The Coded Radiation Tracers (also known as Critters) are technologically modified micro-organisms created by Foaly that are mostly used for search missions. They're a distinct orange color and are bathed in radiation of the same frequency as the object being looked for. They locate an item, move to its position (leaving behind a trail) and clear any surrounding debris or other obstruction. Once their mission is complete, they will fizz out and die.
They appear in The Time Paradox when Foaly is desperately trying to get Holly Short to use it to find the underwater box which monitors the Kraken's health. They locate the box and scrape the moss off the machine so Holly can clearly view the lights. Foaly says while the critters are doing their job, 'They swim, they fly, they burrow. God bless their tiny hearts.' He is obviously quite proud of his creations.