Figure 8. A model for mechanisms underlying spider-head segmentation.

An autoregulatory signalling network in which At-hh, At-ci, At-otd, and At-opa participate is proposed. Expression of At-otd and At-opa is controlled by short- and longer range activities of At-hh, respectively, via At-ci, and activities of At-otd and At-opa are required for the dynamic regulation of At-hh expression during head segmentation. At-otd activity mediates positive feedback to maintain the head generative zone, and At-opa activity promotes the At-hh—At-otd circuit to enter the splitting phase through an as yet unknown mechanism in the head generative zone. The stripe splitting may involve coordinated regulation of gene expression and cell rearrangement. At-otd may directly and/or indirectly regulate levels of At-opa expression and the development of its pattern.