Ventral nerve cord · Lobeworks/17

The ventral nerve cord is the part of an insect's central nervous system that runs from the neck through the thorax and abdomen, below the gut; it is the insect equivalent of the spinal cord, where the rhythms of the legs and wings are generated and the senses of the body are first processed.

Ventral nerve cord. The ventral nerve cord is the part of an insect's central nervous system that runs from the neck through the thorax and abdomen, below the gut; it is the insect equivalent of the spinal cord, where the rhythms of the legs and wings are generated and the senses of the body are first processed.

In the fruit fly the brain and the nerve cord are joined by a thin bundle at the neck, the neck connective, through which descending neurons carry the brain's commands down and ascending neurons carry the body's news up. The brain decides that the fly should walk or groom; the circuits of the nerve cord turn that decision into the timed contractions of dozens of muscles in each leg.

It is missing from the first whole-brain map. FlyWire stops at the neck, with 1,303 descending and 2,362 ascending neurons cut, so whole-brain models of that fly cannot move a leg by themselves and borrow controllers from a body model instead.

It has been mapped separately and together. The female nerve cord was imaged by serial-section electron microscopy collected on tape, and the male central nervous system published in 2026 includes brain and nerve cord in one volume with the neck intact, the first map that runs from the eyes to the legs.

Questions: What does the fly's nerve cord do that the brain does not? It turns a choice into movement. Descending neurons from the brain tell the nerve cord what to do, walk, groom or take off, and the circuits of the nerve cord produce the timed contractions of the muscles in each leg and wing. A brain alone can select a behaviour but cannot perform it, which is why a model of the brain without the nerve cord needs a body model to move. Why did the virtual fly of 2026 borrow its legs from a body model? Because the brain map it ran stops at the neck. FlyWire cuts 1,303 descending neurons there, and the nerve cord that would turn their commands into steps is not in it, so the simulation read the descending neurons and handed their choice to controllers written or trained for a body model. The male connectome of 2026 includes the nerve cord, which makes a model that walks by its own wiring possible to attempt.