Upper Size Limits for Insects?

Since I’m at work and really should be working, I haven’t read the entire thread, nor have I looked at the links. Please forgive me if this has been discussed.

Yes, the chitin’s strength grows more slowly than mass. But if a Really Big Bug were to exist, wouldn’t it have evolved something to compensate for it? Why couldn’t it develop a buttressed chitin, for example? Something that has internal supports? The buttresses – a “honeycomb” perhaps? – could be sandwiched between inner and outer layers of the chitin so that the insect could molt.

In high school biology, the question was asked: why can’t humans be the size of elephants? The answer was that we would overheat. I didn’t mention it at the time, but I was thinking: Well if humans were the size of elephants, eouldn’t we have evolved a cooling system to support us at that size? Why are we the size we are? If we were half our size, would we argue we couldn’t be this size because we’d overheat? Obviously we’ve adapted.

I think an insect could be the size of a building if it evolved the appropriate support structure. I think we don’t have Amazing Colossal Insects because they have not needed to evolve to that size, and that there are more survival advantages to being small and numerous.

Right. Off to my meeting…

In the words of Foghorn Leghorn “It’s a joke son! A joke!” The “creationist” comment wasn’t serious, it’s simply that a creationist biology text would not be likely to list dinosaurs as examples of the large terrestrial creatures and would possibly stop at elephants. On a more serious note, however, it has been recognized for, at least most of this century, that elephants were dwarfed by the larger dinosaurs.

With respect to mass see

Per the link the heaviest dinosaurs were very close to the weight of the blue whale. This must have required an amazing bit of biological architecture for a terrestrial animal. Over 100 feet long and 220 tons! It would have been an amazing sight just to have see an animal that big move around.

I do hope thay get that mosquito in amber, blood DNA extraction project done soon so I can ride one of these beasts. I currently have to make do with the monkapotomus I’ve got in the back yard, which can be trying, as a creature with the digestive tract of a hippo and the throwing strength of a gorilla makes it’s own “ammo” so to speak.

220 tons… DOH! At 220 tons that Dino needs to diet.
Try 100+ tons.
I will preview my posts.
I will preview my posts
I will preview my posts.

Wow! Thanks everyone! You all have restored my faith.

I was kinda afraid that some of my favorite alien civilizations were gonna be dx’d due to bugs not being able to advance in size enough to possibly gain intelligence at some point. (I know that I didn’t ask that question but it was irrelevant)

Future civilizations of RHFI’s salute you!

Oh! I see…

I just said elephants because they are the largest living land animals.

I love dinosaurs, but I didn’t feel I had enough data to use them for examples.

Anyone ever been to Dinosaur National Monument in Colorado/Utah? They’ve got one museum built right on the site of a dino-excavation. An ancient river bend had collected thousands of bones as well as tons of sediment. After being buried, the whole area petrified, then was geologically uplifted to about a 45[sup]o[/sup] angle. After uncovering the find, they built this enclosed observation area with an attached museum. It’s pretty cool to just sit and watch the paleontologist work.

My meaning for “beyond recognition” in this context is along the lines of "the enlarged version could not be visually identified as a relative of the small version, except in the most general ways (e.g. it has six legs and a hard carapace). As an exercise, let’s engineer an elephant-sized roach and see what we get. Come, DDGor! (Who says I’m less fun?)

  1. First, we have got to work on the leg style. Roach legs point out laterally, then down from the joint. That means that the stress on the upper section and on the joint is increased by leverage–it’s like holding a heavy weight at arm’s length as opposed to holding it close in. Those legs need to be entirely upright (like an elephant’s) so that the strain can be supported by the entire leg.

  2. The actual leg structure is out, too–it needs to be reinforced. Chitin is lighter than bone, but it’s also weaker, so we’re going to need more cross-sectional area of chitin to support our roach. That means that the legs are going to have to be larger than elephant legs, most of it exoskeleton. We’ll have to add some endoskeletal supports as well, but not many; we have to have room for a lot of muscle in there.

  3. I’m not brave enough to greet You-Know-Who in the middle of a giant roach.

  4. The body-leg joints need to be reinforced–the exoskeleton bears the entire weight of the body at these points. The extra chitin will make these joints very bulbous and will restrict the traverse of the legs. Dislocation may be a problem.

  5. The carapace is going to cause problems–even with lightweight chitin, it’s going be heavy. Since it forms an arch, the weight will be displaced to the point where the upper carapace meets the lower (?plastron?). Reinforcement is required to prevent our uberroach from cracking at the seam. The simplest way is to thicken the edge, which will also flatten the arch and lessen the displacement effect. Ditch the wings and wingcases–this sucker is never going to fly in Earth gravity, and this will save a little weight.

  6. We just added considerable extra weight with that buttress–better reinforce the legs a bit. They’re probably thicker than elephant legs, now, and the “knee” joint is fused, or nearly. I can’t think of any way to externally reinforce it enough and still leave it any real flexibility. Better expand the joints at the body some more, too.

