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\title{The Moth That Reads the Milky Way}
\author{The Science Journaling Club}
\date{Field Notes --- Paper Analysis --- Animal Navigation}

\begin{document}
\maketitle

\begin{abstract}
Once a year, an Australian moth with a brain you could lose in a poppy seed flies up to
1,000~km to a handful of alpine caves it has never seen, packs in among roughly 16,000
other moths per square metre of cave wall for the summer, and then flies the whole
distance back to breed and die \cite{warrant2016}. In 2025 a team led by David Dreyer and
Eric Warrant tethered these moths inside a 50~cm planetarium, projected a real, moonless
star field over their heads, and switched the Earth's magnetic field off with a bank of
Helmholtz coils, nulled below 100~nanotesla --- a reduction our own arithmetic puts at
somewhere north of 250-fold \cite{dreyer2025}. The moths kept flying the correct
seasonal bearing anyway. Rotate the projected sky $180^\circ$ and the moths turn with it,
in both directions, in both seasons (six trials, $n$ from 49 to 70 each). Scramble the
same stars into random positions and the moths go nowhere in particular. Recordings from
the moths' own brains turned up 28 neurons, out of 79 sampled, tuned to the rotating sky,
almost all of them firing hardest when the moth faced within a few degrees of due south
--- in spring and in autumn alike, headed toward the caves or away from them. This is,
by the authors' own account, the first invertebrate ever shown to steer by the stars. We
think that is a bigger deal than it sounds, and this note is our attempt to say exactly
why, using nothing but the numbers in the paper and a calculator.
\end{abstract}

%============================================================================
\section{An Insect With No One to Learn the Route From}

The Bogong moth, \textit{Agrotis infusa}, breeds across the lowlands of southeastern
Australia. Each spring a fresh generation of adults, only weeks old, sets off after dark
toward the Australian Alps and does not stop until it reaches a specific set of limestone
caves, where it packs onto the rock walls at densities the companion literature puts near
16,000 animals per square metre and waits out the heat of summer in a torpor called
aestivation \cite{warrant2016}. Come autumn, the same individuals wake up, fly the entire
distance back, breed once, and die. Nobody teaches them the route. There is no older
generation making the trip alongside them, no return flight to correct a bad first
attempt, and by the time they are capable of flying at all their parents are already
dead. Every leg of an up-to-1,000-kilometre round trip is a genuinely novel journey for
the animal making it \cite{dreyer2025}.

Compare that with the two other famous long-distance insect migrants. Monarch butterflies
split their journey across several generations, so no single monarch ever completes the
whole loop, and the ones that do fly the long leg use a sun compass that reads the time of
day off circadian clocks in their own antennae \cite{merlin2009}. Migratory songbirds, the
other classic star-navigators, get to practise: many species make the trip repeatedly over
a multi-year lifespan \cite{emlen1975}. A Bogong moth gets one shot, in each direction, and
has to get it right the first time using a nervous system built in a matter of weeks.

\textit{The question this paper actually answers is not ``can a moth see stars.'' It is
``can a moth's genes, with no help from experience, wire up a working compass out of
them.''}

%============================================================================
\section{Building a Planetarium for an Animal the Size of a Fingernail}

Each animal was chilled for five to ten minutes to hold it still, had a small patch of
scales removed from its thorax with a micro-vacuum, and had a fine tungsten stalk glued to
the bare cuticle with contact cement. That stalk clips, through a short length of soft
rubber tubing, onto the shaft of an optical encoder, which lets the moth rotate freely
around its yaw axis while the encoder logs that heading 5 times a second at a resolution
of $3^\circ$ \cite{dreyer2025}.

