The segment polarity network is a robust development module

All insects possess homologous segments, but segment specification differs radically among insect orders. In Drosophila, maternal morphogens control t he patterned activation of gap genes, which encode transcriptional regulato rs that shape the patterned expression of pair-rule genes. This patterning cascade takes place before cellularization. Pair-rule gene products subsequ ently 'imprint' segment polarity genes with reiterated patterns, thus defin ing the primordial segments. This mechanism must be greatly modified in ins ect groups in which many segments emerge only after cellularization(1). In beetles,and parasitic wasps, for instance, pair-rule homologues are express ed in patterns consistent with roles during segmentation, but these pattern s emerge within cellular fields(2-4). In contrast, although in locusts pair -rule homologues may not control segmentation(5,6), some segment polarity g enes and their interactions are conserved(3,7-10). Perhaps segmentation is modular, with each module autonomously expressing a characteristic intrinsi c behaviour in response to transient stimuli. If so, evolution could rearra nge inputs to modules without changing their intrinsic behaviours. Here we suggest, using computer simulations, that the Drosophila segment polarity g enes constitute such a module, and that this module is resistant to variati ons in the kinetic constants that govern its behaviour.