44 | New Scientist | 8 February 2020it, descending and ascending at various
points along the way. This could allow them
to estimate the dimensions of the space.
“Maybe they’re somehow able to tell how
much silk there is and how much time they
spent exploring. That could potentially
give an indication of how many times they
passed through an area and how big it is,”
says Hesselberg.
The ability to form a mental representation
of space has been documented in many
mammals and birds, but is almost completely
unheard of among invertebrates. Until now,
the best evidence was in bees, which are
thought to use mental maps to help them
navigate – but even that has been contested.
Hesselberg’s preliminary experiments
suggest that the cognitive maps of orb web
spiders aren’t very detailed: they seem to
include big obstacles but not little ones, for
instance. Yet the ability to generate even a crude
representation of their environment would
help to explain other behaviours that appear
to demand a certain level of foresight and
planning. For instance, orb web spiders alter
the size and structure of their webs according
to the remaining silk reserves in their glands,
ensuring that they don’t run out midway. They
are also sensitive to the weather: in low
temperatures, they make simpler structures
with bigger gaps between the spirals to avoid
spending too much time exposed to the cold.
Cognitive sophistication is evident in the
ways that spiders maintain and adapt a web
after construction, too. They tweak the tension
of silk strands in the areas most likely to trap
unsuspecting insects, based on their previous
catches. Increased tension boosts the
transmission of vibrations from a struggling
insect, allowing the waiting spider to respond
more quickly. They also alter the dimensionsSocial
webs
We tend to think of spiders
as solitary creatures, but
around 20 species live in
colonies of thousands where
they share tasks such as
foraging, web building and
maintenance and care of the
young. This lifestyle mostly
occurs in the tropics and, in
some species, may have
evolved to cope with
damage to webs caused by
heavy rainfall. Protection
against raiding ants is
another possible benefit.
Colonial spiders are also
better equipped to catch
the larger prey insects that
are found in these regions.
Consider Stegodyphus
dumicola, which lives in
colonies of up to 2000
members in southwestern
Africa. The colonies consist
of two silk structures: the
small and ball-like “retreat”,
where the spiders live, and
the “capture web”, which is
large and flat and intended
to catch prey. When the
spiders sense the vibrations
of an insect that has ventured
into this trap, a few will
come out of the retreat to
inject it with venom. “When it
stops moving, more spiders
come in, start eating it and
drag it back to the retreat,”
says Noa Pinter-Wollman at
the University of California,
Los Angeles.of the web according to the size of prey found
locally. They can even learn from near misses:
if their prey hits the web but then escapes,
spiders will lay down more sticky silk to ensure
this doesn’t happen in future. You could almost
think of webs as instruments spiders carefully
“tune” according to their experiences. “There
is no doubt that they are much more flexible
than we once believed,” says Hesselberg.
Spiders are a varied group, of course,
but species with very different lifestyles are
proving equally smart. Take jumping spiders
belonging to the genus Portia. Their preferred
prey is other spiders, and these can be up to
twice their own size. They catch them by
stealthily roaming their environment, and
have evolved astonishing eyesight to do this.
“They hunt like tiny cats,” says Fiona Cross at
the University of Canterbury, New Zealand.
This often involves premeditation and careful
route planning, as her research reveals.Mental maps
Working with her colleague Robert Jackson,
Cross placed individual Portia on a tower in
a water tank – like cats, they hate getting
wet. From this viewpoint, they could see two
floating pathways, one of which led to prey on
another tower. Although the prey was out of
sight once the spiders had descended, they
invariably chose the correct route, suggesting
they held a map in their working memory.
In another experiment, Cross and Jackson
provided two routes: both led to the prey,
but one was shorter. Most spiders chose the
quickest path. “For us, that suggests that Portia^
are making a plan of where to go,” says Cross.
In the wild, the spiders use this kind of
premeditation to ambush dangerous prey
from behind.WV
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CS
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AG
ES Jumping spiders
have evolved
astonishing
eyesight to help
catch prey