Arachnid Respiratory System: Organs & Structure

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Have you ever thought about how arachnids breathe? They are animals, so they need to breathe just like you and me. In this lesson we'll take a look at the organs of the arachnid respiratory system and how they facilitate gas exchange.

Breathing Rocks

Before you start this lesson, take in a deep breath, hold it for a while, and then let it out. Congratulations! You've just used your respiratory system to exchange gasses in your body. You breathed in air, which contains oxygen, and that oxygen is used for numerous cellular processes in your body. When you exhaled, you released carbon dioxide as a waste product.

All animals need to breathe in order to survive, but the way they go about it differs. For example, we breathe air into our lungs through our mouth and nose, but fish take in oxygen from the water through their gills. Dolphins and whales breathe through their blowholes on top of their heads, and earthworms exchange gasses right through their skin.

Arachnids (also known as class Arachnida), which are the spiders, scorpions, ticks, mites, and daddy longlegs of the world, also need to breathe to sustain life. So let's take a look at their respiratory systems to better understand how they exchange gasses in their bodies.

Arachnid Respiratory Organs

Arachnids have two types of respiratory organs. The first are called book lungs, which get their name because they look like stacked pages of a book. Book lungs are similar to book gills, which can be seen on the underside of a horseshoe crab. However, book lungs are internal structures, while book gills are external. The second type of structure is called the trachea, which is a tube or set of tubes that carries air. You have a trachea in your throat that is also known as your windpipe.

Book Lungs

Book lungs get their name because they look like stacked pages of a book
book lungs in a spider

Book lungs are actually stacks of many flat, hollow plates, which are saturated with hemolymph. Hemolymph is like blood for arachnids, and it's blue because it contains hemocyanin, a copper-based substance.

Much like the red blood cells in our bodies, hemocyanin binds to oxygen and transport it to areas in the arachnid's body where the concentration of oxygen is lower. It also takes carbon dioxide waste to places where it can leave the body. Hemolymph helps with gas exchange in the book lungs because it flows through the plates and exchanges oxygen and carbon dioxide through passive diffusion. Arachnids that have only book lungs include scorpions, whip scorpions, and tailless whip scorpions.


The trachea is a hollow, air-conducting set of tubes. These tubes open to the outside of the animal through pores called spiracles, which are found on the animal's abdomen. Here, the animal can both take in air and release carbon dioxide waste. The tracheal tubes are lined with chitin, which is a hard substance that is also found in the exoskeletons of insects.

What's interesting about the trachea is that it's believed to be a relatively modern anatomical feature in arachnids. Those that use only a tracheal system include pseudoscorpions, daddy longlegs, mites, and ticks.

The tracheal system in arachnids leads to the outside through spiracles, usually found on the abdomen
arachnid trachea

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