From Movement to Mechanism: Understanding a neural network through one fly leg movement

A connectome is a map of connections between nerve cells. It gives us an extraordinary starting point, but a map alone does not tell us how the network produces an action. We wanted a small experiment that would let us ask that question directly. We established one repeatable movement of a simulated fly leg, driven through a network derived from maleCNS: a wiring map of a male fruit fly's central nervous system. Then we changed the network and followed the resulting movement difference backwards: first to four motor outputs, then to an inhibitory neural route, and finally to the timing of a signal arriving at a motor cell. This report offers that worked example. Its value is a concrete connection between a visible movement and testable claims about the network producing it. It also shows how to investigate those claims when a rule of the simulation changes. Our goal is understanding: using a manageable experiment to learn how this model works, and giving others something precise to build on. This is a computational research preprint, not a peer-reviewed report or experimental validation in a living fly. Version 0.3 contains six data-grounded illustrations, a technical appendix, and a guide to supporting studies 058-063. Files: the approved version 0.3 manuscript in PDF and editable DOCX, a slowed leg-comparison animation, and a figure-data supplement. The supplement contains all six figures as PNG/SVG, the exact P7 schedule, exported trajectories and intervention summaries, selected motor traces, a study-to-file guide, source hashes and licence information. It supports inspection and replotting of these exports; it is not a complete simulation rerun package and does not include the full 058 handoff, all 059-063 study records, executable research code or imported body assets. Author declarations: Tomáš Kovářík is the sole author and an independent researcher. No external funding was received. The author declares no competing interests. AI assistance in research, analysis and writing is disclosed in the manuscript. These declarations were confirmed on 16 September 2026; the unchanged manuscript's author-review wording predates that confirmation. CC BY 4.0 applies to the paper and the author's original documentation and figure data. Third-party materials retain their own rights; no blanket licence is applied to the separately retained simulator and its imported assets.