Snow white and the seven dwarfs: a multivariate approach to classification of cold tolerance.

Two main cold hardiness strategies of insects - freeze tolerance in some species, and overwintering in a supercooled state without tolerance of freezing in many others - were recently reclassified. However, I present several problems with the current systems. My suggested classification is based on clearer definitions of the causes of cold injury. I recognize three main mortality factors: freezing of body liquids, cold shock, and cumulative chill injury. Presence or absence of each of these factors produce eight combinations. I have named the eight classes after Snow White and the Seven Dwarfs to avoid nomenclatural confusion. Some of these classes are probably not used as tactics against cold injury by any insect species. Other classes contain so many species that they might be reclassified in more detail, using values of supercooling point and other quantitative parameters. However, widely comparable parameters, like the upper limit of cold injury zone and the sum of injurious temperatures are still rarely published, thus we still lack comprehensive data for multivariate analyses. Every cold hardiness strategy should be characterized by a meaningful class or subclass together with the physiological, biochemical, and behavioural mechanisms employed by the insects. I also point out the existence of strategies that combine two tactics - either a switching strategy (during preparation for winter, population "chooses" which tactic will be used), or a dual strategy (individuals are ready to use one of the tactics depending on the prevailing environmental conditions).