How Insects Breathe

The Tracheal Respiratory System Explained

Unlike mammals, insects don’t use lungs to breathe.They don’t rely on blood to carry oxygen either.Instead, insects breathe through a vast network of tiny air tubes that deliver oxygen directly to every cell in the body. This system is called the tracheal respiratory system, and it’s one of the main reasons insects can be so active despite having such small bodies.

Rather than inhaling air into lungs, insects let air flow straight into their tissues.

Spiracles: The Gates of Gas Exchange

Along the sides of an insect’s body are small openings called spiracles, usually arranged in pairs on each body segment.

These act like adjustable valves.

When spiracles open:
• Oxygen flows inward
• Carbon dioxide flows outward

When spiracles close:
• Water loss is reduced
• The insect conserves moisture

This is crucial for survival, especially in dry environments.

Some insects can even coordinate spiracle opening in rhythmic patterns, creating airflow through the body similar to breathing.

Tracheae & Tracheoles: Oxygen Delivered Cell by Cell

Once air enters the spiracles, it moves through larger tubes called tracheae, which branch repeatedly into microscopic tubes known as tracheoles.

These tracheoles weave directly between muscles, organs, and even individual cells.

Oxygen diffuses straight into tissues without ever entering the bloodstream.

This means:

• Oxygen delivery is extremely fast
• High energy activities like flying are possible
• Circulation isn’t needed for respiration

But it also places limits on insect size. Oxygen must diffuse efficiently, and over long distances diffusion becomes slow. This is one major reason insects can’t grow to the size of mammals today.

(Fun fact: when oxygen levels were higher hundreds of millions of years ago, insects were much larger.)

Breathing Without Drying Out

Opening spiracles constantly would cause insects to lose water rapidly.

To solve this, many insects use discontinuous gas exchange:

Spiracles stay closed for long periods
Carbon dioxide builds up internally
Spiracles briefly burst open to release CO₂ and take in oxygen

This strategy allows insects to breathe while conserving moisture especially in deserts and hot climates.

A Quick Note on Book Lungs (Not Insects, But Relatives)

Book lungs belong to arachnids like spiders and scorpions, not insects.

They are stacked, leaf like membranes inside the body that allow gas exchange with circulating blood.

Insects evolved the tracheal system instead, which bypasses blood entirely.

This difference highlights how arthropods adapted respiration to different body sizes and lifestyles.

Key Definitions

Spiracles
External openings on an insect’s body that allow air to enter and exit.

Tracheae
Large air tubes that carry oxygen from spiracles throughout the insect’s body.

Tracheoles
Tiny branching tubes where oxygen diffuses directly into cells.

Diffusion
The movement of molecules from high concentration to low concentration (how oxygen moves into tissues).

Discontinuous gas exchange
A breathing pattern where spiracles remain closed for long periods and open briefly to exchange gases.

Book lungs
Layered respiratory organs in arachnids used for gas exchange with blood.

Activity: Build a Breathing Insect Model

This helps visualize how oxygen moves through the body.

Materials:
Straws (large and small)
Tape
Paper or cardboard

Steps:
Create a main “trachea” using a large straw.
Branch smaller straws off in many directions.
Label tiny ends as “tracheoles.”
Draw spiracles on the cardboard insect body where air enters.

Discussion prompts:
Why does branching increase oxygen delivery speed?
What happens if the insect gets larger but tubes stay the same length?
How does this system limit body size?