The blood-like fluid circulating in an insect’s open circulatory system is hemolymph.
Hemolymph is the arthropod equivalent of blood in appearance and several functions, but it is not identical to vertebrate blood. It fills the insect’s hemocoel, the main internal body cavity, and bathes organs directly rather than remaining confined to a network of veins and arteries. A dorsal heart and aorta help move it through the body.
Hemolymph contains a liquid plasma-like component, dissolved nutrients, proteins, hormones, and immune cells called hemocytes. It helps transport materials and can also act hydraulically, helping insects expand newly molted body parts before their outer cuticle hardens.
The most important distinction is respiratory. In most insects, hemolymph does not carry oxygen around the body. Air reaches tissues through a branching tracheal system, while hemolymph mainly handles nutrients, chemical signals, waste products, and immune defense. “Plasma” is therefore only one component of hemolymph, not the complete answer.