Why did giant insects become small?

Difficulty finding enough food to support body mass.
Changes in environment/temperature/atmosphere needed to support bodily requirements
Easy prey for predators due to large size/harder to hide

I'm rather glad there aren't any more of these around! Rather reminds me of my brother-in-law, though....

This is about as big as they get today.



This one's eating a scorpion. But scorpions used to get pretty big as well (ancient aquatic scorpions).

Fascinating...or it could be that a giant meteor wiped out the large insects and the smaller survived

The upper maximum size of the insecta is governed by the relatively inefficient respiratory system. They breathe by diffusion, and cannot concentrate the ambient atmosphere to extract oxygen, as is done in a set of lungs. It is entirely possible that the present-day size of members of insecta represents the largest size at which the most efficient oxygen usage is taking place--larger individuals may not use oxygen in a sufficiently efficient manner to succeed as well in gathering food.

Those all sound like reasonable guesses. But I wonder if it was one thing more than others, or something else not ye mentioned.

I always learned that insects could only reach a certian size before their exoskeleton began to weigh more than their respiratory system could provide power for. And since giant insects used to exist, I expect that modern day physics would allow them to reach the same proportions as their ancestors, but they don't. So something besides physics is keeping them small. Perhaps multiple things.

I'm just not up-to-date on the theories and information regarding the permian/carboniferous fauna and was wondering what the prevailing theories are.

Because if they remained that large, can you imagine the size that the Orkin Man would have to be!

Gigantism in the Arctic and calcium availability.

Set nailed it. Insects have a primitive respiration system that has essentially only evolved as far as "needed"
The oxygen levels in geologis time were governed by 2 Le Chat equations on the distribution of Carbon, Sulfeur, Calcium and Iron, in an O2 medium

1 CO2+H2O =(reversible) CH20 +O2


2 Fe2O3+16Ca(ionic)+16HCO3(ionic bicarb)+*SO4(ionic)+(reversible)4FEs2+15)2+16CaCO3+(lotsa*)O2

Balmer, up at Yale has made the environmental availability of O2 his lifes work , and hes plotted the atmospheric availability of free O2 in the Phanerozoic (last 550 million years). A curious spike in O2 occurs in the Carboniferous where O2 reaches as high as 35%. Bugs respiration was free to be as weird as they wanted and theyd grow huuuge. Cockroaches as big as snapping turtles and dragonflies the size of buzzards.Yeh, life was cool in them days.

The way we can ferret out this is to look at the availability of plant life. We have a huge surfeit of large plants and coal swamps bounded by the Mississipean and ending in the Permian vulcanism. The coal measures contained internal (bench)limestones and pyrite deposits . Graham and AGuilar (Nature 375) 1995, developed a balance sheet for gigantism in plants and bugs during this anomalously high O2 time.

Bugs , using a simple O2 diffusion, had to rely on external, rather than internal means to assist their growth patterns. Id seen a fossil roach in the lower Pennsylvanian coals of Montana. ^This sucker was about the size and shape of a standard football. He needed no help to raid the fridge, hed just pick it up and take it home.

Sorry about my spelling, Im late for a meeting, Ill come back with some links, or you can look up O2 in the Phaberozoic atmosphere, and look for the NAture article.

Oh. So the atmosphere way back when was different. A higher concentration of O2.

I knew bug size was governed by respiration, but I didn't know the O2 content of the atmosphere was that different back in the good old days.

Actually, now that I think about it, I guess I did know that atmospheric O2 concentrations had changed over the eons, but I had never connected the two things (bug size and O2 in the atmosphere).

Cool stuff. Thanks

I am curious though.. .what caused this O2 spike? It was probably a gradual process, I take it?
I was always taught that life developed from smal life forms to larger, more complex life forms. This would seem to suggest that the big bugs evolved from their smaller cousins?

What would be the incentive of bugs to grow bigger though? This is truly fascinating stuff, too bad I'm an ignorant layman in these matters

Naj

naj. The opportunities to grow big or small are pretty much a morphological response to an environmental condition. The fact that bugs respration grew from earlier arthropods that could be beached' and whose spiracles could take in and allow oxygen to diffuse within the body mass was just an body plan that was never really improved . Unlike fish, where swim bladders probably became a plan for lungs (we see that mudskippers have the ability to exchange O2 within their swim bladders)

The guy at Yale was Berner , not Balmer. The governing equations that I posted (while not strictly balanced since I forgot the HsO) showed that the excess of plants in a CO2 environment, led to an excess of oxygen and Pyrite and limestones. I would wager that the environment was quite acidic until the process was rolling and lots of limestone and pyrite were deposited and fixed.Oxygen Spiking during the Carboniferous. This was from Berner

As an object increases in length, its surface increases by the square, and the volume increases by the cube. So the volume of the body soon outgrows the ability of the spiracles and tracheae to carry oxygen to all the tissues. An insect the size of a human would be virtually all tracheal tubes; no room for organs. In the tiniest of insects diffusion of air through the tracheae is sufficient, but in larger insects there is a mechanical movement of the body to move the air through the tracheae.

Here's an excellent reprint of a Natural History Magazine article by Stephen J. Gould discussing this very topic: http://www.naturalhistorymag.com/master.html?http://www.naturalhistorymag.com/editors_pick/1974_01_pick.html

if insects were gigantic we'd pretty much be on the food chain wouldn't we?

i think we already are on the food chain of mosquitos, fleas, etc. and indirectly on the food chain of all the crop eating insects.

Given sunny weather a colony of maggots can eat fifty pounds of human flesh a day.

well, its almost supper time, thanks for sharing that Noddy

Farmerman--

You're welcome.

So, what caused this spike in O2? Too many plants?

The chemical reaction I posted led to an excess of both CO2 and O2, the plants utilized the Co2 and , from the fossil record(Im talking out my ass here so this part is only Speculation) there has never been a worldwide mass of plant material accumulating in the lowland marshes except in one time period, the Carboniferous . The lowland marshes existed because the reconstruction of the post Taconic world had already evidenced that the erosion of the really Ancient appalachians occured once and the time between the devonian and permian is loaded with "flat land" swamp deposits all over the world. Contemporary deposits ofCoal(made only out of plants) is accumulated in North America , Europe, Asia, Australia and Antarctica So weve got lots of green plants cranking out O2 and the Co2 is gettin sequestered in the formation of terrigenous limestones (most limestones are oceanic except for a few exceptions like the coal measures). Then when the big volcanoes happened the O2 levels declined quickly(Imagine that the O2 wasnt really high when the vulcanism occured-we may have had to start all life over). The reason that this theory seems plausible is because the treal killer of animals was in the ocean, where dissolved oxygen is critically held in an equilibrium and available to fish. Thats why a relative depletion of oxygen can occur by an algae bloom in a pond and then all the fish die. The level in water is ata delicate balance to aquatic life.

Oxygen goes up, bugs get big, oxygen goes down, bugs shrink.