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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While perhaps not as impressive as the square miles of fields and trees that have been totally covered in spider webs in New Zealand, Pakistan, Tasmania and Southern Australia over the past year or two, New England does have its share of fields adorned with spider silk. The silk in foreign lands was the result of spiders ballooning (floating aloft on gossamer they spin that is lifted by the wind) in spring – an effective means of dispersal. The silk we see highlighted in fields in the early morning dew of autumn in New England serves as webs, or traps, for unsuspecting insects. A majority of these webs are made by grass spiders, many of which weave a horizontal sheet of silk that have a funnel often on one side leading down to a spider hide-a-way. When vibrations alert the spider to a potential meal that is caught in its web, it rushes out, injects the insect with digestive enzymes, and drags it back into its retreat where it begins to feed.
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)
Although many spider eggs hatch in the spring, there are some that hatch in the fall. Most spiderlings stay within the egg sac until they undergo their first molt – their small cast skins can be seen inside the old egg sac. After molting they emerge and cluster together, still living largely upon the remnants of yolk sac in their abdomens. In several days the spiderlings are ready to disperse, which is necessary to avoid competition for food and prevent cannibalism among the hungry siblings.
Some species, especially ground dwellers, disperse by walking, often over relatively short distances. Others, particularly foliage dwellers and many web builders, mainly disperse by ballooning. To balloon, spiderlings crawl to the top of a blade of grass, a twig or a branch, point their abdomens up in the air and release a strand of silk. Air currents catch the silk, often called gossamer, and lift the spider up and carry it off. Aerial dispersal may take a spiderling just a few feet away or much, much farther – spiderlings have been found as far as 990 miles from land. (Charles Darwin noted spiderlings landing on the rigging of the Beagle, 62 miles out to sea).
Female wolf spiders provide both their eggs and young with a considerable amount of maternal care. They actually carry their egg sac around with them, attached to the spinnerets at the end of their abdomen, as they hunt for food and go about their lives. Careful to keep her egg sac from touching the ground, the mother makes sure it receives a sufficient amount of sunlight each day, presumably to enhance incubation. She also mends any tears that appear in the sac. The eggs hatch in one to two weeks, and 4 to 22 days later, the mother perforates the egg sac either part way or all the way around the seam by rotating the sac with her legs as she makes tiny holes in it with her chelicerae (mouthparts). Within three hours of this, spiderlings crawl out of the sac through the holes made by the mother, climb up onto her abdomen (see photo), and remain there for days or weeks, depending on the species.
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