Has anyone tried to quantify this? They say to let the spiders live because they’ll keep the insect population down around your home, but I often see webs that seem to me to be in useless locations for the catching of food. How do I know they’re not just lazing around all day, spinning strands of web for me to run into?
Simple - if they weren’t successful they would starve. They exist, therefore they nourish on something.
Periodically clean the cobwebs. If the spiders are healthy and eating bugs, they’ll make new ones, and meanwhile you’ll be eliminating the abandoned ones. Also, don’t worry about killing an occasional spider. If there’s food for them, more will appear!
Bottom line: when cleaning, go ahead and clean the webs, as a normal part of your routine. But no need to be crazy about it, searching out and destroying every cobweb on a daily basis.
Moderation in most things!
Look underneath the web. You’ll often find a pile of dead insects. I wouldn’t be surprised if spiders averaged one kill per day in a good location, but I don’t have time to look for a cite right now.
Of course, there are also spiders that hunt without webs - tarantulas, wolf spiders, jumping spiders, etc. For these spiders, webs are a way to make their home more cozy and they’re not expecting to catch anything.
On average, every mating pair of spiders is successful at producing two offspring who survive to the age of reproduction. It does not follow that all (or even most) spiders are good at catching insects. The continued existence of many species, spiders included, depends on the production of hundreds or thousands of offspring from a single mating, with the expectation that nearly all of them will die before reproducing.
So if a female spider creates an egg sac with a 1000 eggs in it, it’s entirely possible that most of them will spin webs in stupid places, e.g. in some dark corner of my basement that rarely sees insects. The existence of a spider, or a web, is not by itself an indicator that that spider is good at catching insects.
OTOH, a particular spider may have picked a good spot to live, and may have chosen to build (or rebuild) a web just before you stopped by; that would explain the existence of a web that doesn’t have a pile of desiccated insect corpses below it.
Damned lazy eight-legged layabouts, sitting around all day waiting for food to just fall into their webs! :mad:
They are clearly not part of the 47% identified by Romney.
Spiders are pretty close to the bottom of the food web - lots of other things eat THEM, hence the high amount of offspring.
Makes me wonder what spiders do all day. Ants seem to always be searching around, same with bees and beetles. I don’t often see spiders running around. Are they actually mostly hanging around the ole web waiting for dinner to fly to them? Or are they doing other stuff?
IIRC, spiders as a species have been around for over 130 million years, so pretty obviously they are successful in catching and eating SOMETHING. 
They outlived the dinosaurs after all.
I have too many spiders in my house, and there doesn’t seem to be a lot of insects to feed on, yet my spiders prosper. My theory is that when they don’t have bugs to eat, they make do with eating each other. (WAG)
Plotting.
Specifically, plotting how best to jump into your mouth while you’re sleeping.
There are many different kinds of spiders. Some make webs and wait for insects to fly into them. But they are constantly monitoring the web for signs of vibration. If something flies into it, they have to rush out as fast as they can, bite it, and wrap it in silk before it can get out or break the web.
Some make burrows in the ground which they line with silk and equip with a trap door. If something walks by and stumbles across their tripline, they rush out and grab it.
Some spiders like wolf spiders and jumping spiders don’t catch insects in webs, but instead chase them down.
Bolas spiders make a sticky ball of silk on a line which they throw at insects to snare them, and then reel them in. One of my colleagues named a new species Mastophora dizzydeani (number 8 on this list) after the baseball pitcher, because these spiders make their living by throwing a ball fast and accurately.
They’re pretty good sometimes: Lunchtime!
phreesh:
They spy. Duh!