The majority of Cyp18a1¹ homozygous embryos develop normally to first instar larvae. Most animals pupariate but fail to complete metamorphosis. Delayed or complete failure to displace the abdominal air bubble during pre-pupal development is observed. During the prepupal to pupal transition, animals die in three phenotypic classes. Of those that pupariate, 72% of animals die as prepupae or as early pupae after delayed head eversion. A small proportion of mutants die as pharate adults, and a very small number of adult escapers are observed. All animals that pupate display a malformed and short third thoracic leg phenotype, and some animals display a microcephalic head eversion phenotype.