Centipedes are arthropods with many leg bearing segments that vary from 15 to 191 segments in mature adults. Out of thousands of species, none (zero!) have an even number of segments when they are mature. All have an odd number of leg bearing segments. Biologists have puzzled over this odd observation trying to find explanations for this phenomena.
Immature centipedes exist that have an even number of segments and they are perfectly viable. Still, natural selection does not produce a centipede with an even number. The best explanation is that a development process produces an odd number of segments. An elongation process then acts to add two segments at a time. Natural selection acts to preserve the development process, thus the odd number is preserved.
Ariel D. Chipman, Wallace Arthur and Michael Akam. A Double Segment Periodicity Underlies Segment Generation in Centipede Development. Current Biology, Vol. 14, 1250–1255, July 27, 2004