The largest living flying insects have a wingspan of ~10-12 inches, which is very impressive for an insect. But have you ever wondered why insects cannot be as big as birds? Insects do not have lungs, they "breathe" (meaning they exchange gases) through a tracheal system which is a series of tubes of decreasing diameters that start off at the surface of the animal, as openings called spiracles. The tubes go everywhere in the insect's body and gas exchange takes place directly at the cell walls. So to be really big and fly, they need a lot of oxygen. In fact, in the Carboniferous, 300 million years ago, there were dragonflies that reached over 2 feet in wingspan! The oxygen content of the atmosphere was 35% back then, compared to 21% now. In fact, fossil insect wingspan correlates very well with the % of oxygen in the atmosphere, until the Jurassic, when oxygen started climbing up from an all-time low of 15%, but winged insects became smaller, not bigger. This is because there was a new type of flying predator in town: birds. If insects were too big, they would have lost the maneuverability contest with fast birds who had by then started to evolve an alula, a small front wing projection that allows birds to maneuver at low flying speeds without crashing. The double whammy of lower oxygen and fast aerial predators kept flying insects at more or less the current size. To read more, go to Ed Yong's blog, Not Exactly Rocket Science, here.
Soft-bodied animals such as worms started to appear on the sea floor during the Ediacaran Period, 635 million years ago, and around 542 million years ago, life started to exhibit a more “shelly” appearance. The fossil record shows an “explosion” of life forms during the Cambrian period. Many strange creatures that would look alien to us today developed during this time period. For example, Anomalocarids, such as the one shown in the following animation, were among the top predators.
Before the Ordovician Period, the only living creatures could be found in the sea. Meanwhile, life continued to evolve into forms more like those we know today. Fish were abundant during the Devonian Period, around the same time that arthropods were making their first significant excursions onto land.
I love that animation of Anomalocarid!
Good question, great answer =)
That answer is the reason why I doubt that somewhere out amongst the stars there is an intelligent life form that its form is based on a insectiod body pattern, due to their system of oxygen intake. Which would exclude them from some of the severe environments such as high altitudes.
Humanoids on the other hand is another matter as they can survive in a large range of differing environments.
True. Hollywood likes giant insects as aliens because they look so damn scary.
Another Reason of Small Body Sizes
ScienceDaily (June 4, 2012) — The mosquito is possibly summer's biggest nuisance. Sprays, pesticides, citronella candles, bug zappers -- nothing seems to totally deter the blood-sucking insect. And neither can rain apparently.
Even though a single raindrop can weigh 50 times more than a mosquito, the insect is still able to fly through a downpour.
Georgia Tech researchers used high-speed videography to determine how this is possible. They found the mosquito's strong exoskeleton and low mass render it impervious to falling raindrops.
The research team, led by Assistant Professor of Mechanical Engineering David Hu and his doctoral student Andrew Dickerson, found that mosquitoes receive low impact forces from raindrops because the mass of mosquitoes causes raindrops to lose little momentum upon impact. The results of the research will appear in the June 4 issue of the Proceedings of the National Academy of Sciences.
"The most surprising part of this project was seeing the robustness this small flyer has in the rain," Dickerson said. "If you were to scale up the impact to human size, we would not survive. It would be like standing in the road and getting hit by a car."
What the researchers learned about mosquito flight could be used to enhance the design and features of micro-airborne vehicles, which are increasingly being used by law enforcement and the military in surveillance and search-and-rescue operations.
To study how mosquitoes fly in the rain, the research team constructed a flight arena consisting of a small acrylic cage covered with mesh to contain the mosquitoes but permit entry of water drops. They used a water jet to simulate rain stream velocity and observed six mosquitoes flying into the stream. All the mosquitoes survived the collision.
"The collision force must equal the resistance applied by the insect," Hu said. "Mosquitoes don't resist at all, but simply go with the flow."
The team also filmed free-flying mosquitoes that were subjected to rain drops. They found that upon impact the mosquito is adhered to the front of the drop for up to 20 body lengths.
"To survive, the mosquito must eventually separate from the front of the drop," Hu said. "The mosquito accomplishes this by using its long legs and wings, whose drag forces act to rotate the mosquito off the point of contact. This is necessary, otherwise the mosquito will be thrown into the ground at the speed of a falling raindrop."
To view a video on this research, click here: http://www.youtube.com/watch?v=LQ88ny09ruM&feature=youtu.be
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