A negative feedback loop between the three primary pair-rule genes Tc-eve, Tc-run, and Tc-odd are responsible for their initial double-segmental periodic expression. Later, they resolve into segmental periodic expression through a yet to be identified genetic mechanism. This later segmental primary pair-rule expression regulates downstream genes to ultimately position segment polarity genes. An arrow represents positive regulation and a blunt line represents negative regulation. G= gnathal segment, T= thoracic segment, A= abdominal segment. It is wothnoting that these regulatory relationships were verified for the anteriormost segments. Further expriments are needed to verify it for the following segments (using embryonic RNAi).