The Gokiraji is a small remote-controlled roach that doesn't do much more than crawl around, but it does so in rather convincing fashion. With tiny antennae flailing about, and a small light at its back for dramatic effect, deploying a small swarm of these on an unsuspecting victim would likely deliver the startled reaction you're looking for. Charged via USB, the robotic pest is available for $21 here, and you can see the Gokiraji in action in the video below.