Re'tu

The Re'tu are a fictional alien race from the sci-fi television show, Stargate SG-1.

The molecules of a Re'tu's body exist 180 degrees seperated from our reality, so that they cannot be seen by humans and can only be interacted with physically to a limited degree (they can see humans just fine, however). They can however be exposed by radiation emitted by a TER device. When exposed, Re'tu appear as shadowy aliens with a humanoid torso, spider-like lower body, and insect-like head with two bug eyes on stalks. In many ways their appearance is similar to that of Species 8472 from Star Trek: Voyager.

The Re'tu only actually appear in one episode, but they are mentioned several times throughout the show. Whenever something bad happens in plain view yet no visible cause is apparent, the invisible Re'tu are usually amongst the prime suspects.