Outside of an SF movie, you never see an insect or spider or scorpion larger than a rat, nor a crab or lobster very much larger than a cat. I have never even seen a giant-arthropod fossil in a museum. Whereas vertebrates can get as big as whales. Why is that? Did arthropods simply never evolve into those niches successfully, or is their something about their exoskeletal design that just doesn’t scale up well under the cube-square law?
I’m not an expert, but I seem to recall that it has something to do with gas transmission through the exoskeleton and the cube-square law.
I’ve personally seen crab shells with bodies the diameter of large pizza pans and legs ~6 feet long each. This was in a museum & I’m not sure if it was a current species or something from ancient times.
ISTR a Discovery or something show which included mention of a prehistoric lobster-like thing roughly 6 feet long.
Neitehr are as big as a whale, but certainly bigger than a cat.
If I remember right, insect used to be a LOT bigger in prehistoric times. I think it was because oxygen levels used to be a lot higher. IIRC, insects have pretty inefficient respiratory systems, so their size is pretty much limited because of that. Here’s a linky to a giant scorpion fossil.
Thank you google for backing up my fuzzy memory! 
Oxygen is definitely the key limiting factor for these guys.
However, an exoskeleton is fairly inefficient compared to an endoskeleton. First, you have to shed it if you want to get bigger. As a small animal, you can kind of get by in the day or two it takes another one to harden, but a land animal the size of a horse or elephant would crush itself under its own weight when it tried to molt. Second, when you scale up the sizes, an exoskeleton becomes weaker per unit of weight than endoskeleton. (Like a modern skyscraper, you can scale up internal column supports quite easily; a wall-supported structure like a pyramid or stone tower quickly becomes little else but support).
On a similar tack, note that the really big arthropods (with a few exceptions of course) are primarily aquatic. A big crab that just molted won’t be crushed if it’s underwater, where it’s not far from being neutrally buoyant. For respiration, underwater arthropods can use gills and a circulatory system, which cuts around the oxygen/size problem.
You may have seen this, the Japanese Spider Crab.
I was terrified just looking at it in a museum. I shudder to think how I’d act seeing one alive, in person.
Probably good eatin’, though. Might need the entire tin of Old Bay, however :-).
Well, those things live 300 meters under water so chances are slim you’ll come face to face with one.
The coconut crab, on the other hand, lives on land. They will definitely give you nightmares.
BTW…they are the largest land dwelling arthropod there is.
Coconut crab. That’s the one I was trying to remember. Apparently they are sold as pets.
Hmmm . . . Can they be attack-trained?
here’s one for scale. Sleep tight 
ETA: I see this picture was already posted. Oh well.
Previous threads:
http://boards.straightdope.com/sdmb/showthread.php?threadid=24669
L. Sprague De Camp used these suckers in a horror fantasy story.
Reanimated, undead versions, risen from a museum display case.
So are humans and animals going to get smaller because our oxygen supply is getting lower? We had 35% 300 million years ago and 21% now. Or is it a really, really slow decline?
I remember reading this too. After a certain size, living things would be crushed by their own weight.
Their size is also limited by how their joints work. They use muscles to contract their joints, but hydrostatic pressure (blood pressure) to extend them. Really large insects wouldn’t be able to move their legs.
As an aside, this is why arthropods tend to curl their legs when they die. They lose blood pressure and the joints contract.
Edit: And yes, blood is actually called hemolymph in insects.
No, we have lungs. Arthropods don’t have true lungs, they are limited (on land) by the Square-cube law and the O2 content. wiki *“Arthropods have a wide variety of respiratory systems. Small species often do not have any, since their high ratio of surface area to volume enables simple diffusion through the body surface to supply enough oxygen. Crustacea usually have gills that are modified appendages. Many arachnids have book lungs. Tracheae, systems of branching tunnels that run from the openings in the body walls, deliver oxygen directly to individual cells in many insects, myriapods and arachnids”
*
http://discovermagazine.com/2007/nov/attack-of-the-giant-historical-insects
Bugs used to be bigger, in part due to higher concentrations of oxygen. Unlike vertebrates, insects absorb oxygen directly through networks of air tubes (yes, it’s a series of tubes). To find out how this mode of oxygen transport affected the size of insects, researchers at Argonne National Laboratory X-rayed beetles large and small. They found that the bigger the bug, the greater the proportion of the body dedicated to air tubes. Getting oxygen to the beetles’ legs was particularly constraining on size, however. In order to absorb enough to survive in today’s atmosphere, a beetle rivaling the size of its prehistoric ancestors would need tubes bigger than its leg openings could hold.
Getting smaller is one way to adapt, the way the arthropods took. Evolving a different and more efficient method of taking oxygen from the air is another. No, we won’t be getting smaller. Not due to this at any rate. See: elephants and whales, frexample.
I would like to eat a coconut crab, preferably with a mallet and tons of clarified butter.