Survival of Heleomyza borealis (Diptera, Heleomyzidae) larvae down to -60 degrees C

Heleomyza borealis Boh. (Diptera, Heleomyzidae) overwinters as larvae in Ar ctic habitats, where they may experience winter temperatures below - 15 deg rees C. The larvae freeze at c. - 7 degrees C but in acclimation experiment s 80% survived when exposed to - 60 degrees C. Of the larvae exposed to bet ween - 4 and - 15 degrees C, only 3% pupated. However, when cooled to - 20 degrees C this increased to 44%, with 4% emerging as adults. Larvae maintai ned at 5 degrees C contained low levels of glycerol, sorbitol and trehalose , which did not increase with acclimation to low temperatures. However, lev els of fructose increased from 6.1 mu g mg(-1) fw in control animals to 17 mu g mg(-1) fw when exposed to - 2 degrees C for 1 week. Larval body water (2.2 +/- 0.1 g/g dw, mean +/- SD, n = 100) and lipid content (0.22 +/- 0.00 2 g/g fw, mean +/- SE) showed no significant change during acclimation to l ow temperatures. Larvae maintained at a constant 5 degrees C survived for o ver 18 months with little loss of body mass (from 7.5 +/- 1.2 to 7.0 +/- 1. 2 mg fw, mean +/- SD, n = 20), but none pupated. Heleomyza borealis larvae appear to feed and grow until they reach a body mass of about 7.5 mg and th en become dormant. They remain in this state until they experience a low te mperature stimulus (< - 15 degrees C) followed by a warm period (approximat e to 5 degrees C). This ensures that the larvae pupate and adults emerge in early summer, allowing the maximum growing period before the following win ter. Heleomyza borealis are adapted to survive long winters in a dormant la rval state. They have a low metabolic rate, can conserve body water even at subzero temperatures but do not synthesize large quantities of cryoprotect ants.