How do insects breathe and Why are Some Insects bigger than others?

How do insects breathe cover image

Every living thing in this world has some form of the breathing process, even though it is not visible. Plants breathe with their stomata, taking both oxygen and carbon dioxide depending on the time. You are breathing as you read this article, but what about the little spider in some corner of your attic? How do insects breathe? 

What’s more important is how breathing and the amount of oxygen in the atmosphere affect the size of the insect. This article will focus on how insects breathe, how respiration takes place, do they have blood or lungs, and how some insects breathe underwater. But before we get into that, we have to know the difference between breathing and respiration. 

Breathing vs respiration

One misconception that people have is they consider breathing and respiration as one process. These two are related, but definitely not the same. 

Breathing is the act of contracting and relaxing the muscles and diaphragm to push the air in and out of the lungs. Breathing is done to draw the fresh air in (with more oxygen) and push the stale air (with more carbon dioxide) out. 

Respiration is the act of using up the oxygen to produce energy and it is a very complicated process involving electron transport channels, etc, and would require a separate article to explain that in detail.

Now that is explained, this article will discuss the procedure and pathways of breathing in insects. The organs of insects for breathing. Let’s begin.

Insect’s breathing organs

Two organs are used for breathing in insects; Spiracles and trachea. These are the two major organs and the finer organs will be discussed later. 

Spiracles are small openings present in the thorax and abdomen of the insects. These tiny openings are covered with a valve that can be opened and closed. This is where the air enters the body. 

Image of spiracles of Indian moon moth by Dean Morley
The orangish spots are the spiracles. Image: Dean Morley. CC-BY-ND 2.0

The trachea is a tiny pipe-like structure that are present inside the body. Imagine these as the wiring inside a computer, connecting different parts of the system. This is the pipe where air travels. 

The trachea further divides into finer tubes that are spread all inside the insect body. These structures are called tracheoles. They are the equivalent of blood capillaries. This is all we need to know how breathing occurs. Let’s look at the process. 

Breathing in insects

The spiracles are guarded by valved which can be closed or opened depending on the requirement of oxygen. As the valves open, air from the surrounding gets inside the spiracles and the trachea. 

A closer look of the spiracles of the Indian moon moth
A closer look at the spiracle of Actias selene. Image: Dean Morley. CC-BY-ND 2.0

Air moves from the atmosphere inside the body due to the process of diffusion. Diffusion occurs when air (or liquid) moves from an area of higher concentration to an area of lower concentration. 

The body of the insect has a lower concentration of air and the atmosphere has a higher concentration. Hence air moves inside the body. But that’s not all. The problem is insects don’t want just the air, they need oxygen. And oxygen is in a deficit. Let me explain. 

How oxygen content affects the size of the insect

While the process of diffusion works and does not require much effort, it isn’t very efficient. The oxygen molecule is surrounded by other air molecules (nitrogen, hydrogen, particles, etc). It is estimated that one oxygen molecule bumps into another molecule 6 billion times in just one second!

Take a look at this brilliant GIF that shows how many times an oxygen molecule (colored red) collides with other molecules.

This is the reason why oxygen is unable to travel deep inside the insect’s body. Think of it this way. Suppose oxygen can only travel only a distance of 2-inches. The insect must be around that size so that diffusion can distribute oxygen across the entire body. 

In case you do not understand, let me give you a more easily understandable explanation using modern terms such as Wi-Fi and mobile phones. Let’s say you get a Wi-Fi router that can send signals up to 2 meters in all directions. So, two meters is the radius of this signal bubble that’s being created by the router.

Now if you have a phone to browse the internet (and read more Spacefarer articles), you’d want to sit somewhere inside this bubble. If you get out of it, there, no signal. And if there are 20 of your friends who want to surf the internet too, they’d have to sit inside this imaginary bubble. No matter what, you’ll have to sit inside this bubble if you want to surf the internet. You cannot go out of it even if you want to.

This is the case with insect size and availability. The cell phones are the cell and the wifi signal is the oxygen. And surfing the internet is life. The router is the spiracle. So, the cells, in order to live, must be close to the spiracle.

