Latitudinal cline in segment number in an arthropod species, Strigamia maritima

Arthropods vary more than 30-fold in segment number. The evolutionary origi ns of differences in segment number among species must ultimately lie in in traspecific variation. Yet paradoxically, in most groups of arthropods, the number of segments is fixed for each species and shows no intra- or interp opulation variation at all. Geophilomorph centipedes are an exception to th is general rule; and exhibit intraspecific variation in segment number, wit h differences between individuals being determined during embryonic develop ment and hence independent of population age structure. Significant differe nces in segment number between different geographical populations of the sa me species have been previously reported, but insufficient sampling has bee n conducted to reveal any particular geographical pattern. Here, we reveal a latitudinal cline in segment number in the geophilomorph species Strigami a maritima: segment number in British populations decreases with distance n orth. This is the first such dine to be reported for any centipede species; indeed as far as we are aware it is the first such dine reported for any a rthropod species. In vertebrates, fish are known to exhibit a latitudinal d ine in segment number, but interestingly, this is in the opposite direction ; fish add segments with increasing latitude, centipedes subtract them.