Mutation of invected has no phenotypic effects. In itself, this phenomenon is not novel.
Nature appears to tolerate unequal division of labor.
In such cases, a mutation in the second gene demonstrates that the first can do the job alone. Perhaps current techniques are not sensitive enough to detect the role of invected, or maybe the proper environment in which to test its effects has not yet been found. When both invected and engrailed are missing, things go slightly haywire. When engrailed does not function in the posterior compartment, its normal site of expression, additional posterior compartments can form. On the other side of these additional posterior compartments, a new anterior compartment may also form.