Spiders do not possess red blood in the same way vertebrates do. Instead, they have a unique circulatory fluid that performs similar functions, but its composition and circulation method result in a very different appearance. This fluid is crucial for their survival, transporting necessary substances throughout their bodies.
How Spiders Circulate Fluids
Spiders utilize an open circulatory system. In this system, a tubular heart, located dorsally within the abdomen, pumps the circulatory fluid through a few main arteries. Once pumped, this fluid flows into open spaces within the body cavity, called sinuses, directly bathing the organs and tissues. The fluid then returns to the heart through small openings called ostia, which have one-way valves. This hydraulic system also plays a role in a spider’s locomotion, as pressure from this fluid helps extend their legs.
What is Hemolymph?
The circulatory fluid in spiders is called hemolymph. It is primarily composed of water, along with various dissolved substances such as ions, carbohydrates, lipids, amino acids, and hormones. Hemocyanin, a protein responsible for oxygen transport, contains copper. Unlike hemoglobin, which contains iron and gives blood its red color, hemocyanin causes hemolymph to appear blue or pale blue when oxygenated. Deoxygenated hemolymph can appear nearly clear or even slightly greenish.
Beyond oxygen transport, hemolymph carries nutrients to cells, removes metabolic waste products, and distributes hormones throughout the spider’s body. It also contains specialized cells called hemocytes, which are analogous to immune cells in vertebrates. These hemocytes play a crucial role in the spider’s immune system, defending against pathogens and initiating wound healing processes.
Spider Healing and Injury Response
When a spider is injured, hemolymph leaks from the wound. Due to their open circulatory system, even a small injury can result in significant fluid loss, which can be life-threatening for the spider. To counter this, spiders have developed effective mechanisms to prevent excessive hemolymph loss and seal wounds. This involves a rapid coagulation or clotting process.
Hemocytes are central to this clotting response. They quickly aggregate at the site of injury, forming a plug that helps to seal the breach. Other factors within the hemolymph, including hemocyanin, also contribute to the formation of this clot. This immediate immune and clotting response is vital for minimizing fluid loss and preventing infections from entering the body cavity. Some spiders can even self-amputate a leg, a process called autotomy, to prevent further bleeding and escape from predators.