Centipedes are a familiar group of arthropods characterized by a long body made up of individual segments, each with its own pair of legs. Despite their familiarity, they actually represent a developmental mystery. The number of leg pairs in different centipede species varies between 15 and 191 pairs, but this number is always odd. This suggests that the range of body forms that are theoretically possible is restricted by constraints we have not yet recognized.
In a new paper, University of Cambridge developmental biologists Dr. Ariel Chipman and Prof. Michael Akam, from the University's Museum of Zoology, in collaboration with Prof. Wallace Arthur from the National University of Ireland at Galway, have provided a possible explanation for this puzzle. Chipman and colleagues studied the millimeter-sized embryos of a long and thin centipede, Strigamia maritima, collected on the coast of north-eastern Scotland, and looked at how the segments of this animal are formed. The number of segments in individuals of this species varies from 45 to 53, but again the number of segments is always odd.
The researchers found a series of genes that initially define a two-segment periodicity in the forming trunk of the S. maritima embryo. This periodicity then resolves to give single segments, which later develop a pair of legs each. If segments are defined two at a time, evolutionary changes would add or remove segments in pairs, so that only certain numbers of leg-bearing segments are possible. Why then odd numbers rather than even? The authors leave this question open, but they suggest that additional segments that do not bear legs are also formed with this mechanism. Most noticeably, the centipede trunk includes a pair of poison claws that are probably modified legs. If these are counted, centipedes actually have an even number of trunk segments.
Ariel D. Chipman, Wallace Arthur, and Michael Akam: "A Double Segment Periodicity Underlies Segment Generation in Centipede Development"
Published in Current Biology, Volume 14, Number 14, 27 July 2004, pages 1250-1255.