Non-sentient creatures, similar to gastropods and arthropods, that co-exist with us beyond our visible spectrum. Though 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.
The insects are drawn to an interdimensional visibility device, apparently Ancient in origin. When the device is active it gives humans who have come in physical contact with it -- or with someone else who has -- the ability to perceive the creatures.
Sight Unseen - Upon activating an alien piece of technology, S.G.C. personnel begin seeing creatures appearing near the device.