SPIDERS MAKE FIERCE PREDATORS
August 12, 2002
Spiders are all around us during late summer, and when an inattentive
bug gets caught in a spider's web a fascinating predator-prey drama
unfolds. If a type of spider known scientifically as Portia
happens to be around, the ensuing event takes on added thrills as
a spectator sport. The lifestyles of several species of these jumping
spiders are the ultimate in stealth, deceit, and fierceness.
Most spiders eat mainly insects, but Portia spiders prey primarily
on other spiders (which are arachnids, not insects). And they capture
their prey not by ensnaring them in webs but by pouncing on them.
The tactics they use to outwit and capture web-building spiders, as
well as members of their own species, are intriguing.
Portia's strategy for attacking web-building spiders involves
a deceptive appearance and some misleading behavior. As part of their
tactical design, they are almost undetectable to their prey, yet have
excellent visual abilities themselves--plus fangs and venom. Four
pairs of eyes that detect motion and provide keen vision give them
better eyesight than most spiders. With their dark brown color and
hairy-looking legs, Portia spiders can look like a piece of
debris, allowing them to go unnoticed when invading another spider's
Being inconspicuous is only part of the design that allows Portia
to enter another spider's lair and attack the inhabitant. They also
mimic trapped insects. Many web-building spiders stay in a protected
area at the base of their web and monitor vibrations. A trapped insect
makes predictable movements while trying to escape. The spider thus
knows it has captured a particular prey item, like a fly or grasshopper.
Portia enters another spider's web, being careful not to make
vibrations that are detectable as anything more than that of a breeze
or a falling leaf. The cunning predator then creates vibrations that
mimic an entangled insect. As the web-building spider emerges from
its hiding place intent on making a meal of a trapped victim, the
tables are turned. The would-be predator, unprepared for an attack,
can become the prey. By assuming a home field advantage against a
defenseless insect, the web-builder may be caught off guard and killed
by the Portia.
The little invaders, however, are not always successful. Most spiders
have fangs, and spiders intended as victims are often much larger
than Portia. If a spider moves in quickly, Portia may
be killed before it has an opportunity to attack. Hence, these small
warriors of the spider world sometimes use other tactics.
The Portia may employ camouflage, by looking like a piece of
trash. As the larger spider approaches, Portia may change its
drumming pattern on the web in a manner that belies its presence.
For example, it may make the web move as it would in a gentle breeze,
so the bigger spider does not attack immediately, assuming that a
piece of debris has fallen into the web. Awaiting the right moment,
Portia can attack effectively by catching the other spider
Not content with eating other species of spiders, Portia spiders,
especially females, will also eat their own kind. Sometimes when females
encounter each other they will fight until one kills the other. Plus,
both adult and juvenile females on occasion release chemical signals
that attract males--as meals, not mates. This ploy works because a
signaling female may actually have mating in mind. Apparently, the
odds are good enough to keep attracting males, a phenomenon some might
consider to be universal among all species of animals.
The Portia spider saga is impressive, and although none may
live in your region, all spiders offer an intriguing glimpse into
the hidden world of invertebrates, which few of us notice. Equally
amazing is that scientists with patience and an understanding of natural
history continue to uncover enthralling facts about the lives of plants
and animals. Captivating natural events are happening in your backyard.
Look around; you may be surprised at what you see.
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