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We hate to admit it, but at some point in our lives, every one of us has squashed a spider. It might have been an accident; you may have thought it was dangerous, or you may even have done it without realizing it! If you’ve ever seen a squashed spider (R.I.P. Our Arachnid Friend), you may have noticed it doesn’t seem to have any blood.
Perhaps you assumed the volume of blood in its body was too little to be seen, or maybe you didn’t care enough to take a second look (arachnophobia is a real thing).
Now you find yourself wondering, do spiders actually have blood?
The simplest answer is no; spiders don’t have ‘real’ blood. Like most arthropods, including some insects, arachnids, and crustaceans, they have a liquid called hemolymph. Hemolymph acts as a replacement for blood, ensuring oxygen can reach all the cells in a spider’s body. However, unlike blood, hemolymph does not contain red or white blood cells or even platelets.
Despite serving similar purposes, hemolymph isn’t the same as blood, nor is it formed from the same components.
This article investigates exactly what hemolymph is, why spiders don’t have real blood, what a spider’s circulatory system looks like, and how it works. Plus, we find out whether or not a spider actually has a heart and exactly how oxygen reaches its organs.
If Spiders Don’t Have Blood, How Do They Survive?
Unlike other animals, spiders don’t have real blood or veins. Instead, they only have arteries, which contain hemolymph. The hemolymph is pumped through the arteries to provide the body with oxygen. Unlike humans, hemolymph doesn’t stay inside a spider’s arteries but seeps through its body to reach all its vital organs and muscles.
This system is discussed in detail later and serves as a perfect example of species adaptation to optimize energy usage.
What is Hemolymph?
Hemolymph is a liquid consisting mainly of plasma (a mixture of water, carbohydrates, proteins, fats, other chemicals, and salts like sodium, chlorine, and calcium), hemocyanin, and hemolymph cells, called hemocytes.
It serves as a replacement for both blood and lymph and transports oxygen-carrying proteins, immune response cells, and essential chemicals alike.
The three main components of hemolymph are essential to a spider’s body not only for transporting oxygen but for responding to diseases and infections and transporting nutrients to the cells that need it.
What is Hemolymph Used For and How Does it Work?
Plasma, hemocyanin, and hemocytes are essential to a spider’s survival. They enable several crucial functions and reactions in a spider’s body. Each component of hemolymph has a vital role to play and is further explained below.
Hemocyanin
Hemocyanin is the spider version of hemoglobin found in red blood cells. Unlike hemoglobin, they aren’t found inside cells but move freely through the plasma of the hemolymph. Their purpose, however, is analogous to that of the hemoglobin in actual blood.
Each hemocyanin protein contains two copper atoms. These two copper atoms bind to one oxygen molecule and enable the transport of oxygen throughout the body. When the hemocyanin reaches the body’s tissues, the oxygen molecule detaches and diffuses into the cells that need it the most.
Hemocytes
Hemocytes are a spider’s equivalent to white blood cells. They enable an immune response to infections, injuries, and diseases the spider may contract throughout its lifetime.
When bacteria enter a spider’s body, the hemocytes use the hemolymph to reach the location of the infection and either destroy or encapsulate the bacteria. There are many types of hemocytes, and they all serve a particular role in keeping the spider healthy.
Plasma
Plasma works like a river, transporting the essential nutrients and hormones a spider needs to whatever part of the spider may need it. Without plasma, both hemocyanin and hemocytes would not be able to move anywhere. They’d be stuck in one place, and the spider would die almost instantly.
Plasma is mostly water, allowing all the things it is transporting to dissolve in it or be carried safely to where it has to go.
What Are the Other Uses of Hemolymph?
Movement
Spiders don’t have a lot of muscle tissue in their legs. To straighten and bend all eight legs so they can move, hemolymph is pumped into the legs. This increase in pressure pushes the legs straight and allows them to take a step forward. When the hemolymph drains from the leg, it automatically bends, pulling the spider’s body forward.
Transporting Waste Products
Although spiders don’t poop or urinate like other animals, they do excrete a waste product that combines the two. To remove waste products from the body, they must first be transported to the right organs to be processed and expelled. The hemolymph allows these waste products to be dissolved in the plasma and safely removed from the spider’s body.
Aids Molting
Spiders grow by molting – shedding their exoskeleton to make place for a new one. This involves the production of molting fluid, which forms a layer separating the new exoskeleton from the old one. Much of the components needed to produce molting fluid come from hemolymph first. [Source]
Heat Distribution
The chemical processes and effects of movement in a spider’s anatomy produce heat. This heat is dissipated through the hemolymph and spread throughout the spider’s body, allowing it to self-thermoregulate.
What Color is Hemolymph, or Spider Blood?
Because hemocyanin contains copper instead of iron, like hemoglobin, spider blood will usually appear blue.
