let-7^KO.C mir-125^KO.C double mutants enter the pupal stage with normal timing and viability and develop as superficially normal pharate adults. 100% of let-7^KO.C mir-125^KO.C double mutants initiate eclosion behaviour with the same timing as age-matched controls, but although they carry out the typical pattern of abdominal and thoracic contractions, these contractions are largely unproductive and only 31% of these animals are able to eclose. Those animals that do eclose have a severely shortened lifespan (50% of double homozygous females die just 8 days after eclosion). The double mutant adults show locomotor defects (no flight, uncoordinated movements and trembling).

let-7^KO.C mir-125^KO.C double mutant wing discs contain numerous dividing cells at 24 hours after pupariation, in contrast to wild-type discs, which contain few mitotic cells at this time. The number of apoptotic cells in the double mutant wing discs is also increased compared to wild type. The wings of eclosed let-7^KO.C mir-125^KO.C double mutant adults are significantly smaller, and contain more, smaller cells than wild-type wings.

Double mutant let-7^KO.C mir-125^KO.C clones in the wing contain 25% more and smaller cells than wild-type clones.

At 54 hours after puparium formation (APF), the extent of nascent neuromuscular junctions (NMJs) on dorsal abdominal muscles of let-7^KO.C mir-125^KO.C double mutant animals is similar to that of wild type. However, the mutant NMJs grow at a significantly slower rate than controls over the next 42 hours and are 50% smaller than NMJs from age-matched controls at each time point studied. The NMJs in abdominal muscles of mutant flies just before eclosion contain significantly fewer and smaller boutons than normal.

The reduction in bouton number at the NMJ that is seen in let-7^KO.C mir-125^KO.C double mutant animals at 96 hours APF is significantly rescued by ab^1D/+.