Identified neurons in an insect embryo: the pattern of neurons innervating the metathoracic leg of the locust.

The population of motorneurons which send axons to the metathoracic leg of the locust embryo has been stained by a cobalt backfilling technique. Embryogenesis lasts 11 days and an adultlike distribution of motorneuron cell bodies is seen as early as day 6: Several identified adult motorneurons can be recognized in the embryo. The total number of filled cells, the overall distribution of stained cell bodies, and the relative positions of identified neuron cell bodies do not change between 6 days and hatching. No motorneurons are stained which are clearly not leg motorneurons, and an adult pattern of axon branching at the level of peripheral nerve roots is already present at day 6. These observations provide information about the timing of motorneuron production and motor axon growth into the embryonic limb. The development of central dendritic arborizations can also be followed. The results are discussed with respect to mechanisms for neuron determination and differentiation.