336 M. Butler etal. 



macrobenthos. The Carex habitat is less stable, as the low water levels at 

 the end of each summer may expose the sedge beds. The non-dipteran 

 insects, the snail Physa, and the water mite Lebertia are rarely found in the 

 pond centers. Oligochaetes and midges occupy both habitats, but various 

 species show a great deal of site specificity. Chironomids in the subfamily 

 Orthocladiinae, the principal midge group reported from most arctic 

 habitats, are most often found among the vegetation. The large midge 

 Chironomus dominates the pond centers along with some of the smaller 

 Tanytarsini. 



Over 36 species of chironomids have been collected from these ponds; 

 many of these were either first described from Barrow or were new to 

 science. Several species have adapted to the windy arctic environment by 

 dispensing with the normal adult swarming behavior. Instead, mating 

 takes place on the pond surface and males show morphological 

 modifications such as a reduced antennal plume. Many genera include two 

 or more species, and congeneric larvae can not always be distinguished. 



In most cases emergence is very synchronous within a species and 

 sibling species emerge at different times; this makes species identifications 

 easier. Chironomus pilicornis, the most abundant species in the pond 

 centers (up to 15,000 m"^), had two distinct emergence pulses. The two 

 pulses may represent two distinct sibling species as the animals had two 

 slightly different adult morphologies. 



The timing of the emergences is apparently controlled by tempera- 

 ture. The same sequence of emergence was observed in all ponds every 

 year, though certain species were sometimes missing or actual dates of 

 emergence were sometimes shifted by early or late thaws. Synchrony of 

 emergence was highest in species emerging early in the season and in at 

 least one case was altered by nematode parasitism. Such synchrony 

 maximizes mating success in these insects with short-lived adult forms and 

 permits timing of emergence to act as a reproductive isolating mechanism. 



Total chironomid emergence over the entire season varied from 500 

 to over 5000 adults m ^ Pond to pond differences were considerable, but 

 between-habitat variability within a pond was greater; the highest numbers 

 emerged from the Carex beds. 



The life history was studied in detail only for Chironomus pilicornis. 

 Coexistence of seven distinct cohorts implied a 7-year life cycle for this 

 species, with the larvae diapausing for seven winters in the frozen 

 sediments. Tanytarsus inaequalis showed a 2-year life cycle. The life cycles 

 of other species are presumed to fall between these two extremes, as no 

 species appeared to reach maturity in less than 2 years. 



The midge fauna appears to include many so-called "absolute spring 

 species" which stop feeding and growth prior to their last winter, then 

 pupate and emerge soon after thaw. There are also some species which 

 feed, grow, and emerge during their final summer. 



