While spiders do not possess blood like humans, they have a fluid that serves similar purposes. This fluid circulates throughout their bodies, performing essential life-sustaining functions.
Hemolymph: The Spider’s “Blood”
Spiders circulate hemolymph, a fluid similar to blood but with distinct characteristics. Hemolymph, a mixture of water, salts, sugars, and amino acids, nourishes the spider’s tissues. It also contains specialized cells called hemocytes.
Unlike vertebrate blood, spider hemolymph does not contain red blood cells or hemoglobin for oxygen transport. Many spiders utilize copper-based hemocyanin, which binds oxygen and gives the hemolymph a blue color when oxygenated. Hemocyanin circulates freely within the hemolymph, rather than being confined within cells.
Circulation and Function
Spiders have an open circulatory system, meaning hemolymph is not always enclosed within vessels. A tubular heart pumps hemolymph through arteries. Hemolymph then flows into open spaces (sinuses or the hemocoel), directly bathing internal organs and tissues.
After circulating through the body cavities, hemolymph returns to the heart through small openings. This system transports nutrients and hormones to cells, collecting waste products for removal. Beyond these transport roles, hemolymph plays a significant part in the spider’s immune defense through its hemocytes, which can initiate coagulation and respond to pathogens.
Hemolymph’s unique function in spiders is generating hydrostatic pressure. This pressure is crucial for various movements, especially the extension of their legs, as spiders lack extensor muscles in many leg joints. By increasing hemolymph pressure, spiders extend their limbs, crucial for molting and maintaining body shape.
Key Differences from Vertebrate Blood
The primary distinction between spider hemolymph and vertebrate blood lies in their oxygen-carrying mechanisms and circulatory system structures. Vertebrate blood contains red blood cells filled with iron-based hemoglobin, which gives blood its red color and efficiently transports oxygen. Spider hemolymph, however, uses copper-based hemocyanin for oxygen binding, giving it a blue or colorless appearance.
Unlike vertebrates with their closed circulatory system, where blood continuously flows within vessels, spiders have an open system where hemolymph directly contacts tissues in body cavities. This open system in spiders is less efficient for oxygen transport per unit volume compared to the high-pressure closed systems of vertebrates. The hydrostatic function of hemolymph for leg extension is also a notable difference, as vertebrates rely solely on muscle action for limb movement.