Around that moth sits a transparent Perspex cylinder 50~cm across and 35~cm tall, and onto
the inside of that cylinder the team projected a real night sky --- generated in the
planetarium software Stellarium, moonless, dated to actual spring and autumn nights
(1~October 2018 for spring; 21 and 27~March 2018--2019 for autumn), filling $160^\circ$ of
the moth's field of view in the behavioural rig and $100^\circ$ in the electrophysiology
rig, both centred on the zenith \cite{dreyer2025}.

\begin{figure}[htbp]
\centering
\begin{tikzpicture}[line width=0.9pt, font=\footnotesize]
  \draw[bcmag, dashed] (-3.2,-2.1) rectangle (3.2,2.1);
  \node[bcmag, anchor=south west] at (-3.15,2.15) {3-axis Helmholtz coils, 900$\times$835$\times$775 mm};
  \draw[bcmag, dashed, opacity=0.6] (-2.7,-1.7) rectangle (2.7,1.7);
  \draw[fill=white] (-1.4,0.75) ellipse (1.4 and 0.28);
  \draw (-1.4,0.75) -- (-1.4,-0.9);
  \draw (1.4,0.75) -- (1.4,-0.9);
  \draw (-1.4,-0.9) arc (180:360:1.4 and 0.28);
  \draw[dashed, opacity=0.5] (-1.4,-0.9) arc (180:0:1.4 and 0.28);
  \node[anchor=south] at (0,1.15) {\scriptsize arena: 50 cm $\times$ 35 cm};
  \node[bcstar, anchor=south] at (0,0.55) {\scriptsize projected night sky (Stellarium)};
  \draw[bcmoth] (0,0.75) -- (0,0.05);
  \filldraw[bcmoth] (0,0) circle (2.3pt);
  \draw[fill=white] (-0.28,0.6) rectangle (0.28,0.78);
  \node[anchor=north, font=\tiny] at (0,-1.05) {tethered moth, free to choose any heading};
  \node[bcmag, anchor=north] at (0,-2.15) {\scriptsize field nulled below 100 nT within 4 cm of the moth};
\end{tikzpicture}
\caption{The rig. A moth flies in place inside a projected sky while three pairs of
Helmholtz coils cancel the Earth's own magnetic field around it. Every dimension is
quoted directly from the paper's methods \cite{dreyer2025}.}
\label{fig:rig}
\end{figure}

%============================================================================
\section{Turning the Magnetic Field Off, on Purpose}

The three pairs of Helmholtz coils in Figure~\ref{fig:rig} do not just weaken the ambient
field, they actively cancel it, driven by constant-current supplies and checked with a
magnetometer before each run, holding the field below 100~nanotesla in a small volume
around the tethered moth \cite{dreyer2025}. Earth's surface field runs roughly 25,000 to
65,000~nanotesla depending on latitude, a range that is common background in geomagnetism
rather than a number from this paper. Divide those ourselves and 100~nanotesla is
somewhere between a 250-fold and a 650-fold reduction --- comfortably below anything a
magnetoreceptor could plausibly use as a directional signal.

%============================================================================
\section{Six Trials That Do the Real Work}

Fly a moth under the field-nulled, projected night sky and see which way it points. Then
fly a fresh moth under the identical sky flipped $180^\circ$. Then fly a fresh moth under
the same stars with their positions scrambled into a random arrangement, keeping
brightness and density the same but destroying every constellation. Run all three
versions in spring, when free-flying moths head roughly south toward the caves, and again
in autumn, when they head roughly north back to the breeding grounds.

\begin{table}[htbp]
\centering
\small
\begin{tabular}{llrrll}
\toprule
\textbf{Season} & \textbf{Sky condition} & \textbf{$n$} & \textbf{Mean heading} & \textbf{Rayleigh $R^{*}$} & \textbf{$p$} \\
\midrule
Spring (target $\approx 170^\circ$) & natural sky            & 70 & $168^\circ$ & 2.01 & $6.3\times10^{-5}$ \\
Spring                              & rotated $180^\circ$     & 68 & $355^\circ$ & 2.09 & $2.7\times10^{-5}$ \\
Spring                              & stars scrambled         & 58 & disoriented & 0.60 & $0.35$ \\
\midrule
Autumn (target $\approx 341^\circ$) & natural sky            & 54 & $341^\circ$ & 2.51 & $9.0\times10^{-9}$ \\
Autumn                              & rotated $180^\circ$     & 56 & $167^\circ$ & 1.72 & $1.8\times10^{-3}$ \\
Autumn                              & stars scrambled         & 49 & disoriented & 0.02 & $0.91$ \\
\bottomrule
\end{tabular}
\caption{Six trials, magnetic field nulled throughout. Flip the sky and the moths flip
their heading with it, in both seasons; scramble the stars and orientation collapses.
Values quoted directly from \cite{dreyer2025}.}
\label{tab:sixtrials}
\end{table}

Notice what the rotation trial actually rules out. A moth attracted to the brightest patch
of projected light, or repelled by some artefact of the projector itself, would not flip
its heading when the sky flips, because the physical hardware around it has not moved. A
moth reading a magnetic cue would not respond to the sky at all, since there is no field
left to read. The only thing left standing after both of those explanations fail is the
pattern of stars itself.

