Ghost Hunting: Insects Retain Ancestral Abdominal Legs in a Truncated Form

An iconic feature of insects is the apparent lack of legs on the abdomen, which is believed to be due to the repression of the leg-patterning gene Distalless (Dll) by abdominal Hox genes. However, in contrast to these molecular observations, it is not widely appreciated that the embryos of most insect groups do in fact form paired protrusions on most abdominal segments, which degenerate to form the abdominal exoskeleton. These embryonic abdominal legs appear to be homologous to the thoracic legs. To resolve this discordance between molecular and morphological observations, the expression patterns of pannier and araucan, genes known to distinguish proximal leg segments in all arthropods, are examined in embryos of the flour beetle Tribolium castaneum. In Tribolium embryos, paired protrusions are observed on most abdominal segments and the stripes of pnr and ara expression that delineate the proximal leg segments of the thorax are seen to continue unabated through all abdominal segments. Thus, insects retain abdominal legs in a truncated form that were inherited from their crustacean ancestors. These cryptic, truncated abdominal legs appear to serve as an important wellspring of new structures and functions in insects, such as caterpillar prolegs, gills, and leaf-like camouflage structures.