Spiders possess a blue-tinged circulatory fluid scientifically termed hemolymph, which is not technically blood like that found in vertebrates. This unique fluid serves multiple functions beyond simple circulation. Its pale blue color is a direct consequence of the spider’s distinct internal anatomy and the specific chemistry used for oxygen transport throughout the body.
Hemolymph: The Spider’s Circulatory Fluid
Hemolymph is the bodily fluid that circulates within the spider, transporting nutrients, wastes, and respiratory gases. Unlike the closed system of arteries, veins, and capillaries found in mammals, spiders possess an open circulatory system. The heart, a long, tube-like structure in the abdomen, pumps the hemolymph into a large body cavity called the hemocoel.
The hemolymph flows freely within the hemocoel, directly bathing the internal organs and tissues. This direct contact facilitates the exchange of substances before the fluid is collected and returned to the heart through small, valved openings called ostia. Due to this open circulation, hemolymph acts as both blood and the interstitial fluid surrounding cells.
The Chemistry of Blue Blood
The distinctive blue coloration of spider hemolymph results from the respiratory protein hemocyanin, which carries oxygen. Hemocyanin is a metalloprotein that relies on copper atoms to bind oxygen molecules. When oxygen attaches to the copper center, the molecule absorbs light differently, causing the fluid to appear blue or blue-green.
This mechanism contrasts with the red blood found in vertebrates, which uses the protein hemoglobin. Hemoglobin uses iron atoms as its oxygen-binding center, reflecting a red color when oxygenated. The difference in the metal ion used—copper instead of iron—is the reason for the color difference.
When oxygen is released to the spider’s tissues, the copper center reverts to its deoxygenated state, causing the fluid to lose its blue hue and become colorless or pale grey. Hemocyanin is not packaged within specialized cells like human red blood cells. Instead, this large, complex protein is dissolved directly into the hemolymph, floating freely throughout the hemocoel.
Hemocyanin molecules are significantly larger than hemoglobin, and they can bind multiple oxygen molecules. The concentration of this copper-based protein can account for a substantial portion of the hemolymph’s total protein content. This free-floating arrangement allows for efficient oxygen delivery throughout the spider’s open circulatory system, supporting the arachnid’s metabolic needs.
More Than Just Oxygen Transport
Beyond its function in gas exchange, hemolymph plays a crucial mechanical role, acting as a hydraulic system essential for the spider’s movement and locomotion. Spiders lack the extensor muscles necessary to push their legs outward at certain key joints, such as the femur-patella and tibia-metatarsus joints.
To extend their legs, spiders rapidly increase the hemolymph pressure within their cephalothorax, the fused head and mid-section. This pressure forces the fluid into the legs, hydraulically extending them to enable walking, running, and jumping. During normal walking, leg pressure is regulated between 4 and 8 kilopascals, but it can surge to 60 to 130 kilopascals during explosive movements.
This reliance on hydraulic pressure explains why a deceased spider’s legs curl inward; without internal fluid pressure, the flexor muscles contract unopposed. In addition to this mechanical role, the hemolymph is the foundation of the spider’s immune system. Circulating cells called hemocytes are dispersed throughout the fluid and are responsible for various defense mechanisms.
Hemocytes recognize and respond to foreign invaders, performing cellular actions such as phagocytosis, where they engulf and destroy pathogens. The immune system also relies on a sophisticated clotting cascade within the hemolymph to prevent fluid loss when the exoskeleton is damaged. This clotting mechanism works with the secretion of antimicrobial peptides, specialized proteins that attack and neutralize bacteria and fungi.