%============================================================================
\section{The Night the Stars Moved and the Moths Didn't}

A separate set of trials, run outdoors under the real night sky with the coils switched
off entirely, asks a harder question: can the moths keep their heading as the real sky
drifts across the night, the way it always does because the Earth keeps turning
underneath it?

Ninety-five moths were flown outdoors early in the evening, averaging 20:49 local time,
and pointed a mean $5^\circ$ --- essentially due north, the correct autumn bearing. The
same 95 animals were flown again roughly three hours later, averaging 23:42, and still
pointed $355^\circ$, a shift of only $10^\circ$ that a likelihood-ratio test could not
distinguish from no change at all ($\chi^2=2.663$, $p=0.103$) \cite{dreyer2025}. We wanted
to know how much the actual star field had moved in that gap, so we did the arithmetic
ourselves, using a fact the paper does not need to state because every astronomer already
knows it: the sky appears to rotate around the celestial pole at essentially the Earth's
own rotation rate, about $15^\circ$ per hour.

\begin{center}
\fbox{\begin{minipage}{0.92\linewidth}
\small\textbf{Club arithmetic, not in the paper.}
\begin{align*}
\text{time between readings} &= 23{:}42 - 20{:}49 = 2\,\mathrm{h}\,53\,\mathrm{min} = 2.883\,\mathrm{h} \\
\Delta\theta_{\text{sky}} &\approx 2.883\,\mathrm{h} \times 15^\circ/\mathrm{h} \approx 43^\circ \\
\Delta\theta_{\text{moth}} &= 5^\circ \to 355^\circ = 10^\circ
\end{align*}
A moth reading off one fixed star's position, with no correction, should have drifted
with it by roughly $43^\circ$. It drifted by less than a quarter of that.
\end{minipage}}
\end{center}

Meanwhile 44 moths flown under overcast skies, with neither stars nor moon visible, still
oriented at a mean $13^\circ$ (Rayleigh $R^{*}=2.83$, $p=7.6\times10^{-10}$), statistically
indistinguishable from the clear-sky result ($\chi^2=0.24$, $p=0.89$) \cite{dreyer2025}. Here
the magnetic field was never nulled, so this is Earth's own field acting as a fully
functional backup compass, not a vestige.

That combination --- stars alone sufficient with the field nulled, magnetism alone
sufficient with the sky hidden --- is the whole argument for two independent compasses,
either one enough on its own, and for something smarter in the stellar one than ``fixate
on one bright dot.'' Either the moths track a broad pattern, such as the shape of the
Milky Way, that rotates as a rigid whole and stays recognisable as it turns, or they run a
genuinely time-compensated star compass, of the kind already documented for the sun in
monarch butterflies \cite{merlin2009}. The paper does not settle which mechanism it is. It
does rule out the naive one.

%============================================================================
\section{A Compass Wired Into a Brain the Size of a Poppy Seed}

To find out how, not just whether, the team repeated the projected-sky experiment while
recording electrical activity directly from neurons inside the moth's own brain. They
sampled from 79 neurons in total. Of those, 28 --- about 35\% of the sample, arithmetic we
did ourselves on the paper's own raw counts --- passed the criteria for reliable,
repeatable tuning to the rotating sky \cite{dreyer2025}.

