Why don't we see coconut crab sized insects?

January 30, 2008 2:03 PM

Why don't we ever see insects as large as a coconut crab walking around?

I've read that the thing limiting insect size is the rate of oxygen diffusion through their tissues. Do arthropods have the same limitation? If so, why no coconut crab-sized cockroaches? (Not that I'm hoping for that or anything) If the coconut crab has a different kind of breathing apparatus, how does it differ from an insect's?

posted by fnerg to science & nature (15 comments total)

Check out The Biology of B Movie Monsters for a better explaination of the mechanical limits of an exoskeleton.
posted by kuujjuarapik at 2:08 PM on January 30, 2008

Particularly this passage:
One estimate suggests that mammalian joints are called on to withstand forces as much as 100 times the animal's body weight during normal locomotion. (In humans, the peak forces on the knee during running can be 15 times body weight.) Arthropods, even though they have more legs than mammals, have it even worse; their joints may see forces as great as 3,000 times body weight, 30 times higher than mammals. Because joint contact areas are much lower in arthropods than vertebrates, the difference in stresses must be much greater. Now we come to the heart of the matter. As you scale up an ant, body mass must increase faster than joint surface area--indeed, the stress on the joint should increase in direct proportion to size.
posted by kuujjuarapik at 2:11 PM on January 30, 2008

Shit, come down to sunny Florida if you want big bugs! We have some doozies, like the Huntsman spider which can get as big as a pie pan with all it's creepy leggy things. It jumps, too. Oh, and some of our cockroaches fly!
posted by 45moore45 at 2:17 PM on January 30, 2008

Because joint contact areas are much lower in arthropods than vertebrates, the difference in stresses must be much greater. Now we come to the heart of the matter. As you scale up an ant, body mass must increase faster than joint surface area--indeed, the stress on the joint should increase in direct proportion to size.


But crabs, being crustaceans, are arthropods too, so what goes for the ant would go for that huge land crab.

This is a really good question,and I suspect that the upper limit of Arthropod size is that land crab, so I am wondering if crustaceans respiratory system is similar to insects. i suspect it might not be because crustaceans are mostly water-based. (Fish and mammals are in the same phylum but very different respiratory systems), but hopefully someone who knows this stuff can answer.
posted by xetere at 2:23 PM on January 30, 2008

You're right about the diffusion of oxygen.

Coconut crabs have organs called branchiostegal lungs (these are chambers containing gill like things that require water - and must therefore be replenished), whereas insects have organs called spiracles which deliver air (and therefore oxygen) directly to the tissues by diffusion. Max body size is limited by the amount of O2 that can diffuse through the system. In the past the amount of O2 has been higher, so the O2 could diffuse further enabling larger body sizes to develop.

BTW insects are arthropods, and cockroaches are insects. The size of cockroaches is presumably limited by the spiracle thing.
posted by jonesor at 2:28 PM on January 30, 2008

I'm not really asking why we don't see giant insects, just large insects. The coconut crab is real, live, living, disturbingly large critter. Some orb weavers are big, but not as big as that crab. Even camel spiders don't compare as far as I know. (They're both arthropods, so I guess the breathing question isn't just arthropods, but coconut crabs in particular)

Why is it that only this one weird-assed land crab thing has gotten this big? What keeps other insects and crawly things from getting that big?
posted by fnerg at 2:30 PM on January 30, 2008

Why don't we ever see insects as large as a coconut crab walking around?

They're extinct.
posted by Cool Papa Bell at 2:31 PM on January 30, 2008 [1 favorite]

I think that insects, and arthropods in general have embraced the idea that they are better off evolutionarily in a numbers over size model. This also improves their natural genotypic variance, to avoid a natural selection disaster. Obvious exceptions are usually niche fillers.

I for one, would really enjoy having an arthropod as a pet for 60-70 years!
Thanks for letting me know about the coconut crab.
posted by schyler523 at 2:53 PM on January 30, 2008

If the coconut crab has a different kind of breathing apparatus, how does it differ from an insect's?

Crabs can apparently pull more oxygen per minute out of the water than they can the air, so I think you could attribute the attainment of this size to some advantage due to size they get in the water, which does not incur the penalty there it does in air. According to an article assessing aerobic and anaerobic metabolism in crabs from two different habitats

Individuals of P. crassipes had a decreased metabolic rate during air exposure. Aerial Vo2 was 35% lower than the rate during submersion, but whole body lactate content was the same. Submerged crabs of both species maintained vigorous activity longer than those exposed to air, suggesting that depressed metabolism can influence activity.

posted by jamjam at 2:59 PM on January 30, 2008

I was watching some special recently on the History Channel about the pre-dinosaur life on earth. The particular segment I saw was about these giant insects: 11' long millipedes, 3' long dragonflies, etc. It said the primary reasons these things got so big was the higher concentration of Oxygen in the air, and the lack of other large predators. Many areas were swampy, yielding greater oxygen in the air.
posted by blastrid at 8:20 PM on January 30, 2008

This and this may help. This is more technical and you'd need a PNAS subscription to access it.

