How Do Spiders Breathe? | Book Lungs And Tracheae

Spiders possess a respiratory system entirely different from humans. They do not inhale through a nose or mouth, nor do they use a diaphragm to pump air in and out of massive lung sacs. Instead, they rely on a combination of passive diffusion and slow muscular contractions to move air through specialized organs located on their abdomen.

Understanding these systems reveals much about their behavior, their size limits, and how they survive in diverse environments ranging from underwater caves to high-altitude peaks. The mechanics of their respiration dictate everything from their hunting style to their ability to conserve water in arid deserts.

How Do Spiders Breathe?

Spiders breathe by allowing air to enter their bodies through small slits on the underside of their abdomen called spiracles. Once air enters these openings, it travels to one of two respiratory organs: book lungs or tracheal tubes. Most ancient spider species, like tarantulas, rely heavily on book lungs. Modern spiders often use a combination of book lungs and tracheal tubes to distribute oxygen directly to their tissues.

The air enters, and oxygen diffuses across thin membranes into the spider’s blood, known as hemolymph. Unlike human blood, which is iron-based and red, spider blood is copper-based and turns blue when oxygenated. This hemolymph circulates through an open body cavity, bathing the internal organs in oxygen rather than traveling through a closed network of veins and arteries. This open circulatory system works in tandem with their respiratory organs to sustain life.

Respiratory Organs Comparison

The following table breaks down the primary differences between the respiratory structures found in arachnids and how they function compared to other systems. This overview provides the biological context needed to understand their limitations and capabilities.

Feature	Book Lungs	Tracheal Tubes
Structure	Stacked, leaf-like plates (lamellae)	Branching network of hollow tubes
Primary Method	Passive diffusion into hemolymph	Direct gas delivery to tissues
Location	Underside of the abdomen (anterior)	Throughout the body (posterior origin)
Gas Exchange	Occurs at the surface of plates	Occurs at the tube endings
Water Loss Risk	High due to large surface area	Low due to narrow openings
Spider Types	Tarantulas, Scorpions, Primitive Spiders	Jumping Spiders, Wolf Spiders, Active Hunters
Ventilation	Some muscle movement (atrial expansion)	Mostly passive diffusion
Oxygen Carrier	Hemocyanin (Blue Blood)	Direct diffusion (Minimal carrier reliance)

The Anatomy Of Book Lungs

Book lungs are the most distinctive respiratory organ in the arachnid world. They get their name from their physical structure, which resembles the pages of a folded book. These organs are located inside an air-filled cavity (atrium) on the ventral side of the spider’s abdomen. Each “page” of the book is a hollow, leaf-like plate called a lamella. A single book lung may contain dozens of these lamellae stacked neatly on top of one another.

Air enters the atrium through a spiracle. It then flows between the lamellae. The inside of each lamella is filled with hemolymph. As air passes over the surface of the plate, oxygen crosses the thin membrane and enters the blood, while carbon dioxide exits. Small pegs keep the lamellae separated, ensuring that the air gaps remain open even when the spider moves. If these plates were to collapse, the surface area for gas exchange would disappear, and the spider would suffocate.

This system relies heavily on surface area. The more plates a spider has, the more oxygen it can absorb. However, book lungs have a drawback: they lose moisture rapidly. This is why many spiders with large book lungs, such as tarantulas, prefer humid environments or stay in burrows during the heat of the day. The large surface area required for oxygen intake also acts as a large surface area for evaporation.

Tracheal Tubes And Direct Delivery

Evolution drove many modern spiders to develop a second system: tracheae. These are systems of branching tubes that start at the spiracles and extend deep into the body. In some active hunting spiders, these tubes branch out so finely that they touch individual muscle cells.

Tracheae function differently than book lungs. Instead of oxygenating the blood at a central point, the tubes carry air directly to the organs that need it. This reduces the spider’s reliance on hemolymph circulation for oxygen transport. It allows for higher metabolic rates, which is necessary for active hunters like jumping spiders (Salticidae) that need bursts of energy to pounce on prey.

The tracheal system also helps with water conservation. The tubes are narrow, which limits the amount of water vapor that can escape. This adaptation allows spiders to colonize drier habitats where a creature relying solely on book lungs might desiccate and die.

Spider Breathing Mechanisms And Evolution

The diversity in breathing apparatuses among spiders links directly to their evolutionary history. The most primitive spiders, the Mesothelae, and the Mygalomorphae (which include tarantulas and trapdoor spiders), typically possess two pairs of book lungs and no tracheal tubes. Their large size and sedentary ambush tactics fit well with this respiratory setup. They do not chase prey over long distances, so they do not require the rapid oxygen delivery that tracheal systems provide.

