A bunch of dancers and their possibly sleazy manager, Gary, are on their way to Singapore (to dance I guess) when a toy plane is violently torn asunder and they find themselves drifting on the open sea. The good news is they are saved when they happen upon an island. The bad news is the island is filled with spiders (just a couple actually) and horrors (actually just one). Thus the title, Horrors of Spider Island!
Later on, a couple of researchers also show up and the movie momentarily becomes some crazy teenagers dancing on the beach in the darkness movie. Not for long though, because there is a monster- a hideous half man, half spider beast (a veritable Spider-Man!)- stalking everyone. Plus the spiders (though they are not much of a problem after the first half of the movie).
I have not seen a lot of these 1950s/1960s black and white pop horror/scifi movies, so I cannot compare it to others and I apologize for that. I can tell you that this movie was surprisingly entertaining for about the first half, but then it dragged on for the second half. Like seemingly a lot of pop horror/sci fi movies from this period, the film is short at under 90 minutes but you wouldn't realize it for the pacing.
It's almost like two movies were smooshed together. The first half is a decent "bunch of people who don't like each other stuck in an awful situation" movie, but then the scientist hunks show up and the girls are basically just making out on the beach or screaming and fainting all over the place. Too bad, if they had maintained the tension from the first half of the movie this would have been a great little thriller.
My sources ("the internet") tell me this was originally a nudie flick from Germany. Of course the version I saw was the US release, which has all the good stuff cut out, though you do still get a catfights between girls in skirts and even some stripping down to bathing suits, which I imagine must have just caused massive swooning in the aisles back in puritanical 1967 USA.