Giant Insects Might Reign If Only There Was More Oxygen in the Air

The delicate lady bug in your garden could be frighteningly large if only there was a greater concentration of oxygen in the air, a new study concludes. The study adds support to the theory that some insects were much larger during the late Paleozoic period because they had a much richer oxygen supply, said the study’s lead author Alexander Kaiser.

The study, “No giants today: tracheal oxygen supply to the legs limits beetle size,’’ was presented at Comparative Physiology 2006: Integrating Diversity.

The Paleozoic period, about 300 million years ago, was a time of huge and abundant plant life and rather large insects -- dragonflies had two-and-a-half-foot wing spans, for example. The air’s oxygen content was 35% during this period, compared to the 21% we breathe now, Kaiser said. Researchers have speculated that the higher oxygen concentration allowed insects to grow much bigger.

Tubes carry oxygen

First, a bit of background: Insects don’t breathe like we do and don’t use blood to transport oxygen. They take in oxygen and expel carbon dioxide through holes in their bodies called spiracles. These holes connect to branching and interconnecting tubes, called tracheae, Kaiser explained.

Whereas humans have one trachea, insects have a whole tracheal system that transports oxygen to all areas of their bodies and removes carbon dioxide. As the insect grows, tracheal tubes get longer to reach central tissue, and get wider or more numerous to meet the additional oxygen demands of a larger body.

Insects can limit oxygen flow by closing their spiracles. In fact, one reason insects are so hardy is that they can close their spiracles and live off the oxygen they already have in their tracheae. Kaiser recalled a caterpillar that fell into a bucket of water in his lab. When the creature was discovered the next day, lab workers thought it had drowned. But when they removed its apparently lifeless little body from the water, they were surprised to see it crawl away.

Tracheae grow disproportionately

This experiment was designed to find out:

• how much room the tracheal system takes up in the bodies of different-sized beetles
• whether tracheal dimensions increase proportionately as the beetles get larger
• whether there is a limit to the size a beetle could grow in the current atmosphere

The researchers used x-ray images to compare the tracheal dimensions of four species of beetles, ranging in size from 3mm (Tribolium castaneum, about one-tenth of an inch) to about 3.5 cm (Eleodes obscura, about 1.5 inches). Beetles were not in existence during the Paleozoic period, but Kaiser’s team used the insect because they are much easier to maintain in the laboratory than dragonflies, which are quite difficult.

The study found that the tracheae of the larger beetles take up a greater proportion of their bodies, about 20% more, than the increase in their body size would predict, Kaiser said. This is because the tracheal system is not only becoming longer to reach longer limbs, but the tubes increase in diameter or number to take in more air to handle the additional oxygen demands.

The disproportionate increase in tracheal size reaches a critical point at the opening where the leg and body meet, the researchers found. This opening can get only so big, and limits the size of the trachea that runs through it. When tracheal size is limited, so is oxygen supply and so is growth, Kaiser explained.

Using the disproportional increases they observed among the beetles, the researchers calculated that beetles could not grow larger than about 15 centimeters. And this is the size of the largest beetle known: the Titanic longhorn beetle, Titanus giganteus, from South America, which grows 15-17 cm, Kaiser said.

And why wouldn’t the opening between the body and the leg limit insect size in the Paleozoic era, too? After all, dragonflies and some other insects back then had the same body architecture, but they were much bigger.

It is because when the oxygen concentration in the atmosphere is high, the insect needs smaller quantities of air to meet its oxygen demands. The tracheal diameter can be narrower and still deliver enough oxygen for a much larger insect, Kaiser concluded.

The research was carried out by Alexander Kaiser and Michael C. Quinlan of Midwestern University, Glendale, Arizona; J. Jake Socha and Wah-Keat Lee, Argonne National Laboratory, Argonne, IL; and Jaco Klok and Jon F. Harrison, Arizona State University, Tempe, AZ. Harrison is the principal investigator.

Source: American Physiological Society

Explore further

X-ray images help explain limits to insect body size

Citation: Giant Insects Might Reign If Only There Was More Oxygen in the Air (2006, October 11) retrieved 22 September 2019 from
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.

Feedback to editors

User comments

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

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more