Myth: Spiders can lay their eggs under human skin in wounds created by their bites.
Fact: In a surprisingly widespread urban legend, a nameless woman is bitten by a spider (usually on her cheek) while on vacation. She later develops a swelling, from which, in due course, baby spiders emerge! Somehow or other, the venom must have transformed into eggs. Spiders, need I say, do not find the human body a suitable site for egglaying, and no actual case anything like this can be found anywhere in scientific or medical literature.
That is, unless you count as scientific literature the book Dancing in the Mind Field by Nobel Prize chemist Kary Mullis. In one chapter, Mullis claims to have been bitten by a brown recluse spider in California (where no such spiders exist), that other spiders came back to feed on the resulting wound, and that female brown recluses lay their eggs in such wounds to feed their babies. All this is sheer fantasy. As the name implies, real recluse spiders avoid humans like the plague if at all possible.
This legend is still alive and well; in September 2008 a correspondent claimed it happened to the usual un-named "friend," who of course didn't preserve the evidence! In subsequent similar messages, it was always a "friend," never the correspondents themselves. I will never believe such stories without physical evidence. Spiders simply don't have the equipment to get their eggs into bite wounds.
In 2006 another person said they'd heard this one combined with the bubble gum legend by a cousin who allegedly "had a swollen jaw, went to the dentist. When the dentist probed the inflamed area, it burst, and millions of tiny spiders ran out. The cousin had gotten the eggs in her mouth from bubble gum!" Yet another person suggested this legend began in Nicholas Conde's 1982 novel The Religion, but it really dates back much farther.
According to 2014 news stories, an Australian tourist in Bali learned from a local doctor that a spider (not eggs or young for a change) was burrowing under his skin! No doubt something was lost in translation, since no spider has that capability.