Science Questions

Why are there no whale-sized insects?

Wed, 31st Jul 2013

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Jim Schaefer asked:

Really enjoy your programme.


Why don't we see insects as large as whales or cows ? Why is the ratio of largest to smallest so much bigger for mammals (blue whale vs kitti's hog nosed bat) than it is for insects (goliath beetle vs fairy fly) ?




Jim Schaefer

Ann Arbor, Michigan, USA



Ian Burgess - Okay, right. Well, an insect depends for its structural integrity on having its external skeleton. Of course, we have an internal skeleton which can be relatively robust and therefore, relatively light. If you had a suit of armour on the outside, in order to get the same sort of robustness, it needs to be relatively thick and is actually quite heavy. If you've ever tried a suit of armour on, or even something very similar, you’ll know how difficult it was fighting 400 years ago.

Now, for an insect to carry that around, it needs a huge amount of muscle and a lot of energy expenditure. So, it requires particular conditions in order to grow bigger and the biggest insects that we have at the moment are things like goliath beetles which just about will cover the palm of your hand or the biggest ones will. Most of them are a bit smaller than that. But back in the Carboniferous which is what, 400 million years ago, the insects had more oxygen, although oxygen is an interesting thing as regards to insects because it can be toxic if they have too much. So, they were able to grow bigger because the conditions enabled them to actually have the energy that they could use and they could grow bigger and support in the heavier atmosphere.

Dave - So, isn't this because, they don’t have lungs so they can't get the oxygen to the – if you had something in the size of cow, the middle of an insect just wouldn’t be able to breathe.

Ian - No, I don’t think that's really the issue. It’s the physical structure of supporting all of that external skeleton. So, the things that did grow big then were relatively lightly constructed – things like dragonflies and if you look at a dragonfly, it’s sort of spindly. It’s like one of the wonderful designs of second World War aircraft. It looks as though it was only just held together. The other insects that grew moderately big, you got some big cockroaches and there was another insect, a big spider at that time as big as a dinner plate, but that's about as far as they could go. Now, if you wanted to get much bigger than that, then you'd have to find some way of supporting then other than necessarily the – just muscles and a big thick armour. So, if we had an insect the size of a whale, it would probably have about a meter thickness of external skeleton just to keep it together.

Chris - That's quite a skeleton isn’t it?

Ian - So, unless it manages to find some way of – because of course we’re dealing with biological materials rather than sort of a sheet metal, so in order to have it that big and support itself, so it has to find some way of getting bouyancy as well. The only reason why it grows as enormous as it does is because it has the water to support it. If you put a whale on land, even a little tiny whale, it suffocates because it can't even support its own breathing.


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The structures that allow insects to walk and fly only allow them to grow to a certain size. It has to do with ratios. Think of the average bicept (a muscle used by the arm to lift). Th lifting capability of such a muscle is a function of the cross section of the muscle, i.e. how thick it is. If the strength of the muscle increases then it requires an increase in the cross sectional area to increase by a fctor of two. However if the shape of the muscle remains the same and the cross section increases by a factor of two then the volume increases by a factor of eight. Eventually the insect is unable to support his weigth using the same structural material and then that has to change to make thing larger. A similar argument applies to flight. An animal can only be so large and still be able to fly for similar reasons. Pmb, Thu, 18th Jul 2013

I believe it also has to do with breathing.  Insects do not have lungs, and there is a limit to the volume of body that insect respiration can oxygenate. Bill S, Fri, 19th Jul 2013

This doesn't explain the cow or whale-sized insects, but it's interesting nonetheless:

Basically it's what Bill said--insects don't have an advanced respiratory and circulatory system like us, so they rely on absorbing oxygen fairly passively from the air.  That means they're limited in size by the concentration of oxygen.  Moreover, a large bird has far more efficient lungs so they can generally hunt much more actively than a similarly sized insect.  Once birds evolved, insects stopped increasing in size, even though oxygen levels increased, indicating small size was an evolutionary advantage (they could move quickly and reproduce in large numbers). jpetruccelli, Fri, 19th Jul 2013

A common breathing organ in insects is the spiracle, and so, many insects grow "longer" in order to be bigger — centipedes, millipedes, caterpillars, beetles, flies, etc. Maybe that's one reason why segmentation is so common among insects. Lmnre, Fri, 19th Jul 2013

There was a time when Earth had big insects.

