The Baby Spiders were introduced in the episode "Web Weirdos." They are very small compared to their parents, who are giant-sized spiders. They were birthed from their mother's egg sac as live young, having developed inside of her abdomen. Upon their birth, they spouted out by the thousands, covering their surroundings, including Finn and Jake, in a heaping pile. Their arrival brought on the reconciliation of their parents.
They look like regular spiders when viewed up close, but from far away, they just look like black spots. Despite being spiders, they only have six legs.