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A COMPARTMENT1 is a precisely demarcated region of an adult insect which is made by a group of related cells. This group (a polyclone2) consists of all the descendents of several founder
cells which were set aside at an earlier stage. The developmental pathway followed by a growing polyclone depends on the local state of a small number of controlling ‘selector’ genes3. One
example is the engrailed gene which is thought to determine the ‘posterior’ rather than the ‘anterior’ pathway of development and is used in several different segments of the developing
fly4–6. The thorax of Drosophila is constructed on these simple principles; each of the three segments consists of two polyclones which generate the anterior and posterior compartments1,7.
The proboscis is similarly subdivided8 but the abdominal segments are probably not9. The segmental status of the head is controversial10–12. Here we summarise three lines of evidence which
show that the main part of the ‘head’—the cuticular parts formed by the eye-antennal disc13—is constructed on the same principles as a thoracic segment.
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