Young flies have seven rhabdomeres per ommatidia but they are arranged irregularly. In older flies only a few rhabdomeres can be seen per ommatidia in ectopically located eyes.

Flies from extra eye stocks show pattern duplications of head morphology in varying degrees; small duplications may appear as a few extra orbital bristles, whereas extreme expression involves well-formed, supernumerary, compound eyes on the vertex with associated orbital structures; ectopic eye is a mirror-image partial duplication of the ipsilateral eye. Occasionally, ocelli may be duplicated and rarely the antennae; a line of mirror-image symmetry often can be recognized between normal structures and their duplicated counterparts; structural deficiencies, particularly in the occipital region, are common. Penetrance is temperature sensitive (72% at 29^oC and 25^oC, 43% at 19^oC); temperature-sensitive periods in mid-embryogenesis and mid-first instar. There is no evidence of cell death in eye-antenna imaginal disc; first evidence of disc hyperplasia in late third instar. Fine-structure anatomy and physiology of photoreceptors in extra eyes appear normal; electroretinograms suggest that photoreceptors in extra eyes can make functional synaptic connections, although neuroanatomical studies show receptor-central-nervous-system innervations only rarely; the rare supernumerary antennae, however, have receptor cells whose axons innervate the ventral brain. ee¹ is influenced profoundly by apparent enhancer mutations on 1, 2 and 3; it behaves as a partial dominant when appropriate enhancers are made homozygous; P-elements also serve as enhancers (Marcey).