Helicomery in Centipede Trunk: Different Processes for the SamePattern

In arthropods, helicomery (or ‘‘spiral segmentation’’) describes the condition of a region of an axial structure (generally the trunk) where transversal (‘‘intersegmental’’) arthrodial membranes do not form distinct closed belts delimiting the set of sclerites belonging to a discrete axial unit (a segment), but form instead a continuous helix that develops around the axis for one or more whirls. Defects in the segmental pattern referred as to helicomery have been described for several arthropod clades. We have studied trunk helicomery in an isolated population of the geophilomorph centipede Stigmatogaster subterranea. In this population, sampled across several years, the incidence of several kinds of naturally occurring segmental abnormalities is high. Our analysis shows that a helicomeric pattern can result from particular cases of different classes of developmental defects. These are defects of ventral trunk segmentation during germ-band stage and, developmentally later, mismatches of the left and right anlagen of dorsal sclerites during the dorsal closure of the embryo. Thus, helicomery, being one of the possible patterns of sclerite arrangement that can result from more than one type of defect, does not represent a class of abnormalities in its own. We compare helicomery in S. subterranea with similar axial patterns described for other arthropods. Helicomery questions the concept of segment as a body unit, as the ‘‘segmental identity’’ of repetitive structures cannot be univocally determined, suggesting that segmentation applies to specific structural components of a body axis rather than to the axis itself.