Answer: giant bugs
After Kong dumps them into the chasm, they're swarmed by giant crickets, spiders, and lampreys. Just as it's looking like our heroes are doomed, they're rescued by some of the sailors, and one actor, Bruce Baxter who either didn't go when our group set out into the island's interior, or turned back after the Brontosaur stampede.
The sequence of the humans being attacked by giant bugs is based on a deleted scene in the original King Kong (1933) featuring a giant man-eating spider. The scene was deleted because the studio producers actually decided that it was too scary to be included!