  7. Now we move inside the body (brief pause to let the gag-reflex die down). The internal organs are all mushed together on the bottom of the carapace, pulled down off the “walls” and “ceiling” by their own weight. We’ll need an infrastructure of chitinous crossmembers to support them, so they’ll function properly. <Looking at the spiracles> Those are big enough for some pretty nasty parasites to crawl into, and they lead straight into the tissues–better put screens on them <adds fragile chitinous screens over the inch-wide holes>. Should we buff up the legs some more to compensate for the skeletal weight?

  8. Up in the head, we’re adding some more internal supports to keep the glorified ganglion that our creation is using for a brain from falling to the floor. We’ll have to reinforce the join between the head and thorax, as well. It won’t be able to turn its head, but it least the head won’t fall off. We’ll shorten the mandibles and reinforce the joints there as well–otherwise they’d break right off the first time the critter bumps 'em into something hard. If we scale up the antennae fully, their weight will pull them out of the skull. They’re going to get integrated into the skull now as strips along the top with sensor filaments.

OK, that’s the Mark I (we’re doing a male to avoid the problems of those ovipositors). What does it look like? It stands fully upright on foot-thick, inflexible legs (which restrict it to moving in tiny little steps). Its body is flat, rather than tapering toward the edge, with huge, bulbous joints along the bottom where the legs are attached. It has no wings, or wingcases, so the thorax-abdomen join is clearly visible (and heavily armored)–its back doesn’t have the arcing patterns typical of small roaches (the pattern is a characteric of the wings). It has a row of small, screened “window” in its sides, through which it breathes (the problems with which we’re not going into here). It’s also disproportionately thick compared to its tiny kin, to allow room for the infrastructure. At one end, it tapers slightly into a massive head (lacking the trademark antennae) with short mandibles. The head can’t be turned or moved. We’ll have to make sure it has a good supply of food and water about 6 feet off the ground, since it can’t raise or lower itself in any way to reach anything else.

In short, the most roach-like giant roach I can imagine…doesn’t look much like a roach. It certainly doesn’t move like a roach. It’s unlikely that it could feed itself, or actively defend itself from most attackers (even assuming that there was nothing wrong with its metabolism). It’s not a viable animal.

That’s my take on it, DDG. I admit that I haven’t done the math on the stresses, but I think my approximations are reasonable. If you’d care to suggest any improvements (aside from stomping the sucker with a really big boot :D), feel free.

*Originally posted by Balance *

Oh Yeah? Here’s one writer that thinks it’s plenty viable!

Kakfa’s Metamorphosis

"What has happened to me? he thought. It was no dream…

“Gregor’s eyes turned next to the window—one could hear rain drops beating on the tin of the windowsill’s outer edge and the dull weather made him quite melancholy. What about sleeping a little longer and forgetting all this nonsense, he thought, but it could not be done, for he was accustomed to sleep on his right side and in his present condition he could not turn himself over. However violently he tried to hurl himself on his right side he always swung back to the supine position. He tried it at least a hundred times, shutting his eyes to keep from seeing his wriggly legs, and only desisted when he began to feel in his side a faint dull ache he had never experienced before.”

It’s too late for me to be logically reading through every argument stated in this thread - though I did skim for key words, if it helps.
My answer to the original question is two words: “Goliath beetle”. Don’t know if it’s the biggest, but sounds like it might be. Do a web search if it interests you.

Insects need to molt to grow don’t they? The mature exoskeleton is a hard, dead shell. Does this create any additional barriers to bigness in bugs?

We were recently discussing the giant dragonflies in a evolutionary paleobio class. One of the answers we got (along with heightened atmospheric oxygen content) was that the cylindrical body shape better facilitated internal oxygen distribution than the roughly spherical shape of other insects.

As for the structural support issue - once you start adding endoskeletal struts of chitin, you don’t have an insect any more. If any critter using this method did evolve, it’s not part of the fossil record that I’m aware of (but I’ve done most of my stuff with small mammals and salamders, so don’t take that as an authoritative comment).


The main limit on how big insects can be is their inefficient spiracle method of obtaining air. The largest insects are from tropical areas where the higher average temperature allows air to diffuse more readily.

But let’s say that somehow an insect evolved the equivalent of a vertebrate’s respiratory and circulatory system. How big could it be then, given just the limits of it’s exoskeleton? At the very least, behold the coconut crab. This creature is about the size of a housecat, and is strong enough to climb palm trees, hence it’s name. The only reason it’s not considered the world’s biggest terrestrial arthropod is that it breathes will gills; it has to return to the ocean when it’s oxygen runs low.

How much bigger could a lung-equipped coconut crab be? Look at a giant tortoise. That has a pretty big, heavy “exoskeleton”. Or for that matter, the dinosaur anklyosaurus. Size of an ox, covered with a heavy carapace of boney armor. I think you could probably get a beetlelike creature that big.

So why don’t we have giant insects today like in the past? Probably, competition and predation from vertebrates. Back in the “giant dragonfly” days, there was probably only amphibians to worry about. But once vertebrate predators evolved (especially flying ones like birds and bats), giant insects were outcompeted.

