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, right?

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 ground.

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 be overcome.

Hey!! You seem like a budding arachnologist. Perhaps you should check out the American Arachnological Society's home page:
americanarachnology!

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 quickly."

"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 web.

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."