Spiders do not possess red blood like humans and other vertebrates, but they circulate a fluid called hemolymph that performs the same function. Hemolymph transports nutrients, waste, and respiratory gases throughout the spider’s body. The difference lies in its composition and the circulatory system it travels through, which gives spider “blood” unique properties. This fluid sustains the arachnid’s metabolic functions.
The Spider’s Circulatory System
Spiders utilize an open circulatory system, unlike the closed network of arteries and veins found in mammals. Instead of remaining entirely confined to vessels, the hemolymph is pumped into a large central cavity called the hemocoel. The heart is a long, tube-like structure located dorsally in the abdomen, which pumps the hemolymph out through arteries.
After leaving the vessels, the fluid bathes the internal organs and tissues directly within the hemocoel, exchanging substances before returning to the heart. Small, valved openings known as ostia allow the hemolymph to re-enter the heart when the pressure drops. This system is highly effective for the spider’s body structure.
The Chemical Basis of Oxygen Transport
The difference between hemolymph and vertebrate blood is the respiratory protein used to carry oxygen. Vertebrates use hemoglobin, an iron-based protein that gives blood its red color. Spider hemolymph relies on a protein called hemocyanin.
Hemocyanin uses copper atoms at its core to bind oxygen molecules, contrasting with the iron in hemoglobin. When deoxygenated, the hemolymph is colorless or a pale grey. When the copper atoms bind with oxygen, the fluid turns a distinct blue-green color.
Unlike hemoglobin, which is packaged inside specialized red blood cells, hemocyanin is dissolved directly into the hemolymph fluid. Hemocyanin is also involved in the clotting and immune responses of the spider.
Hemolymph’s Role in Movement and Molting
Beyond gas and nutrient transport, hemolymph serves as a hydraulic system to extend the spider’s legs. Spiders lack the opposing extensor muscles necessary to push their legs outward at certain joints.
To move, a spider rapidly increases hemolymph pressure in its cephalothorax, the fused head and mid-section. This pressure forces the fluid into the legs, hydraulically extending them for walking, running, and jumping. When a spider dies, this hydraulic pressure is lost, which is why its legs curl inward.
The hemolymph’s hydraulic power is also instrumental during ecdysis, the molting process. The spider uses internal hemolymph pressure to swell and break free from its rigid, old exoskeleton. This pressure is then used to inflate the body and stretch the new exoskeleton before it hardens.