Some species of spiders (including wolf and jumping spiders) overwinter as young adults and mate/lay eggs in the spring. Many spiders, however, mate in the fall, after which they lay eggs and die. Their white or tan egg sacs are a familiar sight at this time of year. One might assume that these species overwinter as eggs inside their silken sacs, but this is rarely the case as spider eggs can’t survive being frozen. Spider eggs laid in the fall hatch shortly thereafter and the young spiders spend the winter inside their egg sac.
Although egg sacs provide a degree of shelter (the interior is packed with very fine, very soft silken threads), the newly-hatched spiderlings do have to undergo a process of “cold hardening” in the fall in order to survive the winter. On nights that go down into the 40’s and high 30’s, these young spiders start producing antifreeze compounds, which lower the temperature at which they freeze. By the time freezing temperatures occur, the spiders are equipped to survive the winter inside their egg sac – as spiderlings, not eggs. (Photos: Black-and-Yellow Argiope egg sac and spiderlings – egg sac had been pecked open by a bird)
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