. The Canadian field-naturalist. 1998 Shaw: Distribution of Silver Spotted Tiger Moth 251 Although the SSTM is known to occur on old- growth trees (Silver 1958), the distribution of larval colonies within an old-growth forest has never been described. If physical, microenvironmental aspects of the environment are important to development of these winter active colonies, then their distribution within the forest should reflect a particular micro- habitat. Microhabitat selection is a basic feature of insect thermoregulation (Casey 1981), while it is particularly important for some caterpillars,
. The Canadian field-naturalist. 1998 Shaw: Distribution of Silver Spotted Tiger Moth 251 Although the SSTM is known to occur on old- growth trees (Silver 1958), the distribution of larval colonies within an old-growth forest has never been described. If physical, microenvironmental aspects of the environment are important to development of these winter active colonies, then their distribution within the forest should reflect a particular micro- habitat. Microhabitat selection is a basic feature of insect thermoregulation (Casey 1981), while it is particularly important for some caterpillars, especial- ly those that occur in cold environments like the Arctic or are active during colder seasons of the year (Heinrich 1993). The Eastern Tent Caterpillar (Malacosoma ameri- camim Fabricius) has been extensively studied in regard to thermoregulation (Fitzgerald 1995). The colonies are active early in the spring when ambient temperatures are below the minimum temperature required for growth. A combination of factors assists the caterpillars in thermoregulating, including direct exposure to sunlight, aggregation of individuals, and tent building (Fitzgerald 1995). Basking in sunlight elevates individual caterpillar temperature, while aggregation aids this by reducing loss of convective heat. The tent again aids in thermoregulation because of the boundary layer created by the tent also reduces convective heat loss. Exposure to the radiant heat of sunlight is necessary for this thermoregula- tion (Fitzgerald 1995). The SSTM exhibit similar behavioral and physical attributes of the eastern tent caterpillar: caterpillars are hairy and dark colored (Figure 1), the larval colonies are active in the colder portion of the year, build tents (Figure 2), and aggregate to bask in sun- light (Figure 3). To accommodate thermoregulation, however, they must be exposed to the sun. Tall stature, old-growth Douglas-fir/Western Hemlock forests have complex structure (Franklin et al. 1981), wit
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