Black widow spiders have a few transformations in their lifetime. One in particular takes longer than the rest, it takes up to two days to complete. Right after she makes the web around her babies, which are still developing within the egg sac, the black widow molts her legs. During this final transformation, the black widow is completely immobilized. It is almost like she’s in a deep sleep. Meanwhile, the babies are defenseless against predators. Flies can easily spot the egg sac and will lay their eggs on their web. If the black widow is immobilized, then the fly will lay it’s babies on the egg sac and feed on the baby black widows. If the black widow is not immobilized yet, she can eat the fly. If the black widow can time this final transformation right, her babies are saved from the baby eating fly pupae. That is what else I have learned about black widow spider females.