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 web!
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 hunting.
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.
So, 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: