Spiders possess a unique internal fluid, often called “blood,” that differs significantly from human blood. This fluid is typically blue or clear, a characteristic stemming from the molecule transporting oxygen throughout their bodies. Unlike red human blood, a spider’s internal fluid relies on a different chemical process for oxygen delivery.
The Unique Color of Spider “Blood”
The fluid circulating within a spider’s body is called hemolymph, not blood in the human sense. This hemolymph appears blue when oxygenated, or clear to pale when deoxygenated. Its distinct coloration is due to hemocyanin, an oxygen-carrying protein.
Hemocyanin contains copper atoms; these bind to oxygen, causing the bluish tint. When oxygen is released, the copper becomes colorless, making the hemolymph appear clear. Beyond oxygen transport, hemolymph contains nutrients, hormones, waste products, and immune cells called hemocytes. Hemocytes contribute to the spider’s immune response and coagulation.
How Spider “Blood” Differs from Our Own
A difference between spider hemolymph and human blood lies in their oxygen-carrying molecules. Humans utilize hemoglobin, an iron-based protein that gives blood its red color when oxygenated. Spiders, conversely, use hemocyanin, a copper-based protein that results in a blue hue when oxygen is bound. This difference in metallic components dictates the visual appearance of their circulatory fluids.
Another distinction is the type of circulatory system. Humans have a closed circulatory system, where blood is continuously contained within a network of vessels like arteries, veins, and capillaries. Spiders, along with many other arthropods, possess an open circulatory system. In this system, hemolymph flows freely into open spaces, directly bathing organs and tissues, rather than being confined within vessels.
While both systems transport oxygen and nutrients, human blood also regulates temperature and has a more complex clotting mechanism. Spider hemolymph, in addition to transport, generates hydrostatic pressure for movement.
The Spider’s Circulatory System
The spider’s open circulatory system relies on a tubular heart located dorsally in the abdomen. This heart pumps hemolymph through arteries into large open cavities within the body, known as the hemocoel or sinuses. Within these spaces, the hemolymph directly surrounds and supplies the organs with oxygen and nutrients. After circulating through the body cavity, the hemolymph returns to the heart through small openings called ostia.
This system also plays a role in spider locomotion. Spiders primarily extend their legs through hydraulic pressure generated by their hemolymph, rather than relying solely on muscles. Muscular contractions in the spider’s body can increase the pressure of the hemolymph, forcing the legs to extend outwards. This is why a dead spider’s legs often curl inward; without the internal hydrostatic pressure, the flexor muscles contract, and the legs lose their ability to extend. During active movements like walking or jumping, hemolymph pressure increases to facilitate rapid leg extension.