posted on Mar, 23 2005 @ 10:45 AM
As previous posters have alluded, there are two reasons why there never were airplane size insects, and one reason why, during the Mississippian,
Pennsylvanian, and Permian eras ther were larger insects thatn we see now:
o The square-cube law;
o Oxygen concentration.
The squeare cube law says that, as the height or width of a critter grows as the square, its mass (and therefore the needs of its support
structure) increase as the cube. What this means is that if a critter is three times as tall as another, it will probably mass nine times as much,
and its support structure (say, its legs) much be much larger in proportion to its body thatn the smaller critter. In other words if you scaled u-p a
mouse to the size of a horse, its legs would have to be much larger, which would make it look more like an elephant or a hippopotamus.
Now an insect or any critter with an exoskeleton doesn't have any hard and strong internal support structure because it doesn't have an internal
skeleton (that's why they're called EXO or 'outside' skeletons). If the mass of the internal organs is measured in milligrams, then they can be
held to the exoskeleton by muscular tissure, but it the animal becomes as big as a cat, say, no amount of musculature can keep the internal organs in
place and the animal would rupture itself and die.
With internal skeletons, which are like rebar in concrete, there is much more of a foundation for heavy organs like the liver, stomach, lungs, heart,
etc., to hold on to, and the animal can gro much more massive without falling apart.
Of course, this is based on the force of gravity on land; marine and aquatic creatures don't have this problem, which is why you can have very large
exoskeleton animals as well as molluscs which don't have any skeleton at all!
Oxygen concentration is a key factor, because the way insects 'breathe', using spiracles works fine when the oxygen can cross membraneous
barriers to get to the tissues by only going a few millimeters.
Given that it's only a few millimeters from the spiracles to most of the insects' tissues, such an inefficient mechanism works fine, but if the
tissues are buried too deeply, the only way the cells could get oxygen is to have smoe means of transporting it over longer distances, i.e., blood and
blood vessels, which the insect doesn't have.
During the last three eras before the Mesozoic, the oxygen content was substantially higer (~35%) according to some estimates, which allowed a less
efficient method (the spiracles) to work ove a slightly longer distance. so during the Mesozoic, one-foot flying animals, as long as they
stayed light and didn't violate the square-cube law, would work.