# What limits the size of arthropods? **URL:** \ **Category:** Factual Questions\ **Created:** [September 16, 2009, 1:54pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319 "2009-09-16T13:54:25Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![BrainGlutton](https://avatars.discourse-cdn.com/v4/letter/b/82dd89/32.png) [@BrainGlutton](https://boards.straightdope.com/u/BrainGlutton)\ **Post date:** [September 16, 2009, 1:54pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/1 "2009-09-16T13:54:25Z")
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? ---
**Author:** ![Whiteknight](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whiteknight/32/4962_2.png) [@Whiteknight](https://boards.straightdope.com/u/Whiteknight)\ **Post date:** [September 16, 2009, 2:04pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/2 "2009-09-16T14:04:31Z")
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. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [September 16, 2009, 2:38pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/3 "2009-09-16T14:38:29Z")
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. ---
**Author:** ![Mr\_Buttons](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@Mr\_Buttons](https://boards.straightdope.com/u/Mr_Buttons)\ **Post date:** [September 16, 2009, 4:06pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/4 "2009-09-16T16:06:18Z")
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](http://news.nationalgeographic.com/news/2007/11/071121-giant-scorpion.html) to a giant scorpion fossil. > [@](#): > > The size of a large crocodile, the 390-million-year-old sea scorpion was the top predator of its day, slicing up fish and cannibalizing its own kind in coastal swamp waters, fossil experts say. > > Jaekelopterus rhenaniae measured some 8.2 feet (2.5 meters) long, scientists estimate, based on the length of its 18-inch (46-centimeter), spiked claw. ---
**Author:** ![Mr\_Buttons](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@Mr\_Buttons](https://boards.straightdope.com/u/Mr_Buttons)\ **Post date:** [September 16, 2009, 4:11pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/5 "2009-09-16T16:11:57Z")
Thank you google for backing up my fuzzy memory! 🙂 [Giant insects might rule the day if there was more oxygen.](http://www.sciencedaily.com/releases/2006/10/061012093716.htm) > [@](#): > > The delicate lady bug in your garden could be frighteningly large if only there was a greater concentration of oxygen in the air, a new study concludes. The study adds support to the theory that some insects were much larger during the late Paleozoic period because they had a much richer oxygen supply, said the study’s lead author Alexander Kaiser… > > The Paleozoic period, about 300 million years ago, was a time of huge and abundant plant life and rather large insects – dragonflies had two-and-a-half-foot wing spans, for example. The air’s oxygen content was 35% during this period, compared to the 21% we breathe now, Kaiser said. Researchers have speculated that the higher oxygen concentration allowed insects to grow much bigger. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [September 16, 2009, 4:29pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/6 "2009-09-16T16:29:14Z")
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). ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [September 16, 2009, 5:16pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/7 "2009-09-16T17:16:44Z")
> [@dracoi](#): > > 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. ---
**Author:** ![davekhps](https://avatars.discourse-cdn.com/v4/letter/d/a5b964/32.png) [@davekhps](https://boards.straightdope.com/u/davekhps)\ **Post date:** [September 16, 2009, 8:59pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/8 "2009-09-16T20:59:03Z")
> [@LSLGuy](#): > > 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. You may have seen this, [the Japanese Spider Crab](http://en.wikipedia.org/wiki/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 :-). ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [September 16, 2009, 9:14pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/9 "2009-09-16T21:14:56Z")
> [@davekhps](#): > > You may have seen this, [the Japanese Spider Crab](http://en.wikipedia.org/wiki/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. Well, those things live 300 meters under water so chances are slim you’ll come face to face with one. The [coconut crab](http://www.naturenet.net/blogs/media/blogs/eating/coconut_crab.jpg), on the other hand, lives on land. They will definitely give you nightmares. BTW…they are the largest land dwelling arthropod there is. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [September 16, 2009, 9:23pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/10 "2009-09-16T21:23:56Z")
[Coconut crab](http://en.wikipedia.org/wiki/Coconut_crab). That’s the one I was trying to remember. Apparently they are sold as pets. ---
