[Onanist (OP)]

What is the largest an invertebrate could theoretically grow to on our planet? On land, or in water. Is there an upper limit? What determines this?

I thought it might be pressure, but since the largest known invertebrate is the giant squid, and it lives in a high pressure environment, I guess that can't be right.

I basically want to know if giant insects could at some point in the future evolve on our planet, and if so, how big they could get.

Also, if there might be giant insectoid life on other planets.

Because giant insects are cool.



[mod edit - subject changed to make it a clear question - please try to do this in future as it makes it easier for others to grasp the nature of the thread]

[RD]

Being too big and consequently too slow to catch prey sets the upper size limit for many creatures.

[Madidus_Scientia]

Mostly, the way they respire limits them in size. Insects don't have lungs or gills but what is called tracheae, air enters through a small hole somewhere on the insect and travels through a branching network of very tiny tubes that go all over the insects body. Oxygen diffuses in, carbon dioxide out. If an insect becomes too big, then oxygen cannot diffuse far enough into the insects tissues from the tracheae.

Also another factor is one I discussed here: http://www.thenakedscientists.com/forum/index.php?topic=13972.0

[Onanist (OP)]

So in a larger scale, insects would not only not be able to breath, but would not be strong enough to support their own bodyweight either?

Disappointing. No giant insectoid aliens for me.

 []

[Bored chemist]

Is there a theoretical limit to a "worm" that is made by sticking ordinary worms end to end? It would need to have more than one mouth, and a number of other holes (not to put too fine a point on it) but I don't see any limit there. Similarly, for a really thin "worm" you could keep it adequately fed by diffusion, no matter how long it was.

[Kryptid]

So in a larger scale, insects would not only not be able to breath, but would not be strong enough to support their own bodyweight either?

Disappointing. No giant insectoid aliens for me.

 []

Theoretically, giant insectoid aliens could exist on worlds with lower gravity than Earth and an atmosphere with a higher partial pressure of oxygen. Imagine a place like Saturn's moon Titan (which has less gravity and a thicker atmosphere than Earth), but with water instead of hydrocarbon lakes and an oxygen-enriched atmosphere.

[TheHerbaholic]

Giant insects lived on earth at one point. It was on one of them programmes like 'walking with beasts'. I remember a millipeed that was as big as a car being on it. It was when earths atmosphere was alot more oxygenated.

[Jerryade]

I personally do not think insects can still evolve.If evolution is to repeat itself now it will be the turn of man to evolve not the incects or any other class of vertebrate or invertebrate this time

[JimBob]

there are fossils scorpions that are well over 8 feet long (http://www.eurekalert.org/pub_releases/2007-11/yu-3sf112107.php)

there ae tube worms living now that are more than 30 feet long

And in the Cambrian, water dwelling soft bodied creatures of very large width and length - over 9 feet - are known to exist.

[Madidus_Scientia]

I personally do not think insects can still evolve.

Why can't they evolve? New species of insect are discovered every day.

If evolution is to repeat itself now it will be the turn of man to evolve not the incects or any other class of vertebrate or invertebrate this time

Evolution is constant, what do you mean by evolution repeating itself? Why is it man's turn to evolve? Different species don't take evolution in turns.
Evolutionary pressure doesn't exert itself on mankind the same way it used to. We are more likely to change our own environment to make ourselves more comfortable rather than adapt to it. One of the only significant evolutionary pressures I can think of is that in the western culture many of us eat high fat foods and turn into blimps, affecting sexual attraction and therefore will be less likely to reproduce. Ironically the reason we gain and hold weight so well is because we evolved through hundreds of thousands of years to be able to tolerate famines.

[JimBob]

If insects couldn't evolve the the common fruit fly, Drosophila melanogaster, would not be one of the most commonly used animals to study genetics. Its short life-span and its ability to rapidly mutate with very little stimulus makes it an invaluable tool for geneticist and scientist wishing to study the effects of chemicals on organisms.

[Onanist (OP)]

I personally do not think insects can still evolve.

Why can't they evolve? New species of insect are discovered every day.

If evolution is to repeat itself now it will be the turn of man to evolve not the incects or any other class of vertebrate or invertebrate this time

Evolution is constant, what do you mean by evolution repeating itself? Why is it man's turn to evolve? Different species don't take evolution in turns.
Evolutionary pressure doesn't exert itself on mankind the same way it used to. We are more likely to change our own environment to make ourselves more comfortable rather than adapt to it. One of the only significant evolutionary pressures I can think of is that in the western culture many of us eat high fat foods and turn into blimps, affecting sexual attraction and therefore will be less likely to reproduce. Ironically the reason we gain and hold weight so well is because we evolved through hundreds of thousands of years to be able to tolerate famines.

I read an article on the way the human digestive tract is adapting to a high carbohydrate diet made up predominently of grain.

Surely this is an example of our continued evolution?

[LeeE]

There seems to be a lot of misunderstanding about the process of evolution.  It isn't a process where existing organisms gradually adapt in response to changing conditions - this would imply that with no change in conditions there would be no evolution.

Evolution is the process where a non-disadvantaging random mutation of an existing organism creates a new organism, and this will continue to occur regardless of conditions.  This is how we end up with trivial differences within species e.g. different hair colour.

However, when conditions are changing, the random mutation may actually confer an advantage over the original organism, and thus become dominent and eventually supplant it.

Evolution is a terribly wasteful method of development because the failure rate is incredibly high - living organisms depend upon a very fine balance of many factors just to be viable and the great majority of potential mutations will be more likely to upset that balance, rather than strengthen it - the vast majority of mutations die.  This is why evolution is a very slow process.

This is not to say that an organism cannot adapt to changing conditions though, but it is more like an unfit person getting fitter by doing regular exercise.  As the person gets fitter, their metabolism will become more efficient and there will be changes to their body chemistry.  However, these changes will lie within pre-existing limits.  Similarly, if the change in diet lies within the existing capabilities of our digestive tract, it can lead to the tract tuning itself for better performance with an non-optimal diet, but this tuning will not be passed on genetically, and so is not evolutionary.

[AllenG]

Microchaetus rappi
The African giant earthworm can reportedly reach lengths of almost seven meters.

[Bored chemist]

Presumably it's not often used as fishing bait.