Image showing w wifi signal emitted as an analogy for understanding dependency of insect size and oxygen need
The phones stay inside the circle. Image: The Spacefarer

If the insect is 10-inches wide and long, oxygen would only reach 2-inches from all the sides. This would result in cell death on the inside and hence, the insect cannot exist.

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How large insects existed

There was once a time when insects were large enough to make you scream and have nightmares. Dragonflies the size of a small eagle. Insects that were so long, they’d fit along the length of your room. The single word to describe them would be “terrifying”. So how did that happen?

Back in the Carboniferous period in the Paleozoic era. This was about 360 to 300 million years ago, so a long time ago. This is the period when trees covered the entire globe (of which the coal is still being used). The oxygen content in the atmosphere was 35%, highest of all time. This allowed insects to grow larger. 

Since there was more oxygen in the air, diffusion would allow oxygen to travel a longer distance. Hence the insects could afford to grow larger. Astonishing how oxygen content affects the size of dragonflies. 

Mechanism of breathing in modern insects

Diffusion is not very great in transporting oxygen across longer distances, hence limiting the size of the insect. But then, how ants are so small and praying mantis and grasshoppers are so terrifyingly huge (yes, I am scared of these two menaces). Well, it’s because of how they breathe. 

Insects like grasshoppers compress and relax their thorax and abdomen sucking in the air in and pushing it out. This breathing allows the air to reach deeper parts of the body and hence the insect can reach large sizes. Take a look at this brilliant animation that shows how humans, birds, and grasshoppers contract and relax their body and how the air goes into the trachea, into the tracheoles, and then expelled out. 

But this is not how all the insects breathe. Spiders breathe using a different organ that resembles gills more than tracheas. Spiders breathe using book lungs that are made of thin layers called lamellae. Oxygen is taken through the air and diffuses via these lamellae and goes into the blood to be distributed to the body. But..blood?

Do insects have blood?

Biology is a science of exceptions. Some insects have blood and some don’t. But the insects that do have blood, it isn’t exactly like human red blood. In fact, it is very different. 

The blood in insects is called hemolymph and it is different from human blood in not having red blood cells. It is a liquid medium for oxygen transport, lubricating the organs, transporting nutrients, and waste material. 

Not just that, but hemolymph has hemocytes that work as immune cells of the body. Unlike human blood which has iron to transport oxygen, insect blood has copper. This also contributes to the blue-green color of the blood. 

Again, biology is a science of exception and there is an exception in the case of blood as well. Some insects such as the bloodworm, midge-larvae of the Chironomidae family have hemoglobin.

Breathing underwater

Many insects have made the water their home, but they don’t have the permit to live in it; they don’t have gills. So the insects have developed some clever ways of breathing in the water world.

Some small insects have a cuticular way of breathing in which they take oxygen through the skin. Since not much oxygen is available, these insects are very tiny and the least motile. 

Some insects have modified gills that allow them to breathe in water. Some use breathing tubes called siphons to access air from the atmosphere while they remain submerged, sort of like, well, exactly like snorkeling. 

mosquito larvae using siphons to breathe
mosquito larvae using siphons to breathe. Image: : James Gathany, CDC. CC-BY-2.5

Some insects carry an air bubble inside with them tucked under their wings to have a portable air supply, like a diver’s oxygen tank. Some have specialized hydrophobic hair that trap air called plastrons. 

Summary 

In summary, insects majorly breathe through a network of chitinous tubes called tracheas that open outside the body through tiny pores called spiracles. 

Some insects such as spiders have book lungs, while other can breathe underwater using specialized organs or just carrying air with them. Insects do have blood called hemolymph but it does not have iron or red blood cells. Some insects, however, such as the bloodworms, have hemoglobin. 

Oxygen content in the atmosphere affects the size of the insect. Higher content of oxygen in the air results in a larger size of the insect body. This concludes the article.

References 

Gullan, P.J. and Cranston, P.S. “The Insects: An Outline of Entomology, 3rd Edition.” Wiley-Blackwell, 2004

Respiration in aquatic insects

Merritt, Richard W. and Cummins, Kenneth W. “An Introduction to the Aquatic Insects of North America.” Kendall/Hunt Publishing, 1978

Meyer, John R. “Respiration in Aquatic Insects.” Department of Entomology, North Carolina State University (2015).

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