Conversely, the Araneomorphae, or “modern spiders,” show a shift. Most have replaced the posterior pair of book lungs with a tracheal system, keeping only the anterior pair of book lungs. Some small species have lost book lungs entirely, relying 100% on tracheae. This shift supports a more active lifestyle and protects them from dehydration.

Scientists classify these configurations into specific groups based on the number of spiracles and lung pairs. This anatomical data helps researchers trace the lineage of different families, showing how spiders moved from aquatic ancestors (who used book gills) to terrestrial conquerors.

Hemolymph: The Blue Blood Factor

Oxygen capture is only half the battle; transport is the other. Once oxygen diffuses across the book lung membrane, it binds to a molecule called hemocyanin. Vertebrates use hemoglobin, which contains iron and turns red when oxygenated. Spiders use hemocyanin, which contains copper. When oxygen binds to copper, the solution turns a pale blue. When deoxygenated, the blood is clear.

Hemocyanin is not as efficient as hemoglobin at carrying oxygen, but it functions well in the cold and functions freely in the open circulatory system. The spider’s heart, a tube located on the dorsal side of the abdomen, pumps this blue blood forward into the cephalothorax (head section) and down to the legs. From there, it flows back through the body cavity, bathing tissues directly before pooling back near the book lungs to recharge with oxygen.

This open system creates hydraulic pressure. Spiders do not have extensor muscles in their legs; they use blood pressure to extend their limbs. If a spider loses too much blood pressure—perhaps due to a wound—their legs curl inward, a characteristic “death curl” often seen in deceased specimens.

How Do Spiders Breathe Underwater?

A common question is whether water kills spiders instantly. While they are air-breathers, many spiders display remarkable resistance to drowning. When a spider is submerged, air gets trapped in the dense hairs (setae) on its body. This forms a silvery sheen around the abdomen known as a plastron. The spiracles open into this bubble, allowing the spider to breathe the trapped air for a significant amount of time.

The Diving Bell Spider (Argyroneta aquatica) takes this to the extreme. It lives almost entirely underwater. It spins a dome-shaped web between aquatic plants and fills it with air bubbles brought down from the surface on its abdomen. This “diving bell” acts as a physical gill. As the spider consumes oxygen from the bubble, the partial pressure drops, causing dissolved oxygen in the surrounding water to diffuse into the bubble. This allows the spider to stay submerged for days without resurfacing.

Other semi-aquatic spiders, like the fishing spiders (Dolomedes), run across the water surface. If threatened, they can dive and remain submerged for up to 30 minutes, relying entirely on the air film trapped against their spiracles.

Control And Regulation Of Air Intake

Spiders are not passive bags of fluid; they have active control over their respiration. The spiracles are equipped with valves and muscles that can open and close. This control is vital for survival. If the spiracles were permanently open, the spider would lose moisture continuously and dry out.

When the spider is resting, the spiracles may close almost completely, opening only intermittently to let in just enough oxygen. When the spider begins to move, hunt, or fight, the oxygen demand increases. The hemolymph CO2 levels rise, triggering the muscles to open the spiracles wider. This creates a trade-off: high activity equals high water loss. This is one reason why many desert spiders are nocturnal—they hunt when the relative humidity is higher, making it safer to keep their airways open.

Respiratory Rates And Heart Rate

A spider’s metabolic rate is generally lower than that of an insect of similar size. Their ability to sit motionless for days or weeks without food is linked to this low metabolic demand. However, during bursts of activity, their heart rate can skyrocket. A tarantula at rest might have a heart rate of 30 to 40 beats per minute. When disturbed, that can jump to over 100 beats per minute to circulate oxygenated hemolymph faster.

If a tarantula is forced to run continuously, it will eventually exhaust its oxygen supply. Because the transport of oxygen via hemolymph is slow compared to a closed vascular system, the spider builds up anaerobic waste products (like lactate) quickly. This is why tarantulas and other large spiders often stop and rest after short sprints; they are physically unable to continue until their respiratory system catches up.

Why Respiratory Systems Limit Spider Size

You will never see a spider the size of a horse, and their method of breathing is the main reason why. Gas exchange in book lungs and tracheae relies heavily on diffusion. Physics dictates that diffusion is only efficient over very short distances. As an organism grows larger, its volume increases much faster than its surface area (the square-cube law).