And why don’t we have humans as big as elephants? Probably, our vertical spines. Spines were originally meant to bear loads horizontally. In humans, this has been juryrigged (and not all that well either) into a vertical load bearing column. Our internal organs, which originally hung down below the spine, now are slung from the vertical spine and ribcage, which is why potbellies are common and washboard stomachs rare.

So even at the modest height of one and a half to two meters, you have hernias and dislocated vertebrae. Beyond that the engineering difficulties are just too great.

I read this and nearly fell off the chair laughing. The way the prose is structured, I pictured Bob Vila saying this on an episode of This Old Roach.


Six-foot bugs?

[Jaws deadpan]
We’re gonna need some bigger boots.
[/Jaws deadpan]

Anyway, I hope you are all enjoying this: Big bugs are one of the few things that send chills cascading through my neural stalk. Beetles the size of housecats? Only if they’re made by VW, thanks.

:D()()() - The happy worm.

Six-foot bugs? Double that and let’s talk!

The Japanese Spider Crab has a claw span of 12 feet. I have always wanted to see one, but no luck so far–probably because I don’t hang out at the bottom of the Pacific Ocean enough. More information on this animal, and on the proto-lungs developed by land crabs, at this link:


Obviously crabs are not insects, but they scuttle and have an exoskeleton. So every time I eat lobster I think, “mmm, ocean cockroach”.

No, seriously, if land crabs managed it, and they can get pretty big, why couldn’t insects develop lung-like cavities and increase their potential size? Is it because water affords faster movement for an animal regardless of size, whereas land does not? Would large insects be easy prey as they waddled around?

Oooo, Balance, I think you should add “Designer of the Fabulous UberRoach” to your sig. Definitely. :smiley:

How nice to know that theoretically at least, it’s possible. Hmmm, a cockroach-shaped tank with legs instead of treads. Now we need to figure out what to feed it. Dog biscuits, I suppose.

Nancy, are you still with us?.. :smiley:

And thanks, Lazarus, that’s exactly the reference I was trying to pin down–Bob Vila. Nailed it in one. :wink:

Re Dinosaur National Monument: Absolutely. Anybody within the sound of my voice, if you have a chance, go see it. It’s worth the schlep through the boondocks of Colorado (or Utah, depending on how you approach it). Er–make sure everybody uses the potty before you get in the car… :smiley:

Abe, there’s no reason that our RFHIs couldn’t have developed lungs and the associated musculature and circulatory, but that’s such a massive and fundamental change that I wouldn’t be willing to call them “insects” anymore. The respiratory system of gilled arthropods is very different from the spiracle system–IIRC, crabs and their kin extract oxygen and distribute it through their circulatory systems. Since the distribution is muscle-assisted, it can be scaled up more effectively. Without muscle-assisted respiration, the RFHIs would have to be flat enough that no part of the internal tissues were more than a couple of feet from a spiracle. That leaves the body of our elephant-sized roach reduced to a thickness of only 4 or 5 feet, including exoskeleton–it would look like a very large, flat disc on tree-trunk legs. Respiratory problems are the first bottleneck.

Another point about the strictly marine arthropods is that they are supported, in part, by their own buoyancy. That greatly reduces the stress on the exoskeleton. Really Freakin’ Huge Crabs [band name alert] have an advantage over our RFHIs, too–an extra pair of legs to help distribute the weight.

Lumpy, we’ve established from the fossil record that arthropods the size of house cats are entirely possible–there are fossils of roaches that size. I’m sure that a creature like the coconut crab could be made significantly larger without changing its fundamental structure; I can’t think of any way to determine just how big you could make it without confining it to water, though. I’ve tried to calculate some of the stresses, and got nowhere–it gets really complex in a hurry. Bear in mind that the support structure of the large vertebrates you mention (tortoises and anklyosaurs) is made of a stronger material than chitin, and that it is in the form of solid (or nearly solid) supports, rather than hollow, tissue-filled pipes. It may also be significant that the bones are protected by the tissue over them, buffering external stresses (such as impacts). Given lungs, though, an ox-sized beetle (with a heavily reinforced exoskeleton) might be possible.

Astro, who are you going to believe: Kafka or some long-winded stranger on a MB who calls himself “Balance”?

Lazarus, that was exactly the image I had in mind when I wrote #8–glad you liked it.

Just for you, DDG. :smiley:

OK, so have we successfully established that: a)insects body systems cannot support large size, and b)if insects body systems were altered to support large size, they wouldn’t be insects anymore???

Just asking because this thread is making me feel all ooogey :eek:

I think we’re pretty much agreed on those points, Nancy (someone can contradict me if they want another marathon post). WRT point B: although they would no longer be insects, they’s still be icky. :wink:

Balance, this is far and away the coolest thing I’ve seen all month.

Threads like this are the true raison d’etre of the SDMB: hugely informative, and hugely entertaining. Hats off to everybody.