Hemolymph, or haemolymph, is a fluid in the circulatory system of some arthropods (including spiders, crustaceans such as crabs and shrimp, and even some insects such as stoneflies) and is analogous to the fluids and cells making up both blood and interstitial fluid (including water, proteins, fats, sugars, hormones, etc.) in vertebrates such as birds and mammals. In addition, some non-arthropods such as molluscs possess a hemolymphatic circulatory system.

Hemolymph fills all of the interior (the hemocoel) of the animal's body and surrounds all cells. It contains hemocyanin, a copper-based protein that turns blue in color when oxygenated, instead of the iron-based hemoglobin in red blood cells found in vertebrates, thus giving hemolymph a blue-green color rather than the red color of vertebrate blood. When not oxygenated, hemolymph quickly loses its color and appears grey. The hemolymph of lower arthropods, including most insects, is not used for oxygen transport because these animals respirate directly from their body surfaces (internal and external) to air, but it does contain nutrients such as proteins and sugars.

Hemolymph can contain nucleating agents that confer extra cellular freezing protection. Such nucleating agents have been found in the hemolymph of insects of several orders, i.e., Coleoptera (beetles), Diptera (flies), and Hymenoptera.

Muscular movements by the animal during locomotion can facilitate hemolymph movement, but diverting flow from one area to another is limited. When the heart relaxes, blood is drawn back toward the heart through open-ended pores (ostia).

Hemolymph is composed of water, inorganic salts (mostly Na+, Cl-, K+, Mg2+, and Ca2+), and organic compounds (mostly carbohydrates, proteins, and lipids). The primary oxygen transporter molecule is hemocyanin.

There are free-floating cells, the hemocytes, within the hemolymph. They play a role in the arthropod immune system. The immune system resides in the hemolymph. As the insect grows, the hemolymph works something like a hydraulic system, enabling the insect to expand segments before they are sclerotized.

The volume of hemolymph needed for such a system is kept to a minimum by a reduction in the size of the body cavity. The hemocoel is divided into chambers called sinuses.

In the grasshopper, the closed portion of the system consists of tubular hearts and an aorta running along the dorsal side of the insect. The hearts pump hemolymph into the sinuses of the hemocoel where exchanges of materials take place.

Coordinated movements of the body muscles gradually bring the hemolymph back to the dorsal sinus surrounding the hearts. Between contractions, tiny valves in the wall of the hearts open and allow hemolymph to enter.

This "open" system might appear to be inefficient compared to closed circulatory systems like those possessed by mammals, but the two have very different demands being placed on them. In vertebrates, the circulatory system is responsible for transporting oxygen to all the tissues and removing carbon dioxide from them. It is this requirement that establishes the level of performance demanded of the system. The efficiency of the vertebrate system is far greater than is needed for transporting nutrients, hormones, and so on, whereas in insects, exchange of oxygen and carbon dioxide occurs in the tracheal system. Hemolymph plays no part in the process in most insects. In a few insects living in low-oxygen environments, there are hemoglobin-like molecules that bind oxygen and transport it to the tissues. Therefore, the demands placed upon the system are much lower. Some arthropods and most molluscs possess the copper-containing hemocyanin, however, for oxygen transport.

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