Trundlebugs were insects native to the Southern Continent. They travel in a straight line, walking over anything in their path, including water, and going straight up tall obstacles, and down the other side. If disturbed from their path, they release an unpleasant stink that lingers for days.
"One of the disadvantages of living in the open was the insect population, in myriad forms: some scratched, even burrowed in flesh if possible; some merely moved in straight lines, like the trundlebugs that were the object of Zaranth's current inspection. A straight line for a trundlebug could also be perpendicular to the ground. They had been observed maneuvering up to the crown of a frond tree and down the other side. Right now, a very large trundlebug-the creatures could become quite large if no natural hazard ended their existence-was under intense draconic surveillance. This one had no fewer than five young still attached behind it, in various stages of maturation in the trundlebug's peculiar reproductive process. Their bodies collected pollen from low-growing shrubs and vines-also the occasional tree-and shed it in their progress to whatever unknowable goal trundlebugs had. What other purpose they served Tai did not know, but they were less of a nuisance than some crawlies and rather curious to watch. Single-mindedness was exemplified in the trundlebug. It had been suggested there was only a female of the species." ~ from "The Skies of Pern "