Some Aspects of Segmentation

Abstract

The body of an insect is subdivided into a number of units called segments, morphologically definable in terms of the pattern elements they contain. In Drosophila, segments are built up by derivatives of the mesoderm and ectoderm, e.g. muscles, tracheae, nervous system, epidermis, whilst endodermal derivatives are neither segmentally organized nor do they display any metamerical organization. Anatomically, boundaries between segments of insects are defined by intersegmental furrows and by apodemes for insertion of muscles and, indeed, in the trunk region of the animal one can readily define the pattern elements of segments, as well as the segmental boundaries, using anatomical criteria. This is not the case in the terminalia, i.e. the procephalon and the tel-son, where segments are rudimentary or have fused. Therefore, we will deal first with the segmented germ band, including aspects of the gnathal segments; the preoral segments and the tail will be discussed at the end of this chapter.