Graphical representation of Delta , Eve1 and Engrailed expression in Strigamia . Formation of the posterior head and trunk segments is under the control of a clock-like mechanism that manifests as a burst of gene expression in the peri-proctodeal area. Delta and Eve1 expression oscillate out of phase and propagate anteriorly through the posterior disk as a cyclic wave of gene expression. Once the primary stripes of either Delta or Eve1 reach the forming germ-band they stop and Eve1 and Delta intercalary stripes appear. Shortly after, Engrailed is expressed in every stripe and a new morphological segment becomes visible. However, when the last nine segments are added to the trunk Eve1 and Delta oscillations cease. At this stage Eve1 is homogenously expressed in the posterior disc, and a single stripe emerges from this domain in the germ band. Delta expression is limited to a stripe that co-localizes with the Eve1 stripe observed in the germ band. As described for the more anterior segments, Engrailed is then expressed and segments are formed.