Among the variety of roles and diverse possible functions that have been attributed to glial cells, the nutritive function is strongly supported by direct experimental evidence obtained in a model of the honeybee drone retina. We have shown that in this nervous tissue, with crystal-like structure, in which glial cells and photoreceptor neurons constitute two distinct metabolic compartments, glial cells transform glucose to alanine and, with proline, fuel the mitochondria of the photoreceptors. Proline supplies the Krebs cycle by making glutamate. The use of proline implies high ammonia production. Pyruvate transamination in the glia fixes ammonia at a rate exceeding glutamine formation. We favor the hypothesis that ammonia rather than K+ is the metabolic signal trafficking between neuron and glial cells.