When expression is driven by Scer\GAL4^how-24B, the ectoderm develops relatively normally, but the muscle founders fail to form. Rounded, unfused myoblasts accumulate, but do not fuse to form muscles. The motor axon phenotype resembles that seen for twi mutants that lack mesoderm: SN and ISN grow out but axons fail to defasciculate and nerves remain unbranched. In segments of the embryo where loss of founders was incomplete, axons defasciculate from the principal nerve trunks and innervate the remaining isolated muscles. The specificity of axon branching was maintained. Nerve branching occurred in the absence of adult muscle precursors, but only in response to founder myoblasts.