Small Worlds in a Small Brain: Testing a Universal Network Principle on the First Complete Fly Nervous System

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Publication Details

Journal
Zenodo (CERN European Organization for Nuclear Research)
Published
2026-09-14
DOI
https://doi.org/10.5281/zenodo.22746402
Primary Topic
Neurobiology and Insect Physiology Research
Type
preprint
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preprint

Small Worlds in a Small Brain: Testing a Universal Network Principle on the First Complete Fly Nervous System

Yasar Syed
Zenodo (CERN European Organization for Nuclear Research)
Neurobiology and Insect Physiology Research
preprint

Small Worlds in a Small Brain: Testing a Universal Network Principle on the First Complete Fly Nervous System

Yasar Syed
preprint en

Abstract

A brain has to solve an odd kind of engineering problem. It needs enough local processing power towork out fine details close to home, but it also needs a way to move information across the wholesystem quickly, because most real behaviour depends on combining information from very differentplaces at once. One proposed solution to this problem, first described mathematically by Watts andStrogatz (1998), is what they called a "small-world" network: a network that is highly clustered, muchlike a tight-knit local neighbourhood, but that also has short average distances between any twopoints, almost like a network built entirely at random. Small-world organisation has since beendocumented in human brain networks and has been linked to real differences in cognitiveperformance (Bassett et al., 2009). It has also already been demonstrated in a fly brain: Lin et al.(2024) reported the clustering coefficient, the path length, and an explicit small-worldness score forthe female FlyWire connectome, which covers the brain only. This paper takes thatalready-established finding and pushes it one step further, using the newly released, completecentral nervous system connectome of a male Drosophila melanogaster. This dataset is the first flyconnectome to include both the brain and the ventral nerve cord (the fly's rough equivalent of aspinal cord) stitched together into one continuous map. Working directly from the real synapticconnectivity data (26 million raw connections among 211,577 confidently identified neurons), webuilt a network graph spanning the entire nervous system and calculated its clustering, its averagepath length, and its small-world score, this time comparing the result to a random network built tomatch the real network's exact connection pattern rather than the simpler random baseline used inearlier fly work. We found that the complete male nervous system is, without question, small-world inits organisation (with a small-world score of 12.7 once we corrected two problems in our firstattempt, described below), and that 99.8 percent of every confidently identified neuron in the animalbelongs to one single, fully connected network. We compare this result to Lin et al.'s (2024)brain-only finding and to the classic result from the roundworm C. elegans (Varshney et al., 2011),and we walk through a methodological mistake we made partway through this project, one thatinflated our own first estimate by roughly 80 percent, because it illustrates something genuinelyimportant about how this kind of analysis has to be done carefully.

Zenodo (CERN European Organization for Nuclear Research)
Maharaja Sayajirao University of Baroda (IN)
Neurobiology and Insect Physiology Research
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