Jennifer Corvino (Jennifer Connely) is a new student at Richard Wagner academy in Switzerland, her father - famous actor Paul Corvino - always traveling around the world paid for her studies there. Quite shy, often unable to make friends as she moves very often, she gets well with Sophie, her roommate (and a fan of her father), but the other girls are very jealous of her, playing all kinds of trick to make her quit the school. On a hard day of bullying, Jennifer breaks down and thousands of flies come knocking on every window of the school. Next night, as Sophie had left the school to join her boyfriend, Jennifer has an episode of sleepwalking, somehow following a night insect, and falls off a rotten ledge. She often has visions of other places, and one night she goes far away from the school and meets Prof McGregor (Donald Pleasance) ; he is on a wheelchair and uses a domesticated chimpanzee to help him. He tells her of his passion (scientific) for insects and she tells him insects are her friends. He is studying human remains for the police and uses insects to know when death arrived, sometimes only with parts of the body. As Jennifer understands some girls of the academy have gone missing, and Sophie's missing too, she and McGregor try to indentify the killer.
Having sen lots of Italian horror (Bird with crystal plumage, Stendhal syndrome) and gore movies (Demons, Cannibal holocaust, City of the living dead), Argento uses usually more psychological horror elements and evocative images than guts / blood like the Fulci / Deodato school. We have here a first part of the movie with getting acquainted with Jennifer, the other inhabitants of the small Swiss town, and the horror comes very progressively, some blood here and there when girls are killed, the body parts McGregor is analyzing. It's only when the climate of paranoia and terror (using the "insect" vision and Jennifer's growing dread mostly) is in full roll that the disturbing images set in and keep you dizzied until the end.
Saw this as a young teenager, and quite often after that so I guess it still works.