The proventriculus is a complex structure comprising derivatives of two germ layers: the ectoderm and the endoderm. It consists of an inner epithelial layer which essentially represents a continuation of the oesophagus; a recurrent layer which is folded back over the inner layer, a ring of imaginal cells representing the border between ectoderm and endoderm parand an outer layer which is continuous with the midgut. The ectodermal imaginal cells will give rise to the oesophagus of the adult fly.