Don't know how correct it is but oxygen content must have played a role. yor_on, Fri, 19th Jul 2013 RD, Fri, 19th Jul 2013

Perhaps the rigid exoskeleton restricts growth of insects - and shedding the exoskeleton in order to grow further represents a phase where it is susceptible to attack? Perhaps an internal skeleton allows steady growth without these major pauses?

Another factor is the strength of skeleton required to support a creature's weight. For Elephants, the leg bones take a large fraction of the diameter of the leg; presumably the ratio would be even more extreme for Apatosaurus.

However, seawater provides support, so the whales and plesiosaurs grew to quite large sizes.
One could speculate that insects could grow larger in seawater - if the sea and lakes weren't already populated by fishes with an internal skeleton, and with flow-through respiration using gills.

evan_au, Fri, 19th Jul 2013

Also, they have too little brain compared to animals that can grow larger. flr, Sat, 20th Jul 2013

Interesting. Meaning that animals need to control their bodies so they don't become blundering, self-injuring and unable to defend themselves (and extinct) ... and that insects have limitation(s) in their nervous systems that evolution cannot overcome? Hmmm. The brain burns a lot of energy, so maybe the respiration, oxygen requirements,brain size, and body size are all related?

On a different line of thought, the mollusca phylum has diverse members, characterized by a shell, external for many species (clams etc), but internalized for many others (octopi, squids, etc). Maybe it's my perception, but the external-shelled animals seem mostly smaller and less intelligent, and the internalized-shell animals seem able to grow larger and more intelligent. Does being more vulnerable (lack of shell, maybe large size, etc) force an animal to compensate — if not by sheer number of offspring, then by intelligence (or both) — in order to survive? This might be the start of a new thread. Lmnre, Sat, 20th Jul 2013

That picture was scary RD :) He wouldn't just be in my boot, I suspect he rather would carry it on his, eh, feet?

Brains are highly convoluted are they not? In the way they are topologically I mean. Somewhat similar to how a lung can be stretched, folded out. A nervous system though is more like a analogue counterpart to some electrical description, able to produce logical reactions to the environment, as when one burn yourself.

Could one imagine a highly complicated array of a nervous-system to act similar to a brain, and if not, what will be missing? Isn't there animals that seem to have 'brains' in several locations too? Think I saw something about it, somewhere.

Where does a nervous system become a brain, the question might be :) yor_on, Sat, 20th Jul 2013

Sure, a distributed nervous system. I'm thinking that, spread out as they are, they lack sufficient connectivity and hierarchy ... and maybe including consciousness. Lmnre, Sun, 21st Jul 2013

Octopuses have quite distributed nervous systems, but also show high intelligence and, potentially, consciousness (it's hard to know). Not sure if they've done the mirror test, but they can learn complex novel tasks by observation... dlorde, Mon, 22nd Jul 2013

Because if they were that big we'd call them whales or cows or dragons. percepts, Mon, 22nd Jul 2013

That question causes me to ask: Why are there on insect sized whales? Ethos_, Tue, 23rd Jul 2013

I wonder about insects and consciousness. The other day I was eating lunch at a picnic table and there was a large dead bug on it. Another insect went up to it, circled around it, paused, and then (I am not making this up) shoved it off the table. Then I watched the bug crawl down the picnic table to where the dead insect was, and haul him away.) How is that not a conscious thought process? And even if you believe it's "instinct" or "programmed" behavior, what test or experimental method would you use to tell the difference between instinctive, automatic behavior and conscious problem solving? cheryl j, Tue, 6th Aug 2013

First you've got to decide precisely what you mean by consciousness in this context. People can mean anything from simple responsiveness to environmental stimuli, through complex adaptive behaviour, to introspective self-awareness. Most people working on concepts of consciousness accept that there is a continuum both of degrees and kinds of consciousness; it isn't a simple thing, you can be conscious to varying extents, and in some ways but not in others.

Personally, I'd be inclined to judge this example in terms of adaptability and creativity in problem solving. I'd do a number of different trials with varying parameters to try to establish how flexible and adaptable this particular behaviour was.