If a spider were the size of a human, its book lungs would need to be impossibly large to provide enough surface area to oxygenate its massive volume of tissues. The tracheal tubes would also become inefficient, as air would take too long to diffuse down the long tubes to reach deep muscles. Without a diaphragm to actively pump air and a closed circulatory system to rapidly transport it, giant spiders are biological impossibilities under current atmospheric conditions.

During the Carboniferous period, when atmospheric oxygen levels were much higher (around 35% compared to today’s 21%), arthropods did grow larger because diffusion was more efficient. Today’s oxygen levels keep spiders firmly in the small-to-medium size category.

Breathing During The Molting Process

Molting is a perilous time for any arachnid. To grow, a spider must shed its rigid exoskeleton. This includes the lining of the tracheal tubes and the book lungs. So, how do spiders breathe when they are shedding their breathing organs?

The lining of the tracheae is shed along with the outer skin. As the spider pulls its old skin off, it pulls the old tracheal linings out of its body, much like pulling a finger out of a glove. During this brief window, gas exchange is compromised. The spider relies on the oxygen already saturated in its hemolymph. This is why the molting process is physically exhausting. If the spider gets stuck, it can suffocate or die from exhaustion before the new exoskeleton hardens.

Oxygen Requirements By Species

Different spiders have different oxygen needs based on their lifestyle. Sedentary web-builders have lower requirements than active hunters. The table below highlights these differences.

Spider Group	Primary Activity	Respiratory Reliance
Wolf Spiders (Lycosidae)	Active Hunting / Running	Strong Tracheal System + Book Lungs
Jumping Spiders (Salticidae)	Pouncing / Visual Hunting	Advanced Tracheal System
Orb Weavers (Araneidae)	Passive Web Waiting	Book Lungs Dominant
Tarantulas (Theraphosidae)	Ambush / Burst Speed	Two Pairs of Book Lungs
Diving Bell Spiders	Aquatic Living	Plastron / Bubble Diffusion
Daddy Longlegs (Pholcidae)	Web / Scavenging	Book Lungs (often one pair)
Trapdoor Spiders	Burrow Ambush	Book Lungs (conserves moisture)

Lung Books vs. Book Lungs

The term “book lung” is precise. The structure is an invagination of the cuticle. Interestingly, the evolutionary precursors to book lungs are book gills, found in horseshoe crabs. Book gills are external flaps used for swimming and gas exchange. As arachnid ancestors moved onto land, these flaps moved inside the body to prevent them from drying out.

This internalization was a major step in terrestrialization. It allowed arachnids to separate breathing from water balance. The basic design has remained virtually unchanged for millions of years because it works perfectly for organisms of their size. The lamellae provide a massive surface area packed into a tiny space. In a large tarantula, the total surface area of the book lung plates can be quite substantial, allowing for sufficient oxygen intake to support a predator that eats mice and lizards.

Circulation And The Pericardial Sinus

The relationship between the heart and the lungs is direct. The spider’s heart is located in the abdomen, suspended in a cavity called the pericardial sinus. The book lungs drain oxygenated blood directly into this sinus. Ostia (one-way valves) on the heart open to suck this oxygen-rich blood in, then close as the heart contracts to pump it forward.

This proximity ensures that the freshest blood goes immediately to the brain and legs. In spiders with tracheal systems, the tracheae often bathe the heart muscle directly in oxygen, ensuring the pump itself never runs out of fuel even if the hemolymph oxygen levels drop. This dual-system redundancy in modern spiders makes them incredibly resilient survivors.

Environmental Adaptations

Spiders live in almost every terrestrial habitat. Their breathing systems adapt accordingly. In caves with low airflow, certain spiders have larger book lung openings to maximize intake. In high-altitude Himalayas, jumping spiders (Euophrys omnisuperstes) live at elevations where oxygen is thin. Their efficient tracheal systems and small body size allow them to persist where larger animals would struggle with hypoxia.

Understanding these mechanisms helps us appreciate the complexity of these animals. They are not simple pests; they are biological machines fine-tuned by millions of years of evolution. Their respiratory systems are a perfect compromise between the need for oxygen and the need to retain water.

The next time you see a spider sitting motionless in its web, remember that it is actively managing its airflow. It is likely keeping its spiracles narrowed to save water, waiting for the moment to strike when it will flush its blue blood with oxygen to power its attack.

For further reading on arthropod biology and respiration, resources like Britannica’s Arthropod Respiratory System provide excellent diagrams and deeper scientific context.