Science Question: Large Arthropods

[Dannyalcatraz]

I know that millions of years ago, Earth's atmosphere had a higher Oxygen content that allowed terrestrial arthropods to grow quite large.

Nowadays, the largest terrestrial arthropod is the Coconut Crab (Birgus latro)- at (supposedly up to) 40lbs and about 3' across, its no lightweight, to be sure.

Coconut crab - Wikipedia, the free encyclopedia

There are other arthropods out there that get quite large, notorious among them the tarantula family of spiders. Yet none of the tarantulas known to science are even close to the Coconut Crab in size. There are stories of spiders big enough to attack dogs and cats from certain areas of the world- creditable enough for some cryptozoologists (legit ones, not the ones who claim to have "Bigfoot" samples) to go looking for them, but as yet, none have been found.

My question is this, though- given the size disparity between the Coconut Crab and the largest known tarantulas, is there some anatomical difference between the crab and the spiders that lets the former have a larger top size?

 

[Atanatotatos]

I think it's a pretty interesting question, especially if there's someone who can answer properly, but actually, isn't it a bit like asking why rhinos aren't as large as elephants?

 

[Achan hiArusa]

The limiting factor in the size of arthropods is their respiratory system (and their exoskeletons). Tarantulas use a delicate set of lungs called book lungs. They are a thin set of 15 folded and attached sheets that deliver air to the circulatory system. The crab's lungs are a lot more complex and somewhat analogous to our own. Thus they are much more able to deliver oxygen than tarantulas. I'm sure the difference in toughness of the exoskeletons play a factor as the stronger crab skeleton is able to support more mass than the softer exoskeleton of a Tarantula.

 

[Umbran]

My question is this, though- given the size disparity between the Coconut Crab and the largest known tarantulas, is there some anatomical difference between the crab and the spiders that lets the former have a larger top size?

Well, given the precedent of to coconut crab, the answer should be that any anatomical differences could be overcome with a bit of evolution. The question is more why they'd do so.

Mammals have a distinct advantage over arthropods in speed and strength at larger sizes. The coconut crab is a scavenger, eating mostly fruit. It doesn't generally tangle with large and lively prey. So, a coconut crab sized spider, competing with mammalian predators in that same size range - mostly felines and canines, probably loses out.

So, as you note, there's stories of big spiders - the suggestion is that there have been big spiders, and that they've gone extinct. Usually, you go extinct due to mishap, or inability to compete.

So - spider have probably been big, and failed in the attempt.

 

[Dannyalcatraz]

Thanks for the responses!

1) Yes, it is a bit like asking about rhinos and elephants. But I already know why elephants are bigger- a combo of becoming too big for most predators to bring down and being able to access food resources that most other critters can't get.

2) I wondered if it was about the book lungs- I knew spiders have them, but I didn't know that crab lungs were more sophisticated. That's probably the key.

3) Most of the big spiders are ambush predators, and their venom helps neutralize the advantages that warm blooded vertibrates have over them. For a "super-spider," a pounce, a bite and a strategic retreat, and Fluffy is dinner.

 

[Goldmoon]

I suspect it has a lot to do with diet. Coconut crabs feed on scraps as perviously stated and most spiders are predators. I would like to say that I live in Guam and I have seen a few rather large ones. Creepy little (relatively speaking) bastards.

 

[Knightfall]

Wow! 0_o

 

[Goldmoon]

This is the one that always gets me.....

 

[Achan hiArusa]

This is the one that always gets me.....

I just want to know how it would taste with butter.

 

[Umbran]

3) Most of the big spiders are ambush predators, and their venom helps neutralize the advantages that warm blooded vertibrates have over them. For a "super-spider," a pounce, a bite and a strategic retreat, and Fluffy is dinner.

Dude, look at that coconut crab, and watch some videos of them in action. It isn't pouncing on anything, even a coconut. Even with the better lungs, it is carrying a lot of weight in skeleton for its size, limiting the speed and mobility.