Prehistoric insects and arachnids

[Thedus]

This thread is inspired by Dutch90's Cloning Prehistoric Animals.

Present day insects and arachnids are limited in size by their physiology.  i.e. - having open circulatory systems and exoskeletons.  Basically, the larger the organism the thicker the exoskeleton would need to be to deal with the weigh of the internal juiciness of the critter.  The thicker the exoskeleton the more difficult it is to get air to the inner most organs.

So with that in mind how is it that prehistoric insects and arachnids grew to be so large?  Dragonfly's with wingspans of 3 feet, scorpions up to a meter long, Arthropleura with grew up to 2 meters long, and so on.

Was it due to differences in physiology between then and now? If so then what changed to cause them to evolve into smaller creatures?  Were the exoskeletons made of some other protein than they are now?  Was it that the earth's atmosphere was so different that oxygenating the innermost portions of their body was easier even through a thicker exoskeleton?  Was it their diet?  or was it a little of each of these?

[Dutch90]

Actually it had to do with oxygen levels in the atmosphere. Arthropods have a very primitive respiratory system compared to terrestrial vertebrates like mammals, birds and reptiles. I'm not entirely sure how it works, but I believe they don't actively breath like we do, they just rely on air molecules drifting into breathing orifices and then filter out the O2 or something like that. Anyhow, this makes them very dependant on oxygen levels when it comes to their size. They were bigger in the Carboniferous period due to the exceptionally high oxygen levels at the time. As oxygen levens decreased in the Permian period and so forth, arthropods decreased in size. This is also why the biggest arthropods today are often found in tropical rain forests, which produce a lot of oxygen.

Respiration in arthropods varies between groups but is usually by gills in aquatic habitats and a tracheal system in terrestrial forms. The tracheal system is composed of tubes that distribute air throughout the body. Some spiders have a specialized lung, called a book lung. Although most arthropods have specialize respiratory structures, diffusion of gases across thin areas of the cuticle also contributes to respiration. The circulatory system is open.

http://bioweb.wku.edu/courses/biol225/225lab5.html

The film Mimic correctly points out that insects (and other arthropods) could be much larger if they possessed lungs, although at some point they would get limited by their exoskeleton (they would at least be able to attain Carboniferous sizes).

I'm not sure there have been major changes in the physiology of the arthropods' exoskeleton. This may be hard to gather from fossilized remains, but I do know that many arthropod groups have changed very little in 300-400 million years.

[Thedus]

Thanks, Dutch90 (+)

This is also why the biggest arthropods today are often found in tropical rain forests, which produce a lot of oxygen.

I never thought about that!  Good point.

The film Mimic correctly points out that insects (and other arthropods) could be much larger if they possessed lungs, although at some point they would get limited by their exoskeleton (they would at least be able to attain Carboniferous sizes).

Actually, the size of the bugs in Mimic always bothered me since even with lungs the Judas Breed (or what ever it was called... the last time I saw it was in the theater) would not have been able to match the size of a human with an exoskeleton.  A six foot insect would have had a difficult time standing upright to mimic a human with the exoskeleton thick enough to support all of it internal fluids and organs.

[Pandorag]

For once, I am very happy we have less oxygen on earth. I have chronic arachnophobia. I leave a room when one the size of a penny is present; I stopped hanging laundry outside one summer because one made a nest on the adjustable thingy of the drying line, I mean, it filled up the hole which was bigger than a loony (one dollar)!! I have to call helplessly my son to kill the bugger!
 
*hangs head in shame for the girly comment*

I love dragonflies though, especially the blue ones!

Do all insect have endoskeleton, such as mosquitoes, flies or even butterflies?

[deezelboy]

All adult insects have exoskeletons.

[Dutch90]

Actually, the size of the bugs in Mimic always bothered me since even with lungs the Judas Breed (or what ever it was called... the last time I saw it was in the theater) would not have been able to match the size of a human with an exoskeleton.  A six foot insect would have had a difficult time standing upright to mimic a human with the exoskeleton thick enough to support all of it internal fluids and organs.

Probably so, that wouldn't be possible in real life. I guess this also means Arthropleura couldn't rear up like it did in the BBC documentaries. I think this creature is about as big as terrestrial arthropods could get, and even under water there'd be limits. Aquatic arthropods didn't get bigger than Jaekelopterus, a eurypterid or sea scorpion (not actually a scorpion) that could be up to 2,70 m long- not much bigger than the terrestrial Arthropleura. For a while they were the top predators, but eventually vertebrates upstaged them because they could get much bigger thanks to their endoskeletons.