Have you recently noticed a roundish, flat, papery, 1/4-inch diameter, metallic-looking structure adhering to the top of a rock? If it has tiny bumps in the center, chances are good that it’s a spider egg sac, most likely that of an antmimic spider (spiders resembling ants that often prey on ants), specifically one in the genus Castianeira. One would assume that the contents of the sac that were causing the bumps were eggs that were going to overwinter and hatch once warm weather arrives. This is true for a majority of spider egg sacs, but some, including those of Black-and-Yellow Argiopes and antmimic spiders, hold spiderlings that have already hatched and will remain in the sac throughout the winter. As long as the temperature stays cold, the spiderlings will be safe and secure until spring. If we have periods of cold interspersed with periods of warmer temperatures, or an exceptionally warm winter, the spiderlings will become active when the thermometer rises, and, not having any insects to eat, will be forced to devour each other.
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