Earlier in the summer, you may have glimpsed a spider carrying its white egg sac around with it, clasping it with the spinnerets at the end of its abdomen. When the spiderlings hatch they crawl up their mother’s legs onto her abdomen, latch onto special knob-shaped hairs, and ride around with her for several weeks (see inset). Only wolf spiders carry their egg sacs and offspring in this manner.
After molting, which occurs mid-summer, the young spiders disperse. Eventually the mother is free to hunt for prey without the encumbrance of hitch-hiking offspring. If you look closely at the pictured wolf spider, you may be able to make out the last lingering spiderling located at the junction of the wolf spider’s cephalothorax and abdomen.
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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, Black-and-Yellow Argiope egg sac, Black-and-Yellow Argiope spiderlings inside egg sac)