**Author:** ![BrainGlutton](https://avatars.discourse-cdn.com/v4/letter/b/82dd89/32.png) [@BrainGlutton](https://boards.straightdope.com/u/BrainGlutton)\ **Post date:** [September 16, 2009, 11:33pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/11 "2009-09-16T23:33:33Z")
> [@Sunspace](#): > > [Coconut crab](http://en.wikipedia.org/wiki/Coconut_crab). That’s the one I was trying to remember. Apparently they are sold as pets. Hmmm . . . Can they be attack-trained? ---
**Author:** ![Superfluous\_Parentheses](https://avatars.discourse-cdn.com/v4/letter/s/8edcca/32.png) [@Superfluous\_Parentheses](https://boards.straightdope.com/u/Superfluous_Parentheses)\ **Post date:** [September 16, 2009, 11:43pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/12 "2009-09-16T23:43:59Z")
> [@Sunspace](#): > > [Coconut crab](http://en.wikipedia.org/wiki/Coconut_crab). That’s the one I was trying to remember. Apparently they are sold as pets. [here’s one for scale](http://www.breakitdownblog.com/wp-content/uploads/2007/10/coconut_crab.jpg). Sleep tight 😃 ETA: I see this picture was already posted. Oh well. ---
**Author:** ![DarrenS](https://avatars.discourse-cdn.com/v4/letter/d/977dab/32.png) [@DarrenS](https://boards.straightdope.com/u/DarrenS)\ **Post date:** [September 18, 2009, 2:50pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/13 "2009-09-18T14:50:41Z")
Previous threads: > **[Upper Size Limits for Insects?](https://boards.straightdope.com/sdmb/showthread.php?t=60812)** > > Is there an upper size limit for a creature with an exoskeleton vs an endoskeleton? If so, why? note: I got thinking about this after recalling the movie “Them” which scared the hell out of me as a young child… [http://boards.straightdope.com/sdmb/showthread.php?threadid=24669](http://boards.straightdope.com/sdmb/showthread.php?threadid=24669) ---
**Author:** ![Bosda\_Di\_Chi\_of\_Tricor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bosda_di_chi_of_tricor/32/16845_2.png) [@Bosda\_Di\_Chi\_of\_Tricor](https://boards.straightdope.com/u/Bosda_Di_Chi_of_Tricor)\ **Post date:** [September 18, 2009, 4:04pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/14 "2009-09-18T16:04:19Z")
> [@davekhps](#): > > You may have seen this, [the Japanese Spider Crab](http://en.wikipedia.org/wiki/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 :-). > [@Whack-a-Mole](#): > > Well, those things live 300 meters under water so chances are slim you’ll come face to face with one. > > The [coconut crab](http://www.naturenet.net/blogs/media/blogs/eating/coconut_crab.jpg), on the other hand, lives on land. They will definitely give you nightmares. > > BTW…they are the largest land dwelling arthropod there is. L. Sprague De Camp used these suckers in a horror fantasy story. Reanimated, undead versions, risen from a museum display case. ---
**Author:** ![BaconAndEggs](https://avatars.discourse-cdn.com/v4/letter/b/f4b2a3/32.png) [@BaconAndEggs](https://boards.straightdope.com/u/BaconAndEggs)\ **Post date:** [September 19, 2009, 9:00pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/15 "2009-09-19T21:00:16Z")
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? ---
**Author:** ![Superhal](https://avatars.discourse-cdn.com/v4/letter/s/b4bc9f/32.png) [@Superhal](https://boards.straightdope.com/u/Superhal)\ **Post date:** [September 19, 2009, 10:03pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/16 "2009-09-19T22:03:41Z")
> [@Whiteknight](#): > > 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 remember reading this too. After a certain size, living things would be crushed by their own weight. ---
**Author:** ![snailboy](https://avatars.discourse-cdn.com/v4/letter/s/97f17d/32.png) [@snailboy](https://boards.straightdope.com/u/snailboy)\ **Post date:** [September 19, 2009, 11:08pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/17 "2009-09-19T23:08:08Z")
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. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [September 19, 2009, 11:55pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/18 "2009-09-19T23:55:30Z")
> [@BaconAndEggs](#): > > 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? 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](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._ ---
**Author:** ![lissener](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lissener/32/6139_2.png) [@lissener](https://boards.straightdope.com/u/lissener)\ **Post date:** [September 19, 2009, 11:55pm UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/19 "2009-09-19T23:55:43Z")
> [@BaconAndEggs](#): > > 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? 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. ---
**Author:** ![FoieGrasIsEvil](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/foiegrasisevil/32/16638_2.png) [@FoieGrasIsEvil](https://boards.straightdope.com/u/FoieGrasIsEvil)\ **Post date:** [September 20, 2009, 12:37am UTC](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319/20 "2009-09-20T00:37:55Z")
I would like to eat a coconut crab, preferably with a mallet and tons of clarified butter. [Next page](https://boards.straightdope.com/t/what-limits-the-size-of-arthropods/510319.md?page=2)