Appendage Development

In direct-developing hemimetabolous insects, leg and wings develop as direct outpocketings from the lateral embryonic ectoderm. Leg buds appear early, just after the completion of gastrulation, whereas wing buds appear later in development, after the lateral ectoderm has grown dorsally. In many metamorphic insects, rather than outpocketing, a cluster of cells that will form the adult leg and wing invaginates below the ectoderm. These cells become the leg and wing imaginal discs and do not undergo any further differentiation until later larval stages.

The molecular basis of positioning the limb primordia within the embryo is also well established in Drosophila and seems to be similar in many respects throughout both hemi-and holometabolous insects. The same information required to pattern the body axis (Fig. 2) is used to pattern the limb primordia. Every segment has the capacity to form a limb, and limbs appear at a discrete boundary formed at the intersection of the segment polarity genes, and the graded signals that are used to pattern the dorsal ventral axis of the embryo. Limb primordia are marked by the expression of the

Distal-less gene. The absence of Distal-less gene function results in the loss of distal limb structures up to the proximal limb segment, which is the coxa. The limbless abdomen characteristic of insects is created by a subsequent repression of Distal-less gene activity by several of the homeotic genes.

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