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Fig. 1. Data applied to test Hawes and Bale's
(Hawes and Bale, 2007)
hypotheses. Fifteen arthropod species for which seasonal variation in cold
hardiness are available were used (see supplementary material Table S1 for the
species list and data). Freeze-tolerant species are indicated with squares,
freeze-avoiding species with triangles, and a solitary chill-susceptible
species (the house spider Acharaenae tepidariorum) with a circle.
Note that two points, for Rhabdophaga strobiloides and Mayetiola
rigidae, overlap completely. The arrows indicate the hypotheses erected
in figure 1 of Hawes and Bale (Hawes and
Bale, 2007). No phylogenetic correction was applied to these data,
although they represent seven orders and 11 families. Following Hawes and
Bale's description, the extent of `evolutionary derivation' was taken as the
extreme lower lethal temperature (LLT, in °C), and the `extent of
plasticity' as the summer (or non-acclimated) LLT minus winter (or acclimated)
LLT (in °C). Note that this procedure might introduce an element of
non-independence of the axes (see Brett,
2004), which would have to be given consideration in further tests
of the hypothesis. The greater variation in LLT of the freeze-tolerant than in
the freeze-avoiding species is likely because of homogeneous nucleation in the
latter (see also Zachariassen et al.,
2004): no insect supercooling points below –54°C have
been reported in either strategy
(Addo-Bediako et al., 2000),
although freeze-tolerant species can survive well below their supercooling
point.
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