Hemolymph

Hemolymph is the fluid that fills the body cavity of insects and other arthropods and of most mollusks. It combines the roles of blood and of the fluid around the tissues, and it is pumped by a tubular heart in an open circulatory system.

Hemolymph is mostly water with dissolved sugars, salts, amino acids and proteins, along with free-floating cells called hemocytes. In an open circulatory system the heart pumps it into the body cavity, where it bathes the organs directly before returning through small openings. In insects it does not usually carry oxygen, because air is delivered by a network of tracheal tubes; crustaceans and horseshoe crabs, in contrast, carry oxygen with a copper-containing pigment, hemocyanin, which turns the fluid blue, as in the Atlantic horseshoe crab.

It transports nutrients, hormones and waste, and it provides hydraulic pressure that helps insects to molt, expand their wings and extend their limbs. Hemocytes engulf invaders and form capsules around parasites, and clotting and melanization seal wounds. Horseshoe crab hemocytes were the source of a reagent used to test medicines for bacterial toxins.

When a mosquito such as Aedes aegypti feeds on infected blood, an arbovirus must escape the gut into the hemolymph, spread through the body and reach the salivary glands before it can be transmitted. The insect's immune responses in the hemolymph, including those used in Wolbachia experiments, shape how well it carries a pathogen, a topic of vector control.

• Arthropod — the animal phylum that uses it
• Salivary Gland — a site viruses must reach
• Hemoglobin — the oxygen carrier of vertebrate blood