Jennifer Connelly said in an interview that she was bitten by the chimp in this film. Apperently during one scene the chimp kept turning around and Dario Argento, not wanting to film her behind, asked Connelly to place her hand on the animal to stop her from turning. But when she attempted to do so, the chimp became enraged and bit her; and then became very hostile toward Connelly for the rest of the film.
The shots of the swarm of insects covering the exterior of the school, and later nearly blotting out the moon were achieved through visual effects. They dumped a bunch of coffee grounds in a tank of water, filmed it, then superimposed the new footage over existing background shots.
The trivia items below may give away important plot points.
For the final scene where Patau is swarmed with flies, over 2 million eggs were bought and incubated in an oven. As soon as they hatched, actor David Marotta was put into the makeup and wardrobe, then rushed to set. To attract the newborn flies, his clothes and covered face were coated in glucose and the flies were released from very fine nets to swarm his body. After completing the scene, the crew opened the studio windows to release all of the flies, but they inhabited the neighborhood businesses for weeks.
working title)Director Dario Argento said that his idea for the look of the killer child came from the real genetic disorder Patau Syndrome, which causes severe deformation of the face. For this reason, Argento calls the child Patau, even though he is never named during the film.