A widely quoted (and debated) example of this kind of thing involves a particular species of sphex wasp that is said to show such rigid action patterns that it can be tricked into repeating them ad-infinitum (see Sand Wasps & Fixed Action Patterns). But this isn't the whole story - other sphex species don't get locked into this repetitive loop, some only briefly, and there seems to be some variation between individuals of a species too; does this mean they're 'more conscious' than those that do? And, of course, fixed, repetitive behaviour is not necessarily diagnostic of lack of consciousness - ask OCD (obsessive-compulsive disorder) sufferers.

dlorde, Wed, 7th Aug 2013

I'm pretty sure the issue here is the effect of heat on diffusion. Insects are cold blooded and don't self-regulate temperature (at least not much). Insects that live in hotter regions can grow larger because their bodies reach higher temperatures and that lets oxygen and other nutrients move through the body more quickly. Similarly insects living in hotter times could also get larger. chiralSPO, Sun, 11th Aug 2013

I've also heard of experiments that demonstrate instinctive, automatic behavior, where the animal seems caught in a loop, like a computer program. But when animals respond to novel situations in a sophisticated way, it becomes trickier to prove it is not instinct. In the past, scientists were accused of anthropomorphizing other species that seemed to be problem solving or displaying emotions. But it seems more unscientific to me to suggest that human behaviors suddenly popped into existence with us and have no roots in other species. It is fascinating, however, when animals or insects demonstrate similar behaviors with very tiny or different brains. cheryl j, Tue, 13th Aug 2013

Yes, but I think it's a general problem for scientists. It is very easy to jump to conclusions about what appears to be obvious and discover you're mistaken after wasting a lot of time and effort based on the mistaken assumption. Anthropomorhising is something we seem to do instinctively, even attributing agency and intent to inanimate objects and natural events. So we must by hyper-vigilant in this respect, and the default approach is not to accept the idea until it has been defined, tested, and verified. Another problem is the difficulty of obtaining a consistent consensus definition for consciousness - how can you test for something you don't have a solid definition for?

In this particular field, there was also the legacy of the quasi-religious idea that humans were quite different from, and superior to, animals - particularly in having consciousness and other advanced faculties (which was associated with the idea that animals were here for our benefit, and could therefore be exploited without qualms). It took a while to get past that worldview.

Yes; you only have to look at other primates and great apes to see we're on a related branch of the tree of life. The more we study other animals, not only mammals, the more we are surprised by their mental faculties and wonder how we could have ignored and missed them for so long. Even octopuses, invertebrates with a differently organised nervous system, show surprising intelligence (e.g. learning complex tasks simply by watching them done) and a degree of self-awareness.

Of course, something the size of most insects doesn't have much room for a highly sophisticated brain with complex adaptive behaviours, so their behavioural adaptability is limited, but can still surprise. I suppose that's why so many insects adopted the hive organism approach, where complex adaptive behaviours can be generated from the interactions of many relatively simple individuals. dlorde, Tue, 13th Aug 2013

As with many scientific questions, asking the inverse question is often illuminating because it takes us back into the realm of the known.

There are bats and shrews that weigh less than 2 grams, and plenty of insects that weigh more, but there's no suitable ecological niche for a tiny aquatic mammal. The surface/volume ratio of small mammals is a limiting factor: tiny mammals (and hummingbirds) spend nearly all their time eating in order to keep warm enough to stay alive in air: they simply cannot eat enough to stay alive in sea water. 

I suspect the opposite may be important in insect physiology: above a certain size, the heat generated by muscle movement might cook the digestive enzymes. Or it may be that with no effective heat sink (due to large size and exoskeleton) the muscles themselves may not work.

Mobility and escape from predators might be important. Yacht racing is ridiculously expensive because the speed of a boat depends on the square root of its length but the cost depends on the cube of its length, so you have to spend a hell of a lot of money to go a tiny bit faster. I suspect the same is true of terrestrial insects - the bigger you are, the thicker your exoskeleton needs to be (the thickness of a girder or roof beam increases with the square of its unsupported length) so armour weight increases disproportionately to the muscle mass available to move it.

But of course there is no size limit to an aquatic invertebrate. It just happens that the ecological niches for very large ones are occupied by creatures with more than six legs.  alancalverd, Tue, 13th Aug 2013

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