What a great question! What you have noticed is
that some behaviors are "harder" or take more
energy than other behaviors that appear "easy".
When something appears to take a lot of energy, as
spinning silk does, it makes you wonder why an
animal isn't behaving differently. Surely there
are more energy efficient ways of capturing food,
There are many ways to answer your
question, so hold on to your keyboard! First of
all, if we could measure the amount of energy it
takes a spider to spin a web of silk versus the
amount of energy it takes to dig a hole, you might
be surprised to find that, in fact, they both take
a great deal of energy. If you had a respirometer
- a tool used by physiologists - you could
actually measure the amount of oxygen a spider
consumed. You might actually find that, in fact,
hole digging takes less energy that spinning a
You question is really great, however,
because it brings up one of the most fascinating
observations about the natural word. That is,
there are many, many ways for organisms to "earn a
living" and, over time, animals have evolved
specialties that other animals don't have. As you
have pointed out, some spiders spin webs to catch
their food. In fact, many other spiders do NOT
spin webs. Spiders that belong to the wolf spider
family (Lycosidae) do not spin webs. Neither do
fishing spiders (Pisauridae), jumping spiders
(Salticidae) or crab spiders (Thomisidae)! All of
these spiders are called "hunting spiders" because
they use other ways of capturing prey besides
spinning webs. They may sit on the ground and wait
for prey to walk past them, or sit well disguised
on a leaf and wait for something to fly by. In
other words, some spiders earn a living by
spinning webs, and other spiders earn a living by
So, some spiders don't spin webs.
Why don't they all make a living the same way? One
reason is because if they all hunted in the same
way they would end up competing for the same prey.
Imagine if all spiders dug holes in the ground and
only captured insects that walked into the hole.
Just think about all those flies that weren't
being caught by webs and eaten by web building
spiders! That's a lot of food going uneaten!
Eventually, spiders would figure out that it's
easier to spin a web and capture all of the flies
in the air than it is to compete for all of the
insects that are simply walking on the
But here is perhaps the most
important - and difficult - thing to grasp.
Ultimately, some spiders spin webs and other
spiders hunt without webs simply because that's
what they are built to do. More precisely, their
genes dictate they way in which they capture food.
Over evolutionary time - many thousands of years -
spiders have developed different methods of
capturing prey, and these methods are now encoded
in their genes. Even if they "wanted" to hunt for
prey differently, they couldn't because their
genes don't let them! Sometimes it's possible for
organisms to evolve new ways of capturing food.
Other times, it's just not possible.
the short answers to your question about why
spiders don't simply dig holes to capture prey
are: (1) digging holes may not be so easy, (2)
even if it is easy to dig a hole, spinning a web
may be the best way to avoid competition for
ground-dwelling prey, and (3) evolution may have
resulted in gene-based behavior that simply cannot
Hey!! You seem like a budding
arachnologist. Perhaps you should check out the
American Arachnological Society's home page:
Scientists believe that the first spiders on earth
used their silk for other purposes, like wrapping
their eggs.It is then believed that spiders used
their silk to catch prey over burrows in the
ground and by creating trip-lines on the ground.
By the age of the dinosaurs it is believed that
spiders were already spinning webs to catch prey.
Although we may think of spinning a web as a long
process and a waste of energy, spiders have
obviously had large success in catching their prey
in spun webs. Sometimes putting a lot of effort
into creating a very successful trap is more
rewarding than spending minimal energy on creating
a trap on the ground which is easily destroyed.
Webs above the ground have advantages in that they
are not as easily torn by animals walking on them
or pouring water over them (like the dinosaurs).
Flying insects cannot see most spider webs and are
therefore easily trapped. Even today, it is
easier for humans to destroy spider webs on the
ground than on the ceiling. The first web-spinning
spiders that caught their prey in webs obviously
had more or at least equal success as those who
trapped their prey over burrows otherwise they
would not have evolved the way they have.
I think some spiders hide in the ground and make
webs over their holes to catch the insects. There
are also spider webs that are dense with a
disorganized layer of spider silk and a silk tube
running down to where the spider hides down below.
I've seen a lot of those in the park where I used
to walk my dog.
I think most insects fly,
so spiders would have a hard time catching them
because the insect could fly away. I emailed your
question to my friend who does lots of research on
spiders. You might say she's a 'spider woman.'
She replied:"What cute questions. Casey will be
so surprised to learn that there are many spiders
that make holes in the ground and that orb-weaving
spiders make their webs rather
"Some small orb-weaving spiders
can spin a web in a matter of minutes (as long as
they're using the same frame as their old web).
Big spiders might take almost an hour. I find
even an hour to be amazingly fast given the amount
of engineering that goes into an orb
Yes, spiders that make burrows and
retreats in the ground often do have trapping webs
or lines. Funnel webs have those broad aprons in
front of them. Spiders in burrows often have trip
lines that radiate out from their burrow entrance.
If a potential prey steps on the trip line, the
spider can feel the vibration and will come out of
the burrow to investigate."
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