Willis O'Brien, creator of the highly regarded effects for the original King Kong, provided the same services here, albeit on a much smaller budget. Pete Peterson, who worked with O'Brien on Mighty Joe Young and would again on Behemoth, the Sea Monster, did most of the actual hands-on animation. O'Brien borrowed heavily from other previous movies for the special effects in this film. The models used for the trapdoor spider and the giant worm with tentacles are the same ones that were used in the famous "Lost Spider Pit Sequence" from the original King Kong. The sounds made by the scorpions are a reuse of the ant sound effect from the movie Them!. A large-scale scorpion "head" was used for closeup reaction shots, but the head's human-like features distracted from the realism of O'Brien's animated models.
The steam locomotive pulling the train that is attacked is clearly a Lionel Corporation toy train. The steam locomotive's tender clearly retains its "Lionel Lines" markings.Three shots of scorpions attacking the city toward the end of the film are actually just empty traveling mattes as the producers had run out of money when the time came to composite these scenes.