Arachnid Circulatory System

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Have you ever wondered how that spider sitting on your deck pumps blood through its body? This lesson will answer all of your arachnid circulatory system questions, and will begin by defining an arachnid and end with the features that make up an arachnid's circulatory system.

Arachnid Defined

We are about to go on a tour of the arachnid circulatory system, or how critters, like spiders, scorpions, daddy long legs and tens of thousands of other species, move blood through their bodies. Before we delve into the circulatory system, let's go over what makes an arachnid, an arachnid. For starters, they are typically carnivorous (meat-eaters), and they have four pairs of legs. They have a tough external covering, known as an exoskeleton, and most species have a body that can be divided into two sections (the cephalothorax and the abdomen).

The spider anatomy, including four pairs of legs (1), a cephalothorax (2), and an abdomen (3)

Arachnids do not have wings or antennae. Some members of the arachnid group (like the mite) are only 0.0031 inches in length, whereas some (like a scorpion in Africa) can reach 8 inches.

Diversity of arachnids

Now that you have a general idea of what an arachnid looks like, let's check out the circulatory system.

Circulatory System

Let's begin our arachnid circulatory system tour with the blood. Unlike your blood, arachnid blood appears blue-ish or clear, as they do not have hemoglobin (which is responsible for your blood's color). Arachnid blood is called hemolymph, and is responsible for transporting oxygen, nutrients, and hormones throughout the arachnid body. In some species, it is also used to increase the blood pressure during molting, or skin shedding.

Like you, arachnids have blood vessels and a heart. But, unlike you, their hemolymph is also found flowing freely throughout the body, bathing all of the organs. Because of this, arachnids have an open circulatory system, meaning that the hemolymph directly surrounds the organs and tissues.

A simple depiction of an open circulatory system

Contrast that with your circulatory system, which is termed a closed circulatory system, which means the blood remains in vessels. Take a look at the table comparing the two.

Open Circulatory System Closed Circulatory System
Organs bathed in blood Organs are not in immediate contact with blood
Blood flows slowly Blood flows quickly
It takes a long time for blood to cycle through Blood cycles through quickly
Does not require a lot of energy to operate Requires a lot of energy to operate

Why the difference? Like everything, there are tradeoffs. While the open circulatory system requires less energy, it doesn't provide the animals with a fast delivery of oxygen and nutrients. For larger animals, like you, the open system would not provide enough oxygen or nutrients to maintain your organs. But for the little guys, like a scorpion, it works just fine.

Okay, let's take our tour to the arachnid heart. It is located in the abdomen and is closer to the arachnid's back than its belly. It pumps the hemolymph into arteries, which lead to open spaces, termed sinuses. These sinuses surround their organs.

Spiders have an additional feature: arteries that lead to their legs. Spiders have muscles that allow them to move their legs inward, but do not have the muscles to move legs outward. The solution? Pump hemolymph through the arteries to the legs. This increases the blood pressure in the legs, and causes the legs to move outwards.

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