Most of those 28 cells, 18 of them, fired in a single broad peak: quiet most of the time,
most active at one particular simulated heading. A further 6 showed the mirror-image
pattern, going quiet at one specific heading instead. A handful of others were sensitive
to which way the sky was turning, and three, recorded in a region that generates steering
commands, fired in two opposite peaks $180^\circ$ apart. Eleven of the 28 cells, roughly
two in five, were filled with a tracer dye and anatomically traced to their exact
location.

\begin{figure}[htbp]
\centering
\begin{tikzpicture}[
  box/.style={draw, rounded corners=1pt, minimum width=3.6cm, minimum height=2.1cm, align=center, font=\footnotesize},
  arr/.style={-{Latex[length=2mm]}, line width=1pt}
]
\node[box, draw=bcstar] (ol) at (0,0) {\textbf{Optic lobe}\\[2pt] primary visual\\ processing\\[4pt] {\scriptsize sharpest tuning,\\ highest SNR}};
\node[box, draw=bcmoth] (cx) at (4.4,0) {\textbf{Central complex}\\[2pt] orientation, navigation,\\ action selection\\[4pt] {\scriptsize shared across\\ insect orders}};
\node[box, draw=bcember] (lal) at (8.8,0) {\textbf{Lateral accessory}\\ \textbf{lobes}\\[2pt] output target of\\ the central complex\\[4pt] {\scriptsize generates steering\\ commands}};
\draw[arr] (ol) -- (cx);
\draw[arr] (cx) -- (lal);
\end{tikzpicture}
\caption{The pipeline. Three brain regions, in the order signals pass through them, each
described in the paper's own terms \cite{dreyer2025}. The central complex is an ancient
structure found across insect orders \cite{honkanen2019}.}
\label{fig:pipeline}
\end{figure}

%============================================================================
\section{Why the Preferred Direction Is Always South}

Pool the tuned cells together and ask which direction each one prefers, and the answer
clusters tightly around one heading: $171^\circ \pm 35^\circ$, almost due south
\cite{dreyer2025}. That number does not shift between seasons
(Mardia--Watson--Wheeler test, $p>0.2$), even though a real moth flying south in spring is
heading toward the caves and a real moth flying south in autumn would be heading the wrong
way entirely. This looks less like a map of ``which way to fly this season'' and more like
a fixed reference axis, read and reinterpreted downstream depending on the time of year
--- the way a compass needle always points to magnetic north regardless of which way you
mean to walk. A close analogy: head-direction cells in streaked shearwater chicks fire
maximally toward magnetic north even though the same chick will fly south on its first
migration \cite{takahashi2022}.

\begin{figure}[htbp]
\centering
\begin{tikzpicture}
\begin{axis}[
  width=12cm, height=6.5cm,
  xmin=0, xmax=360, ymin=0, ymax=1.05,
  xlabel={simulated heading (degrees)}, ylabel={firing rate (normalised)},
  xtick={0,90,180,270,360}, xticklabels={N,E,S,W,N},
  ymajorgrids=true, grid style={dashed, gray!30},
  legend style={font=\scriptsize, at={(0.02,0.97)}, anchor=north west}
]
\addplot[bcstar, line width=1.4pt, mark=none, smooth] coordinates {
 (0,0.00002)(30,0.0003)(60,0.0065)(90,0.0687)(120,0.346)(150,0.835)
 (171,1.0)(180,0.967)(210,0.537)(240,0.1435)(270,0.0183)(300,0.0011)(330,0.00003)(360,0.00002) };
\addlegendentry{tuning to rotating sky, both seasons pooled}
\draw[bcstar, dashed] (axis cs:171,0) -- (axis cs:171,1.05);
\addplot[gray, line width=1pt, dashed, mark=none] coordinates {
 (0,0.30)(40,0.32)(80,0.29)(120,0.30)(160,0.32)(200,0.30)(240,0.32)(280,0.29)(320,0.30)(360,0.31) };
\addlegendentry{scrambled stars: no tuning (SNR far lower, $p=0.00011$)}
\end{axis}
\end{tikzpicture}
\caption{Tuning of the recorded neurons to the rotating projected sky. Curve shape is
illustrative; the peak location ($171^\circ\pm35^\circ$) and the natural-versus-scrambled
comparison are as reported \cite{dreyer2025}.}
\label{fig:tuning}
\end{figure}

%============================================================================
\section{What the Neurons Are Actually Looking At}

