ALL SPIDERS BUILD WEBS
by Whit Gibbons
May 9, 2004
the intrigue of ecology and animal behavior is that the natural world
is full of exceptions. For example, when we think of spiders, we think
of spider webs. But not all spiders rely on spinning a web to capture
their prey. Several species of jumping spiders, known scientifically as
Portia, have a different strategy for obtaining a meal. Stealth,
deceit, and fierceness are all part of the package with these little predators.
eat mainly insects. But Portia jumping spiders rely primarily on
other spiders as a food source. And they capture their prey by pouncing
on them. The tactics they use to outwit and capture web-building spiders,
as well as members of their own species, sound like something out of a
science fiction movie.
strategy includes a deceptive appearance and a cunning attack plan. Part
of their tactical design is to be undetectable, yet they have excellent
visual abilities themselves, plus fangs and venom. With four pairs of
keen eyes that sense motion, they have better eyesight than most spiders.
A dark brown color and hairy-looking legs give Portia the guise
of a piece of debris that can go unnoticed on another spider's web.
As if being
inconspicuous were not in itself sufficient for Portia to enter
the lair and attack other spiders, they extend their behavioral antics
a step further. They actually lure web-building spiders by mimicking trapped
insects. Many spiders stay in a protected area at the base of their web
and monitor vibrations. An insect trapped in a web makes predictable movements
while trying to escape. The spider knows it has captured a particular
prey item, like a fly or grasshopper.
Portia enters another spider's web, not only not getting stuck
but also not making vibrations that are detectable as anything more than
that of a breeze or falling leaf. The tiny 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 unexpectedly 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.
outcome, for the little invaders, however, is not always assured. 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.
may employ camouflage, by looking like a piece of trash. When 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, perceiving that only a piece of debris has fallen
into the web. Awaiting the right moment, Portia can ambush effectively
by catching the other spider completely unawares.
invaders do not limit themselves to 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.
spiders also eat males, especially during the mating season. Both adult
and juvenile females on occasion release chemical signals that attract
males--as meals, not mates. The ploy works because a signaling female
may actually have mating in mind. If a female Portia spider kills
and eats a male before mating with him, it indicates that she has rejected
him as a mate but has accepted him as a meal. The species of Portia
spiders persists because sometimes the females wait until after
they mate before they turn on the male and devour him. Mate- or meal-luring
behavior is the norm for these spider-eating spiders. Apparently, the
odds are good enough to keep attracting males, a phenomenon some might
consider to be universal among all species of animals.
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