If spiders are such effective predators, why aren't there huge ones competing at the top of the food chain?

Answer 1

Several answers to your question, actually:
1) Because they are so effective in their ecological niche, there is no evolutionary pressure to grow any larger -- they do just fine at the size they are currently at. If growing bigger conferred an evolutionary advantage, they would gradually do so. And bigger is not necessarily better; since they are not at the top of the food chain, the bigger they get, the more easily they may be spotted by potential predators and eaten.

2) Spiders are physically incapable of growing much bigger than they currently are, because of how they're put together -- since their bodies are supported by an exoskeleton, all their soft tissues are sort of slopping around inside a hard shell, and if they got any bigger, these tissues would only sag under their own weight and become useless. Also, spiders do not possess a circulatory system like larger animals -- they breathe through "book lungs," which are kind of like gills on the underside of their abdomens and deliver oxygen directly to some of their tissues, while the rest are served by spiracles, little air tubes that lead from various points on their legs through the soft tissues. Grown any larger, and they simply couldn't have enough transport mechanisms to oxygenate their entire bodies efficiently, and they'd be prone to suffocation. You'll notice that the larger a spider gets, the more it tends to become spindly -- those big Mexican cave spiders may have a three-foot leg span, but their bodies are still only the size of a small rat and their legs are just long spindly things.

3) As another poster mentioned, what would they eat? Larger predators require greater energy input to maintain themselves, so unless you're talking about a world full of giant bees and butterflies, it would mean they would evolve to compete in a new niche, hunting small rodents or fish (though one spider actually does catch small fish), competing against snakes, birds, and carnivorous mammals for food. There's a lot more to eat down where they are in the middle of the food chain than there is at the top.

Answer 2

Maybe there used to be ?? Unfortunately they aren't that smart. I've never seen one build a house or design a rifle. They seem to be happy just living along with us. Being at the top of the food chain may be the worst spot to be in. I mean just look at us. We have pretty much neutralized all of the threats to our survival by sheer population, but we can't save ourselves from ourselves. We are our own worst enemy.

Answer 3

They're out there. I'm sure I saw a really bad sci-fi movie a while ago with a super huge spider on some random *** island.

Answer 4

spiders have an exoskeleton which can only support a certaain amount of weight. if they were bigger, they would be slower and more rigid, therefore less effective as predators, and yet would require a larger ratio of food to bodyweight to carry their heavy exoskeleton. like lobsters and crabs.

Answer 5

a spiders body is not constructed to maintain that much weight. they have what is called an exoskeleton, this is good for protection but not so good for supporting a huge body. their diet also restricts their size.

Answer 6

Because they don't grow large and can't breed like humans do to become bigger. (If you look at humans, we've grown taller in the last five hundred years because of globalization).

Answer 7

And what would such big spiders live on ?????????

Answer 8

Because arachnaphobes such as myself will kill them all! :)

Answer 9

Just wait.

Answer 10

There are, they live in my house, oh no, here comes one now, ahhhhhhhhhhh .........