haemo- (relation or resemblance to blood, association with blood;) - lymph (almost colourless fluid that bathes body tissues)
Haemolymph is the "blood" of insects. It is the watery fluid that fills the haemocoel. Haemolymph contains ions, molecules and cells. Often clear and colourless in most insects, some insects haemolymph may contain various pigments, making it appear yellow, blue, green, and in some rare cases of immature aquatic and endoparasitic flies, red due to the presence of haemoglobin.
All chemical exchanges between insect tissues are mediated through the haemolymph. The main difference between insect haemolymph and vertebrate blood is that haemolymph rarely contains respiratory pigments, and has a very low oxygen-transportation capacity. Respiration takes place in the tracheal system, which is mediated by the internal pressure of the haemolymph (the trachea opens and closes, creating a vacuum, as the haemolymph pressure changes).
Haemolymph is important to insect ventilation, thermoregulation, and molting (in breaking the old cuticle and expanding the new cuticle).
Haemolymph is a reserve of water for the insect. The soft-bodied insect larvae can be 20-40% haemolymph by weight, and the adult form is usually a bit less than 20% haemolymph (desiccation is always a problem for adult insects). The main constituent of haemolymph is plasma. Insect plasma is characterized by high concentrations of amino acids and organic phosphates.
Haemolymph not only provides nutrient transfer in the body, it provides protection for the insect. Haemolymph provides protection and defence from physical injury, disease organisms, parasites or other foreign objects entering the body, and sometimes from predation. In some insects, the haemolymph contains distasteful chemicals, which make the insect taste bad to predators (see aposematism). While an insect does have an immune system, please note that it is not as highly specialized as is the complex immunoglobin-based vertebrate immune system.