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ZooNooz Article - december 2004
A tangled web
Why do Redback Spiders like to hang out in our suburban
backyards? WARRICK ANGUS explains.
Photographs Brian Chant
Illustration Katerina Sakkas
Slim Newton's 1972 classic
hit song touches on one of the
mysteries of the spider world - why
it is that Redback Spiders, amongst
the most notorious of Australia's
spiders, are as much a part of our
urban backyard culture as the
weekend barbie, yet so difficult to
find in untouched native bushland.
It all gets back to the science
of habitat selection, vital to any
animal's chances of survival.
A redback's shabby web resembles a tangle of silk threads with the odd leaf or twig suspended in the silk.
Most animals are able to move
between locations to find food.
However, in the case of spiders
that build webs to snare their
prey, they don't have the luxury
of easily moving around to hunt.
Web-building is time-consuming
and utilises precious energy and
resources, so locations must be
chosen that offer the best chance of
protection and shelter and provide
an adequate supply of insects.
So why are our backyards
such a popular habitat choice for
redbacks?
Redback Spiders |
I investigated this question for
my Honours thesis, 'The habitat
selection and feeding ecology of the
Redback Spider', at the University
of Sydney. Taking measurements of
web size, aspect, prey availability,
temperature and even habitat
types, I found that the common
factor for all the Redback Spider
webs I inspected was the type of
surface the web was attached to.
All the webs were built onto a solid
surface such as concrete, bricks,
sheet metal or wood. And when I
gave juvenile redbacks the choice
of building their webs in a variety
of habitats, over 90% of them chose
to build their webs on a concrete
surface rather than on sand, leaf
litter or grass.
Why? Well, it's all to do with
the mechanics of how the web
catches prey.
A redback's shabby web
resembles a tangle of silk threads
with the odd leaf or twig suspended
in the silk. At first glance the web
appears to be in total disarray and
one might ponder how on earth
it could catch any sort of prey.
The thought of a venomous spider choosing to live in our backyards can be a little unnerving - redbacks certainly can give a very painful bite.
However, on closer inspection, amidst
the cobweb some strategically placed
threads of silk can be found stretching
from the body of the web to the
ground. These elastic trap lines are
used to snare small ground-dwelling
animals such as insects, lizards and
even small snakes, each line working
like a kind of reverse bungy-jump. As
prey items brush past the trap lines
they are caught in the sticky silk and
then flung into the web above, where
they become further entangled.
The secret behind this web's
success is the rigid, solid surface that
it is attached to. It would be very
difficult to attach taut trap lines onto
loose surfaces such as sand, leaf litter,
dirt or grass, but pavers, balconies,
chairs, BBQs and other rigid surfaces
in our backyards fit the bill, providing
a perfect Redback Spider habitat.
The thought of a venomous
spider choosing to live in our
backyards can be a little unnerving
- redbacks certainly can give a very
painful bite. However, since the
introduction of anti-venom over
40 years ago there have been no
fatalities linked to the redback.
Redback Spiders |
So next time you're frying a few
sausages in your backyard, keep an
eye out for the insect-eating tenant
that may be living under your BBQ.
She'll be helping you by keeping those
mozzies away, and all you have to do
is relax and enjoy the summer.
Redback Facts
- Females are black
(occasionally brownish)
with an obvious orange to
red longitudinal stripe on
the upper abdomen and an hourglassshaped
reddish-orange spot on the underside.
They have a 1cm-long body and long, slender legs.
- Males are only about 3-4mm long and their red markings are often less
distinct. The body is light brown with white markings on the upper side of
the abdomen and a pale hour-glass marking on the underside.
- Rather than build webs to capture prey, males lurk on the fringe of a
female's web, making overtures to her to discover whether she is ready to
mate. This can prove fatal if she mistakes him for prey!
- Each female can produce about 150 hatchlings in an egg sac and over 10
egg sacs in a season - around 1500 babies per female. Each weighs only
0.0035g (the weight of a grain of salt).
- After hatching, the babies climb to a high vantage point where a slight
breeze is enough to catch the silk that drifts from their spinnerets. When
they land, sometimes many kilometres away, their first mission is to find an
appropriate web site. Those that don't will die.
- Spiders are responsible for eating 99% of all insects in the world. Just
imagine, if it weren't for the spiders in your backyard, that one annoying
mosquito could have been joined by 99 others!
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