that of larvae at the same temperature, and the 

 metabolic response to temperature (Q,o) is greater. All of this increases 

 the likelihood of successful reproduction under the variable and unpre- 

 dictable weather conditions of the arctic summer, a likelihood that is fa- 

 vored by emergence into a high-density population. 



The advantage of synchronous emergence is further enhanced by 

 predation. The abundant avian predators feed upon insect larvae early 

 and late in the season, but switch almost entirely to adult Diptera, especi- 

 ally craneflies, when they are available. During the main period of emer- 

 gence the density of adult flies far exceeds consumption by birds, and the 

 impact of predation is relatively low. As discussed below, predation is 

 more intense upon individuals emerging into low density populations, 

 early and late in the emergence period. The result is selection for syn- 

 chrony of emergence. 



During the midsummer emergence period photoperiodic cues are 

 weak, particularly for soil-dwelling pupae. It appears that the timing of 

 emergence is controlled entirely by temperature as it affects rate of pupal 

 development and ecdysis. For instance, MacLean (1975b) documented 

 emergence at Prudhoe Bay in two seasons (1971-1972) differing by about 

 one week in the time of snowmelt and the onset of activity, and found 

 that emergence differed by a like amount in the two seasons. 



Because of the length of the life cycle, differences between areas or 

 years in the density of emerging adults may reflect differences in the lar- 

 val population density or in the relative abundance of the cohorts com- 

 posing the population. The large difference in emergence of adult P. 

 hannai on the Carex-Oncophorus meadow in 1970 and 1971 (Table 11-4) 

 can be attributed to differences in cohort size rather than total popula- 

 tion; the population was actually much larger in June 1970 than in June 

 1971, as can be easily surmised, since the 1970 population also included 

 the large cohort giving rise to adults in 1971. 



The emergence of adult P. /jcrrtwa/ increased from 1970to 1971 on the 

 Carex-Oncophorus meadow, declined on the Dupontia meadow, and re- 

 mained essentially stable in the polygon trough (Table 11-4); thus, differ- 

 ences in emergence between years do not simply reflect weather patterns. 

 In some years, however, cold weather before or, especially, during the 

 emergence period can delay or even inhibit emergence, as was recorded in 

 the very cold July of 1969 (MacLean and Pitelka 1971, MacLean 1973). 



