The Fly (1958)
I think the reason this isn't seen as much as some classic horror films is the fact that it is much more disturbing than many of its contemporaries. While Andre Delambre (David Hedison) is set up as a tragic dramatic character, his treatment of others in pursuit of his goals can bring up some questions about whether the single-minded insectoid determination at the end is more of a reflection of him than the creature he is fused with.
The story takes place in Montreal, where a nightwatchman finds the body of Delambre with his head and arm smashed in a metal press and his wife Helen (Patricia Owens) fleeing the scene. When question by Inspector Charas (Herbert Marshall) about what happened, she claims that she did it but does not know why, remaining bedridden and acting as if she is insane. She also seems obsessed with catching a certain white-headed fly her son Philippe (Charles Herbert) had caught earlier.
Convinced that there is more to the story than she is revealing, her brother-in-law François (Vincent Price) gets the true story. Andre was working on a teleportation device. Only problem is that organic items that came out the other end weren't exactly right. After accidentally transporting the family cat into another dimension, Andre seems to have worked the kinks out of the device and is uncharacteristically happy and enjoying life.
Within a few days, however, he is once again locked in his lab. Refusing to speak or be seen, he demands his food be blended and left in his lab by Helen, informing her that it is desperate that the white-headed fly be caught and brought to him. It soon becomes apparent to Helen that quite a bit is wrong, and she finds out exactly what when she gets the nerve to take off the hood he wears around her. It turns out that he now has the head and left arm of a fly, and his personality is slowly being usurped by that of the insect.
After the failure to capture the fly, Andre convinces his wife that there is only one thing left to do, leading back to the events at the beginning. François, knowing his brother, believes her, but Inspector Charas is much harder to convince - until a horrifying revealing of the truth at the end.
Keep in mind that this is from 1958, so there are some of the problems you would expect for a modern audience. There is a horribly fake spider in one scene, although it doesn't really detract from how disturbing it is. Surprisingly, the actual human/fly hybrid costume is quite effective. Most of it is in motion, including the mandibles and other parts of the face. Where most such outfits of the time are motionless and obviously rubber, they did a great job on trying to make this as real as possible.
It is also a bit bloodier than most films of its time, especially when it comes to Andre's demise. They made the flattened body as gory as they could get away with.
As for the performances, Hedison is great as the typical mad scientist (and manages to emote inside that fly costume), Owens keeps the hysterics to a minimum and Vincent Price is excellent as always, even if he is a good guy here. The kid is annoying, but they usually are in movies like this.
The Fly (1958)
Time: 94 minutes
Starring: David Hedison, Patricia Owens, Vincent Price
Director: Kurt Neumann