YSM Issue 95.1

Artificial Intelligence
FEATURE
IMAGE COURTESY OF FLICKR
A hackled orb weaver rests on its web. Gordus and his
team used these animals as model organisms to study
the construction of spider webs.
behaviors, the rules, [for each stage of]
web-building?”
In order to study these spiders’
movements, Gordus and his team used
infrared illumination and a high-speed
camera, which captured the minute
motions of each of the spider’s eight
legs. The entire process involved many
attempts, unexpected failures, and an
abundance of perseverance. Gordus
said that they originally tried to study
the spiders under red light, but the orb
weavers refused to build their webs
without complete darkness. The team
then transitioned to infrared light, which
is invisible to both humans and spiders.
To track the orb weavers’ movements,
the scientists placed labels with infrared
dyes on each of the spiders’ legs, a
technique commonly used to examine
fly behavior. However, they were met
with great resistance. “[The spiders]
hated having their limbs labeled, and
they would just spend the whole time
sitting there trying to take it off,” Gordus
said. “And then, they would [sometimes
stop building and would] stick to their
own web, and we would come back, and
they would just be dangling.”
Instead of the labels, the team decided to
use a camera that detected the reflection
of infrared light off of the spiders’ bodies.
They also adopted two recently published
algorithms specifically designed for limb
tracking, called LEAP and DeepLab
Cut. The scientists first trained the
algorithms on several thousand frames of
spider movements, which they manually
tracked. The algorithms were then able to
track millions upon millions of frames,
capturing the minute motions of the
spiders’ legs.
After monitoring six different
orb weavers, the team adopted a
machine learning algorithm, called
the hierarchical hidden Markov model
(HHMM) to deduce patterns in web
construction. The algorithm employed
probability models to predict the spider’s
web-stage based on transitions in its
behavior, without knowing where the
spider was on the web. The researchers
found that the predictions made by
the HHMM mapped onto established
phases of web-building based on the
spider’s position. This solidified the
association between the orb weaver’s
distinct behaviors and specific phases
of construction. Developing the model
involved trial and error—existing
algorithms used to predict fly movements
did not perform as well when applied to
orb weavers, so they had to write their
own code from scratch.
After years of troubleshooting and
diligent work, Gordus’s lab finally
developed a fully-fledged experimental
system. Upon collecting their data and
analyzing the results, the researchers
came to a startling revelation. Contrary
to their expectations, the orb weavers
did not build their webs reflexively,
moving from phase to phase without
pausing. Instead, the spiders revised
their work as they went, returning to
past locations on their webs to rearrange
misplaced strands of silk. Sometimes,
the weavers even repeated entire phases
of web construction before proceeding
again, indicating that they might have
internal models of their webs that they
are attempting to replicate.
“We were surprised [at] how frequently
the spider could go back and try a prior
phase over again,” Gordus said. “[The
spiders are] constantly assessing what
they’re building with this internal goal,
and [they have] a flexible way of trying
to get to that goal.”
Looking ahead, Gordus’s team hopes
to study the effects of certain drugs on
web construction in order to elucidate
the neurological activity associated with
each phase of building. The team is
looking into the effects of two chemicals
in particular: lysergic acid diethylamide
(LSD), a potent psychedelic drug,
and ecdysone, a steroidal hormone in
arthropods that induces molting and
influences decision making.
Already, the researchers have confirmed
that ecdysone causes the orb weavers
to stop building their webs at a certain
stage. They also know that giving the
spiders a microdose of LSD results in the
construction of perfectly symmetrical,
evenly-spaced webs. Gordus said he is
interested in further studying the effects
of LSD on neuromodulatory pathways,
or chemical pathways in the brain that
control internal states.
“If the spiders build really good
webs [after consuming LSD], then we
want to know what changed in their
behavior,” Gordus said. “Are they just
executing the behaviors really well, like
a professional web builder? Or do they
have [obsessive compulsive disorder],
and they’re constantly doing a lot of
error correction? We’d like to know,
what is the behavioral readout?”
By deducing which motor neurons
are activated in the spiders’ brains after
the administration of certain drugs, the
researchers might be able to understand
the effects of these chemicals on human
behavior. For now, though, Gordus
and his team are focused on studying
orb weavers and the graceful, intricate
choreography of their web-building. ■
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March 2022 Yale Scientific Magazine 31