Why did giant insects become small?

I thought it was dinosaurs. With those big guys swatting at flies with newspapers the size of carpet rolls, bugs needed to get big in self defense.

Natural selection .

well, that is an alternative theory that is gaining acceptance in the "Ivies". There will, no doubt, be a Penrose Conference in 2007 to discuss fossil evidence of early Permian newspaper rolls clutched in the arms of Dimetrodonts and Mormoops. Lottsa work is needed to develop this though.

But didn't those jokers have little, tiny arms because they had no shoulder girdles? Them newspapers couldn't have been too big.

Think relative, Set... Little, tiny arms on a multiple ton wighing animal are still quite substantial in my worldview... And how much powah is needed to swat big bugs with huge newspapers anyways?

Gratuitous ad for my other thread

Ok, this was the funniest and most interesting thread of the day.

In addition to the oxygen levels placing a limit on the largest possible size, bugs have evolved into niches that favor a small size (just like synapsids got smaller during the time of the dinosaurs).

("Synapsids" refers to mammals and proto-mammals.)

As stated in the Gould article, another problem with large-sized arthropods is that they must maintain their shape when they shed their cuticle. A six foot terrestrial, immature arthropod would tend to collapse to a puddle of jelly without its external cuticle to maintain its shape. Aquatic arthropods, such as lobsters don't have that problem because of the water pressure to support their bodies.

Thats why , when you look at fossil insects from the pre Permian, all showed either primitive incomplete metamorphosis or aquatic complete metamorphosis.

Regardless of the type of meatmorphosis an arthropod undergoes, it must periodically shed its cuticle to allow for growth until it reaches its last instar, the fully grown adult stage. Just as the internal skeleton supports a vertebrate's body mass, the exoskeleton supports the arthropod's mass. Gravitation pull would misshappen large forms of either skeletal system without its supporting skeleton. Presently, all large arthropods are aquatic, e.g., lobsters.

Still, the problem of Arthropleura exists. If it was truly terrestrial, how did it avoid this problem when shedding? One solution would be to return to the water when shedding, a sort of amphibious life. Or maybe it was aquatic and only came on land temporary for an unknown reason.

Only incomplete metamorphosis even have "instars" . And this is a term used mostly for cockroaches, Limulus, lobsters . crabs etc. Incomplete metamorphosis alows the body to "catch up as the exoskeleton provides room for the new growth.

The millipedes were able to coil, IM uneducatedO could reinforce their own shape when shedding.But remember, the millipedes all have an ample number of spiracles per segment. so their respiration was assured. They didnt spiral out during the late Permian, but in the early Pewrmian when the O2 was still high enough to allow for primitive respiraion. Maybe they did become victims of their own sizes. I really dont recall seeing any fossils of millipedes or cockroaches for that matter. They, in my area of work, have no significance, since the geology of cyclothemic deposits is so conveniently easy to spot, we dont need to bother with fossil evidence

University of Florida Book of Insect Records
Chapter 13 Most Instars
B.R. Sojack
Department of Entomology & Nematology
University of Florida, Gainesville, Florida 32611-0620

8 May 1995


--------------------------------------------------------------------------------
If instar refers to any stage between molts, then the fire brat, Thermobia domestica (Pack) (Thysanura: Lepismatidae), holds this record with 60 molts. If the larval stage is defined as the only stage that contains instars, then the mayfly, Stenacron interpunctatum canadense (Walker) (Ephemeroptera: Heptageniidae), would be our champion with 45 molts.
--------------------------------------------------------------------------------

The first objective was to determine a definition for instar. After asking professors and fellow graduate students, I found there were two ideas of what an instar is. One definition for instar is the stage of the insect between successive molts (Chapman 1982; Borror et al. 1989). This definition does not state anything about immatures or adults. The other definition states that an instar is the stage between molts of the immature insect (de la Torre-Bueno 1989). Therefore I looked for record holders using both these definitions.

Identification of Ladybird Beetles,by instar.

Ill yield on the instar definition in complete and incomplete metamorphism. I always took the definition from my paleo , which the use of "instar" was only from remnants in rock -remember I only deal in fossils.

However, wasnt your point that successive instars may fail to be structurally stable when a bug got to be huge? I thought my guess about insects and other arthropoda were that , (as I took instar to mean) they greww by infilling a shell which, when molted contained a new shell that had to harden before being fully structurally stable. There may have been a time wherein the insect was vulnerble to predation or collapse (like a huge 45 pound lobster when it molts , is soft). Sometimes in a large lobster the shell of the carapae can deform from wieght and this lobster can look like it has "melted" a little, I dont know if thats what you were referring to

The fact is though that these large insects and a rthropods had lived no? Lets go here. Do you know of any examples of Paleozoic complete metamorphism insects that were exhibiting gigantic sizes but were terrestrial? (I dont have any examples but Im not an expert by any means- as I said (fossils are always a biased sample we must remem.ber)before, bugs arent an important environmental indicator (except for amber)

I was then looking at spiders , figuring that they were similar in morphology to todays but found out that most of the sppiders were air breathers but were aquatic in habit, (Geralycosae,Arthrolycosa, Architarbus,, Cryptomartus,polycheras). Now I have no idea how this happens unless they were water striders or carried air bubbles with them In my "treatise on PAleontology' the list of thoughts as to how they could adapt to on land life yet retain aquatic features was to "equalize pressures during molting" However, since there are no fossils from anything but aquatic beds, its difficult to state that no "huge " land spiders lived during the O2 peak time.
Scorpions, it IS known from the fossil record, were large but were all aquatic (paleophonus Eoscorpus) However. in the mega bugs The thoracic structures were all hardened segmented sections sortof like pipe sections. And, they were aquatic juveniles, so where most of the growth took place, they were in a balancing media(stenodycta, meganeura)

That leaves us with the cockroaches and otherwinged insects. In these, the treatise states that significant structural strength may have been afforded by the hexagonal segmentation patterns of ridges in the body segments and the wings. (paleopterans, blattoids, orthopteroids ) the other groups (beetles, neuropteroids, mecopteroids, hymenopteroids, hemipteroids all came AFTER the beginning of the Mesozoic and well after the O2 spike) There were some teeny mecopteroids-proto butterfly types in the upper Permian but these were not of any large size even by todays standards . (they were more like cabbage moths than Polyphemus)

So, Im comfortable that structural elements and probably chitenous exoskeletons kept the molts from collapsing. Also the structural ridges like the hexagonal patterns on early winged insects allowed for growth. Remember, even tody, when a butterfly is emerging from its chrysallis, its totally vulnerable to predation,so maybe there was some evolutionary gimmick like bad taste or " wedging" that protected larger bugs while they molted

God made 'em diff'rent back then.



(Good stuff.)

Obviously

Just in time for the season, this thread has arisen.
I was reading just the other day, a small argument paper about the average decline in O2 levels and how marginally respirant species can even now be seen as a "canary in the mine shaft "

Gunga is questioning ancient O2 levels over here as well.

Recently I watched a video at school it said the the oxygen level back the was alot higher and because insect breath through their skin this allowed them to grow so large. The abormally high oxygen level made the atmosphere really unstable and something as common as lightening could spark an explosion. supposedly a strike of lightening cause an extreamly powerful explosion that wiped out the insects. the oxygen levels decreased over time and since insects dont have lungs they had to adapt by getting smaller.

Thanks for resurrecting this old thread. I really liked it.

As I learned from the responses early on, it was indeed the higher O2 levels in the atmosphere which allowed insects to grow so large.

I'm not sure about lightening strikes causing "explosions" however. Oxygen promotes combustion, but usually not outright explosions.