OVERWINTERING WITH A LOW-LEVEL OF COLD-HARDINESS IN THE TEMPERATE MILLIPEDE POLYDESMUS-ANGUSTUS

Tolerance of freezing and ability to supercool were investigated in al l stadia of the flat-backed millipede Polydesmus angustus. When cooled at a rate of 1 degrees C min(-1), individuals survived sub-zero tempe ratures above the supercooling point (SCP), but did not tolerate freez ing. SCPs were comparatively high, with a mean Value of -5 degrees C i n winter and individual SCPs as high as -2.2 degrees C in the largest specimens. Millipedes starved for three weeks in the laboratory did no t depress their SCP. It is thus concluded that P. angustus has virtual ly no physiological protection against freezing. In cultures under two thermal regimes - with or without moderate frost - nearly all millipe des survived throughout the winter at temperatures just above 0 degree s C but 100% mortality occurred when temperature decreased to c. -6 de grees C. Although the possibility of death from chilling cannot be exc luded, the results are consistent with the supercooling profile of the species and confirm its low level of cold-hardiness. In the field, P. angustus is commonly found in rotting logs, which have been shown to provide considerable thermal buffering in winter. The benefits (possib ility of feeding throughout winter) and limits (in terms of survival a nd geographical range) of the overwintering strategy of P. angustus ar e discussed. The assumption that physiological resistance to freezing should decline in species which avoid frost by behavioural means is ex amined. The evidence is conflicting in insects, but clearer patterns m ay emerge from studies on other classes of arthropods.