OVERWINTERING OF SOME NORTH TEMPERATE AND ARCTIC CHIRONOMIDAE: II. CHIRONOMID BIOLOGY

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Abstract
Several aspects of overwintering were studied in the larvae of selected species of chironomids living in shallow ponds.In two species (Einfeldia synchrona Oliver and Polypedilum simulons (Townes)) from a pond near Ottawa, Ont. (45° N.), development ceased in mid-November and the gut was subsequently evacuated. A species of Chironomus from another pond did not evacuate the gut although feeding ceased. In the laboratory, larval development of Einfeldia collected at any time of year continued except at low temperatures, but pupation occurred only at long day-lengths. Seasonal changes in horizontal distribution were apparently not primarily related to possible winter effects, but to water depth changes. Larvae of E. synchrona penetrated deeper mud layers during winter (as well as in midsummer); those of P. simulans were always confined to superficial mud layers.The later instars, at least in Chironomini, usually predominated during the winter, but season to season variation was marked. In the arctic all instars except the first were present. Closed winter cocoons were found in several temperate and arctic species. Some were made by sealing an existing tube, others by building a more or less transparent tightly applied cocoon. Inside these cocoons, the larvae were tightly folded in different ways more or less characteristic of the species. At Ottawa, cocoons were usually formed well after ice cover, and partly in response to low temperatures. Larvae showed a lower water content in winter than in summer. In the arctic, larvae were also somewhat dehydrated in winter, but there was in addition a different type of dehydration which resulted in shrivelling.There was no significant ability to resist freezing by inoculation with ice. Freezing tolerance was found in some temperate and all arctic larvae and was temperature- and time-dependent. In temperate areas, freezing-tolerant larvae were nevertheless freezing-susceptible in summer.These studies and previous findings are discussed in relation to winter habitat conditions and the remainder of the life-cycle. Generalizations are compromised by complex inter- and intra-species differences and by habitat variations. Freezing tolerance is widely distributed through the genera of most subfamilies except the Tanypodinae, and is probably most general in the Orthocladiinae. Since there are few specializations of larval physiology which explain such tolerance, the Chironomidae can probably be considered as largely pre-adapted to a rigorous winter environment.