How is developmental stability sustained in the face of genetic variation?

The number and arrangement of scutellar bristles on the thorax of Drosophila melanogaster is largely invariant in wild-type stocks. This character therefore appears to be buffered against changes in phenotype, and has previously been described as a canalized character. Mutations that do alter this phenotype increase the variability in bristle number and can reveal otherwise cryptic genetic differences at other loci. This phenomenon is examined and possible mechanisms contributing to stability of this developmental event are discussed, but the notion that the character is canalized is found not to be heuristic.