I've read that the thing limiting insect size is the rate of oxygen diffusion through their tissues. Do arthropods have the same limitation? If so, why no coconut crab-sized cockroaches? (Not that I'm hoping for that or anything)

All insects are arthropods. Not all arthropods are insects. The coconut crab is an arthropod.

Crabs can apparently pull more oxygen per minute out of the water than they can the air

Wikipedia says the coconut crab drowns in water.

If the coconut crab has a different kind of breathing apparatus, how does it differ from an insect's?

The coconut crab has a brachiostegal lung. The lung exchanges gases with the atmosphere and transfers the oxygen to the hemolymph (blood), which then delivers oxygen to the various tissues and organs. In contrast, insects have tiny tubes (trachea) that open to the outside world (as spiracles) and are very efficient. So efficient an insect can shut them and stop breathing to conserve water or avoid taking in CO2-- don't try that at home. BUT the trade-off is that the larger an insect gets, the more of its body must be dedicated to the trachea. As you scale up, it gets less efficient. Those tubes have to get pretty long to reach all the way into the body. Back when the O2 concentration in the atmosphere was higher, there were some larger insects. It's thought that as oxygen levels went down, insect size became more restrained.

Some orb weavers are big, but not as big as that crab. Even camel spiders don't compare as far as I know.

Depending on how you measure, the biggest spider gets kind of close-- the goliath bird-eating spider (doesn't really eat birds) is reported to have a leg span of about 12 in or so for a large female. Most spiders have book lungs, but some have spiracles. Solfugids (camel spiders, an arachnid but not a spider) don't have book lungs.
posted by Tehanu at 5:29 AM on January 31, 2008

I should mention: the insect trachea deliver oxygen directly, not through the hemolymph.
posted by Tehanu at 5:31 AM on January 31, 2008

Weta ...eep!!
posted by mu~ha~ha~ha~har at 10:08 AM on January 31, 2008

I think we're all kind of missing the point in a way. As there are some examples of large arthropods, it's not just oxygen diffusion or exoskeleton mechanics that limit size. One of the strongest reasons that we don't see many insects the size of the coconut crab is that large insects are probably out-competed in their environment by an animal of that size with a more effective body plan, i.e., an endothermic bird or mammal. On Christmas Island, or wherever the coconut crab lives, the crab probably evolved without mammal competition, thereby preserving it's niche. Other arthropods evolving in more competitive environments "chose" smaller size and greater numbers as a survival strategy.
posted by kuujjuarapik at 7:09 AM on February 1, 2008

The main question was a bit muddled by the insect/arthopod confusion. My answer was specifically about insects, even though I made a point about spider size and book lungs to answer some secondary questions. I don't know if anyone has investigated this question for other arthropod groups, including spiders. The answer I gave is the generally supported hypothesis for insects. I do not generalize that answer across arthropods as a whole, which are an extremely diverse array of animals. The coconut crab is not actually the largest known arthropod, even-- just the largest terrestrial arthropod. The title for largest known arthropod (depending on who you ask) seems to be currently held by the Japanese spider crab. These two species aren't even closely related-- the Japanese spider crab is what's called a "true crab" while the coconut crab isn't. Same order, different families, breathing environments about as different as you could ask for.

As far as insects go, other limitations on body size, such as competition with other groups such as kuujjuarapik mentioned, may also apply in the evolutionary histories of specific taxa; however, all the evidence that's in so far pretty strongly suggests that body size among current-day insects is limited by the way they breathe and the current level of O2 in the atmosphere. So in terms of why dragonflies no longer come in giant sizes like they did during the Carboniferous, that's the best answer anyone has, and it's a pretty damn good one, as far as these kinds of answers go in entomology. It really is strongly supported by what entomologists know at the current time. I answered this because I learned it in Entomology class a few years ago, and I still think it's really neat. Other than the arthropod/insect confusion, this is a textbook Entomology question. Why a specific dragonfly species is not any larger is a different question altogether and would of course be due to a combination of general size limitation for insects and then also the specific evolutionary pressures and history of that species.
posted by Tehanu at 6:00 PM on February 2, 2008