A subset of six neurons was tested with two simplified artificial stimuli standing in for
pieces of the natural sky: a single revolving dot, mimicking the brightest patch of the
Milky Way near the Carina Nebula, and a rotating bar, mimicking the Milky Way's whole
elongated band. All six neurons responded to the dot, but its preferred direction drifted
up to $52^\circ$ from the value measured under the real sky. Five of six responded to the
bar, drifting only about $15^\circ$ \cite{dreyer2025}.

\begin{figure}[htbp]
\centering
\begin{tikzpicture}
\begin{axis}[
  width=11cm, height=6cm, ybar, bar width=28pt,
  symbolic x coords={dot,bar}, xtick=data,
  xticklabels={revolving dot (Carina hotspot), rotating bar (Milky Way band)},
  x tick label style={font=\scriptsize},
  ymin=0, ymax=60,
  ylabel={shift from natural-sky preferred direction ($^\circ$)},
  nodes near coords, nodes near coords style={font=\scriptsize}
]
\addplot[fill=bcstar] coordinates {(dot,52)};
\addplot[fill=bcember] coordinates {(bar,15)};
\end{axis}
\end{tikzpicture}
\caption{The extended bar, standing in for the Milky Way's overall shape, reproduces the
natural tuning far more faithfully than a single bright point does --- evidence these
neurons read the galaxy's shape, not just its brightest highlight
\cite{dreyer2025}.}
\label{fig:milkyway}
\end{figure}

A point of light is a much simpler thing to encode than an extended, faint band stretching
across a third of the sky. If the moth's compass worked by fixating on the single
brightest star or cluster, the dot stimulus should have reproduced the natural tuning
perfectly and the bar should have done worse. It went the other way.

%============================================================================
\section{An Old Circuit, Doing a New Job}

None of the three brain regions in Figure~\ref{fig:pipeline} were invented for this task.
The central complex shows up across essentially every insect order examined for it, and a
2019 review co-authored by two of this paper's own scientists lays out its established
repertoire: path integration, a polarised-light compass in desert locusts, and the
transport-orientation system that lets a dung beetle roll a ball of dung away from a
crowded pile in a dead-straight line \cite{honkanen2019}. Dung beetles are, in fact, the
previous headline case of an insect reading the Milky Way --- but only to hold a straight
heading for a few metres away from competitors, not to pick out a specific compass
direction toward a destination a thousand kilometres away \cite{dacke2013}. The Bogong
moth result does not introduce new hardware into the insect brain. It shows old,
well-mapped hardware doing something nobody had caught it doing before.

%============================================================================
\section{Why This Matters More Coming From a Moth Than From a Bird}

Stephen Emlen's planetarium experiments on indigo buntings established fifty years ago
that a migratory songbird can derive a compass direction from the pattern of stars around
the celestial pole \cite{emlen1975}. The 2025 paper's own framing makes the comparison
explicit: as far as its authors know, apart from humans and a handful of night-migrating
songbirds, no other animal had been shown to use the stars to navigate toward a specific
geographic direction \cite{dreyer2025}.

A songbird's star compass sits inside a brain built along the same basic vertebrate plan
as ours, refined over a multi-year lifespan in which the bird gets to fly the route, or
watch the sky rotate, more than once. The Bogong moth's entire tuned population in this
study is 28 cells. Its whole compass has to be assembled, correctly, by a genetic program
alone, inside a brain built in the weeks between metamorphosis and the first flight that
will decide whether the animal lives through summer at all --- and it has to work the very
first time it is used, because there is no second migration to learn from. If a circuit
this small, this fast to build, and this thoroughly disposable can extract a genuine
compass bearing from a pattern as diffuse as the Milky Way, the raw computation involved
may be a great deal less demanding, in neural terms, than its scarcity in the animal
kingdom had suggested.

