The fate of descending brain-thoracic neurons in the metamorphosing supraoesophageal ganglion of the mealworm beetle, Tenebrio molitor, is described. Comparison of the descending neurons of the larval, various pupal, and adult stages outlines a high degree of topological invariance in the structure of descending interneurons; i.e., the basic organization of the imaginal set of descending neurons is anticipated by the structure of the larval neurons. Single descending neurons of analogous clusters of larval and imaginal neurons outline a virtual identical structural organization in both the larval and imaginal brain.
There is a quantitative increase from approximately 70 to approximately 120 brain-thoracic interneurons during metamorphosis. This increase does not cause complex transformations in the structural organization of the descending interneurons. Experiments with the DNA-inhibitor hydroxyurea prove that the described topological invariant structure of the set of descending neurons is based on the persistence of individually descending neurons. There is evidence that the whole set of larval interneurons persists throughout the beetles' metamorphosis.
The essential characteristics of the later imaginal set of descending neurons are qualitatively and quantitatively established within the first 10% of pupation. Structural invariance of the set of descending neurons is discussed with regard to the significance of cellular interaction for the mechanisms of metamorphic reorganization of nervous tissue.