When the hemocyanin is not bonded to oxygen, it looks transparent or slightly yellow-green. However, once oxygen binds to the copper atoms, the molecule formed is blue. Since most hemocyanin particles in the hemolymph are actively transporting oxygen, a spider’s blood will always appear blue.
What is the Difference Between Hemolymph and Blood?
Although human or animal blood and spider hemolymph serve similar purposes, they vary in their components and appearance.
Below is a list of the most striking similarities:
- Both hemolymph and blood consist mainly of water.
- Both contain a large percentage of plasma.
- Both spider hemolymph and blood serve the purpose of oxygen transportation.
- Both contain cells for immune defense.
- Both are pumped through the body through arteries first.
| BLOOD | HEMOLYMPH | |
| ORGANISMS | Vertebrates | Mostly invertebrates |
| RED BLOOD CELLS | Yes | No |
| WHITE BLOOD CELLS | Yes | No, have hemocytes instead |
| TRANSPORTED BY | Veins and arteries | Arteries and cavities called hemocoel |
| COLOR | Red | Blue |
| CONTACT WITH ORGANS | No | Yes |
Why Don’t Spiders Have Blood?
Due to their size, spiders adapted to not having space for parts of humans’ circulatory system, like a large heart and veins. Their circulatory system is perfectly suited to help them survive.
For example, if a spider’s leg is broken off, it can’t bleed to death. This is because there are no arteries or veins in the leg to allow hemolymph to escape. Instead, the few muscles present in the part of the leg still attached to the spider’s body constrict and push the hemolymph back into the body.
What Does a Spider’s Circulatory System Look Like?
Spiders have what is known as an open circulatory system. [Source] This means they don’t have as many vessels in which their hemolymph is transported. Instead, the hemolymph fills spaces called sinuses throughout the body.
Hemolymph is pumped from the abdomen through the arteries, which have valves to block backflow. Once the hemolymph reaches the sinuses, it spreads throughout the body and supplies the tissues with nutrients, oxygen, and other chemicals necessary for survival.
The ’empty’ hemolymph then gets pushed back toward the abdomen, where the process repeats itself.
Do Spiders Have Hearts?
Yes, all spiders have hearts, but they don’t look the same as other arthropods’. Instead of a spider’s heart having chambers and a distinct shape, it looks like a muscular tube. This tube is located in the spider’s abdomen and has a one-way valve at each end to make sure the hemolymph doesn’t flow backward.
The muscles around the heart contract, pumping the hemolymph through the main arteries and into the cavities around the spider’s body. Once the nutrients and oxygen in the hemolymph have been used up, the heart’s pumping action produces negative pressure, which forces the liquid out of the sinuses and back into the heart.
Do Spiders Have Lungs?
By now, you may be wondering how oxygen reaches the hemocyanin so that it can bind to the copper atoms for transportation. Instead of normal lungs, most spiders have two respiratory organs, while some may only have one or the other. These two organs are known as the book lung and the tracheae.
Book Lung
Spiders can have one or two book lungs. They got their name from their similarity to a book’s pages and serve as the main respiratory organ in most spiders. Book lungs are found in a spider’s abdomen, and the alternating air pockets and tissue containing hemolymph allow the diffusion of oxygen necessary for the hemocyanin to bind to it.
Tracheae
Tracheae are like tiny balloons connected by thin tubes. Oxygen fills these balloons, and the hemolymph is brought close enough for the oxygen to diffuse into it. In some cases, spiders will only have tracheae, with no sign of a book lung.
How Does a Spider Breathe?
Unlike humans and other animals, spiders can’t control their breathing. They can’t hold their breath or take a deep breath of fresh air.
Oxygen moves in and out of their bodies through simple diffusion. Once the concentration of oxygen in the book lung and tracheae are too low, oxygen molecules naturally move into the space to even it out. So, a steady supply of oxygen is maintained.
Why do Spiders Have an Open Circulatory System?
There are two main reasons spiders have an open circulatory system.
- They are tiny creatures and do not have the space for complex cardiovascular structures like veins.
- An open circulatory system requires less energy to function than a closed one. This gives a spider a better chance of surviving as it does not need a large food intake to supply extra energy. [Source]
- As oxygen enters the lungs and tracheae through passive diffusion, the process takes far more time than it would if spiders could breathe. An open circulatory system moves hemolymph around at a much slower rate than a closed one would so that both supply and demand of oxygen remain equal at all times.
Conclusion
A spider’s anatomy is very different from that of other animals. Not only does it not have the same lungs or heart as most animals, but it does not even have the same blood.
Hemolymph serves a critical purpose in a spider’s body and has been adapted cleverly to serve this purpose in the most efficient way possible. Yes, spiders don’t have blood, and the hemolymph they do have is blue, but it still does its job and runs the transport system all spiders need to survive.
