The interdimensional creatures, also called trans-phase creatures, are a race of non-sentient creatures, similar to gastropods and arthropods, that co-exist with us beyond our dimensional spectrum.
Although they do not appear to be directly aware of us, they are capable of walking across objects in our dimensional plane, as well as passing through them entirely. Some species are capable of flight. It was speculated by Samantha Carter that the creatures are native to Earth but live in a different dimension from the Tau'ri.
In 2003, SG-1 brought a Dimension observer to Earth from an offworld mission. Jonas Quinn, having first activated the device, was the first to see the interdimensional creatures. The insects are drawn to the Dimension observer, apparently Ancient in origin. When the device is active, it gives humans who have come in physical contact with it--or with someone who has--the ability to perceive the creatures. The ability to see the creatures spread to the general public around Colorado Springs, but ultimately Colonel Jack O'Neill and the United States Air Force were able to contain the situation under the cover story of a toxic chemical spill that causes hallucinogenic side effects. (SG1: "Sight Unseen")