X-ray images help explain limits to insect body size

X-ray images help explain limits to insect body size
X-ray imaging of beetles helps confirm that tracheal system design may limit size in insects. More of the body is filled with air-filled tracheal tubes in larger species, particularly in the legs, and so much larger species than exist today might not have room for enough tubes. Credit: Argonne National Laboratory

Researchers at the U.S. Department of Energy's Argonne National Laboratory have cast new light on why the giant insects that lived millions of years ago disappeared.

In the late Paleozoic Era, with atmospheric oxygen levels reaching record highs, some insects evolved into giants. When oxygen levels returned to lower levels, the insect giants went extinct.

The basis of this gigantism is thought to lie in the insect respiratory system. In contrast to vertebrates, where blood transports oxygen from the lung to the cell, insects deliver oxygen directly through a network of blind-ending tracheal tubes. As insects get bigger, this type of oxygen transport becomes far less effective. But if the atmospheric oxygen levels increase, as they did in the late Paleozoic, then longer tracheal tubes can work. This would allow larger-sized insects—even giants—to evolve.

Recent research published in the journal Proceedings of the National Academy of Science helps confirm the hypothesis that the tracheal system actually limits how big insects can be. The research provides a specific explanation for what limits size in beetles: the constriction leading to the legs.

A collaborative team of researchers from Argonne's Advanced Photon Source (APS), Midwestern University and Arizona State University wanted to study how beetles' tracheal systems change as their body sizes increase. The team took advantage of richly detailed X-ray images they produced at the APS to examine the dimensions of tracheal tubes in four beetle species, ranging in body mass by a factor of 1,000.

Overall, they found that larger beetle species devote a disproportionately greater fraction of their body to tracheal tubes than do smaller species.

The team focused in particular on the passageways that lead from the body core to the head and to the legs. They reasoned that these orifices may be bottlenecks for tracheal tubes, limiting how much oxygen can be delivered to the extremities.

“We were surprised to find that the effect is most pronounced in the orifices leading to the legs, where more and more of the space is taken up by tracheal tubes in larger species,” said Alex Kaiser, biologist at Midwestern University.

They then examined the tracheal measurements of the four species to see if they could predict the largest size of currently living beetles. The head data predicted an unrealistically large, foot-long beetle. In contrast, the leg data predicted a beetle that nicely matches the size of the largest living beetle, Titaneus giganteus .

“This study is a first step toward understanding what controls body size in insects. It's the legs that count in the beetles studied here, but what matters for the other hundreds of thousands of beetle species and millions of insect species overall is still an open question,” said Jake Socha, Argonne biologist.

Source: Argonne National Laboratory

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Jul 29, 2009

A new theory about:
Why insects have small bulks?

Before explanation of my new theory, I point out to those theories which have been presented before on this respect.

Exoskeleton theory:
Insects have exoskeleton and this theory point out that this skeleton isn't able to grow more actually, the insect is imprisoned inside this skeleton and isn't able to get larger.

Defects of this theory: All of arthropod owns exoskeleton, and some of them like crabs are able to own rather large body which is discordant with this theory. On the contrary, there are some insects that own softer exoskeleton, but again aren't able to enlarge. Also in the past (250 million years ago), there were insects that owned big bodies (like dragonfly by length of 75 cm) and this theory can't explain the reason of their existence. Because they owned hard exoskeleton too but they were able to enlarger!

Respiratory system theory:
Insects respiratory system comprise thin and capillary tubes called tracheae that supplies oxygen directly through existent pores on the body, to all insect bodies interior cells. That means, opposite other animals, oxygen isn't been carried by blood. It flows through inside of tracheas by means of propagation phenomenon, and reaches cells. This theory says that the propagation of oxygen inside the cells, is possible only in short intervals and if insects have large body, the length of tracheas will lengthen and oxygen can not reach cells. So insects are forced to own small body.
About existence of large insects in the past, this theory predicts the existence of high percentage oxygen in earth's atmosphere. According to this theory, atmosphere's oxygen amount, 250 million years ago, was higher than the oxygen, would make it possible the propagation of it through the long tracheas of insects. Therefore insects at that time were able to be longer than present insects.

Defects of this theory:
250 million years ago, there were plants from the class of Horsetails and Club moss as the shape of very big trees. But today, the same plants exist only in very small size of some centimeters. Botanists believe that high consistency of atmosphere's carbon de oxide (CO2) is the reason of existence of giant Horsetails and Club moss at that time.
You consider that entomologists predict a high consistency of oxygen and botanists predict a high consistency of CO2 for 250 million years ago. These two have confliction with each other.

This was a brief of theories existing in this respect.
My new theory, in this respect:

Blood circulation system theory:
My theory is based on animals' blood circulation system. The theory compares the power and evolution of blood circulation system for animals and expresses the relation between bulk largeness and power of blood circulation system. According to this theory as much as blood circulation system is stronger and complete, animals' bulk can be larger (bigger). Insect's blood circulation system is too incomplete and there are no blood vessels. Insect's bodies, with lack of blood vessels, aren't able to supply blood consisting nutrition to remote cells, so their body are forced to be small.
In this theory air pressure and gravity, are two physical factors that have impact on function of blood circulation system. The most effort of blood circulation system is used to conquest on gravity. As much blood circulation system is strong, is more able to conquest on gravity and send blood upper into animals' brain. Consequently, animal is able to enlarge more and taller. For example, elephants and giraffes, who own the most evoluted and strongest blood circulation system, are the largest and tallest ones among the animals.
For possibility of existence of insects whit big bodies and other big animals like dinosaurs and mammals in the past, this theory predicts a light gravity at that time. According to this theory, the amount of gravity in the past have been less than the present time, so blood circulatory system of animals was able to conquest better on it and enlarge animals body but along the time, gravity has been increasing and animals are forced to make smaller their body sizes.
As you know, there are some theories such as strike of meteorites or activity of volcanoes for extinction of dinosaurs, which each one has a lot of defects. It has been said ice ages for extinction of mammoths and a lot other theories for other extinctions. My new theory is able to answer all of these questions as a case of a theory and no need to state a different theory for each of these matters. This is my theory's name:

"The relation between gravity and evolution"
This theory is able to answer many questions about animals and plants and their extinction, which haven't been answered yet.
This theory is organized as cast of several articles that the sun of these articles now available as a book. The title of this book is "The theory of gravity increase". It's ISBN is 8-04-7061-964 in 1999 and have 120 page. It's English translation can be found on following site:

Ramin Amir mardfar

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