The early development of flies is governed by two essentially different mechanisms: anterior and posterior. The anterior morphogenetic system in principle could size-regulate through a competitive interaction with the posterior system, and this is enough for a corrective response to changes in egg length or variation in the strength of the posterior system action.
The difference in organizing the two morphogenetic "devices" most likely reflects the evolutionary history of Drosophila, a higher insect. The anterior system is thought to be more advanced. In primitive insects such as the cricket or grasshopper the segment pattern in the anterior part of the body is laid out more or less simultaneously (all at once), as in Drosophila, but the posterior part of the body is formed sequentially, presumably by outgrowth from a posteriorly located growth zone. A possible explanation for the seemingly arbitrary nature of the morphogenetic systems used is that they reflect chance mechanisms for creating morphological change during evolution. The general idea is that ancestor insects start with basically a head and then extend. The principal difference between flies and grasshoppers, is that Drosophila has a rapid establishment of body plan, in comparison with primitive insects. These differences between Drosophila and grasshopper are discernible at the molecular level (Sommer and Tautz, 1993). Segment formation in Drosophila involves the pair-rule genes network which define double segmental periodicities and which have been considered to represent a special adaptation to the high insect type development (Patel et al., 1992).
The Drosophila early segmentation mechanism is thought to be the result of the evolutionary offensive of the anterior system on the posterior one. Namely, the anterior system extended area of action via acquiring segment by segment on the evolutionary pathway from ancestor to modern higher insects.
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