\textit{A brain that has to work perfectly on the first try, with no teacher and no
do-over, is a strange place to discover one of the more sophisticated pieces of astronomy
in the animal kingdom.}

%============================================================================
\section{What the Paper Does Not Claim}

The authors write plainly that it remains unclear exactly how many of the sky's brightest
stars a moth's small compound eyes can actually resolve as distinct points
\cite{dreyer2025}, which means the ``extended shape'' interpretation of
Figure~\ref{fig:milkyway} is an inference from the neural data, not a direct measurement
of the moth's optics. Whether the mechanism is genuinely time-compensated, tracking the
sky's motion the way a monarch's antennal clock tracks the sun \cite{merlin2009}, or
relies on some rotation-invariant feature of the Milky Way's shape, is left open rather
than resolved. And every number in this note describing ``the moth's compass'' comes from
animals tethered in a lab, flying in place under a projected sky.

%============================================================================
\section{What We Would Still Want to See}

First, resolve the compound eye's actual resolving power for faint, extended sky
features, to settle whether ``reading the shape of the Milky Way'' is optically
plausible. Second, run the artificial-stimulus test of Figure~\ref{fig:milkyway} with a
continuous range of stimulus shapes between a point and a full arc, rather than just the
two endpoints tested here. Third, and hardest: find a way to record from, or at least
track, a moth's heading decisions partway through an actual free flight, to check that the
laboratory's $171^\circ$-south reference axis is the same one steering the animal across
real Australian terrain.

%============================================================================
\section{What We Would Tell Somebody Who Read Only the Headline}

Four things. The moth is not ``using the stars'' the way a sailor with a sextant does;
nobody has shown it can identify individual constellations by name, only that it responds
to the rotating pattern as a whole and to an elongated shape resembling the Milky Way in
particular. The magnetic sense has not been demoted: it is a fully functional backup
compass in this species, and the two systems appear to operate independently. The
$171^\circ$ ``south'' tuning found in the brain is a fixed reference axis, not a seasonal
instruction, and something else downstream decides whether to fly toward it or away from
it depending on the time of year. And the headline claim, ``first invertebrate star
compass,'' is not our club's summary or a journalist's shorthand; it is close to the
authors' own words, and every DOI and figure in this note traces back to a paper that, as
far as we can verify, says exactly that.

%============================================================================
\begin{thebibliography}{99}

\bibitem{dreyer2025}
Dreyer, D., Adden, A., Chen, H., Frost, B., Mouritsen, H., Xu, J., Green, K.,
Whitehouse, M., Chahl, J., Wallace, J., Hu, G., Foster, J., Heinze, S. \& Warrant, E.
(2025). Bogong moths use a stellar compass for long-distance navigation at night.
\textit{Nature} 643, 994--1000. doi:10.1038/s41586-025-09135-3

\bibitem{warrant2016}
Warrant, E., Frost, B., Green, K., Mouritsen, H., Dreyer, D., Adden, A., Brauburger, K.
\& Heinze, S. (2016). The Australian Bogong moth \textit{Agrotis infusa}: a long-distance
nocturnal navigator. \textit{Frontiers in Behavioral Neuroscience} 10, 77.
doi:10.3389/fnbeh.2016.00077

\bibitem{honkanen2019}
Honkanen, A., Adden, A., da Silva Freitas, J. \& Heinze, S. (2019). The insect central
complex and the neural basis of navigational strategies. \textit{Journal of Experimental
Biology} 222(Suppl\_1), jeb188854. doi:10.1242/jeb.188854

\bibitem{emlen1975}
Emlen, S. T. (1975). The stellar-orientation system of a migratory bird.
\textit{Scientific American} 233(2), 102--111. doi:10.1038/scientificamerican0875-102

\bibitem{merlin2009}
Merlin, C., Gegear, R. J. \& Reppert, S. M. (2009). Antennal circadian clocks coordinate
sun compass orientation in migratory monarch butterflies. \textit{Science} 325(5948),
1700--1704. doi:10.1126/science.1176221

\bibitem{takahashi2022}
Takahashi, S., Hombe, T., Matsumoto, S., Ide, K. \& Yoda, K. (2022). Head direction cells
in a migratory bird prefer north. \textit{Science Advances} 8(5), eabl6848.
doi:10.1126/sciadv.abl6848

\bibitem{dacke2013}
Dacke, M., Baird, E., Byrne, M., Scholtz, C. H. \& Warrant, E. J. (2013). Dung beetles use
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\end{thebibliography}

\end{document}