Hemolymph orhaemolymph is abody fluid that circulates insidearthropod bodies transportingnutrients andoxygen totissues, comparable with theblood invertebrates. It is composed of aplasma in which circulatingimmunecells calledhemocytes are dispersed in addition to manyplasma proteins (hemoproteins) and dissolved chemicals. It is the key component of theopen circulatory system characteristic of arthropods such asinsects,arachnids,myriopods andcrustaceans.[1][2] Some non-arthropodinvertebrates such asmolluscs andannelids also possess a similar hemolymphatic circulatory system.
In insects, the largest arthropodclade, the hemolymph mainly carries nutrients but not oxygen, which is supplied to the tissues separately by direct deep ventilation through an extensivetracheal system. In other arthropods, oxygen is dissolved into the hemolymph fromgills,book lungs oracross the cuticle and then distributed to the body tissues via thehemocoel.
Hemolymph fills the whole interior (thehemocoel) of the animal's body and surrounds all cells.
In thegrasshopper, 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 chambers — called sinuses — of thehemocoel 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 — calledostia — in the walls of the hearts open and allow hemolymph to enter.
Hemolymph containshemocyanin, acopper-based protein that turns blue when oxygenated, causing the hemolymph to turn from grey to blue-green in color. This contrasts with theiron-basedhemoglobin found in thered blood cells of vertebrate blood which turns a brighter red when oxygenated.
The hemolymph of lower arthropods, including mostinsects, contains nutrients such as proteins and sugars but is not used for oxygen transport. These animalsrespirate through other means, such as tracheas. Ancestral and functionalhemocyanin has, however, been found in the hemolymph of someinsects.[3] Insect hemolymph generally does not carryhemoglobin, but hemoglobin may be present in the tracheal system and may play some role in respiration there.[4]
Muscular movements by the animal duringlocomotion can facilitate hemolymph movement, but diverting flow from one area to another is limited.[5]
Hemolymph can contain nucleating agents that confer extracellular freezing protection. Such nucleating agents have been found in the hemolymph of insects of several orders, i.e.,Coleoptera (beetles),Diptera (flies), andHymenoptera.[6]
Hemolymph is composed ofwater,inorganicsalts (mostlysodium,chlorine,potassium,magnesium, andcalcium), andorganic compounds (mostlycarbohydrates,proteins, andlipids). The primary oxygen transporter molecule ishemocyanin.[7][3]
Arthropod hemolymph contains high levels of free amino acids. Most amino acids are present but their relative concentrations vary from species to species. Concentrations of amino acids also vary according to the arthropod stage of development. An example of this is the silkworm and its need for glycine in the production of silk.[8]
Proteins present in the hemolymph vary in quantity during the course of development. These proteins are classified by their functions: chroma proteins, protease inhibitors, storage, lipid transport, enzymes, the vitellogenins, and those involved in the immune responses of arthropods. Some hemolymphic proteins incorporate carbohydrates and lipids into the structure.[9]
Nitrogen metabolism end products are present in the hemolymph in low concentrations. These includeammonia,allantoin,uric acid, andurea. Arthropodhormones are present, most notably the juvenile hormone.Trehalose can be present and sometimes in great amounts along withglucose. These sugar levels are maintained by the control of hormones. Othercarbohydrates can be present. These includeinositol,sugar alcohol, hexosamines,mannitol,glycerol and those components that are precursors tochitin.[1]
Freelipids are present and are used as fuel for flight.[10]
There are free-floating cells, thehemocytes, within the hemolymph. They play a role in the arthropodimmune system.
Thisopen system might appear to be inefficient compared to the closed circulatory systems of thevertebrates, but the two systems have very different demands 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 thetracheal system. Hemolymph plays no part in the process in most insects. Only in a few insects living in low-oxygen environments are there 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 thecopper-containinghemocyanin, however, for oxygen transport.[11]
In some species, hemolymph has other uses than just being a blood analogue. As the insect or arachnid grows, the hemolymph works something like a hydraulic system, enabling the insect or arachnid to expand segments before they aresclerotized. It can also be used hydraulically as a means of assisting movement, such as inarachnid locomotion. Some species of insect or arachnid are able toautohaemorrhage when they are attacked by predators.[12] Queens of the ant genusLeptanilla are fed with hemolymph produced by thelarvae.[13] On the other hand,Pemphigus spyrothecae utilize hemolymph as an adhesive, allowing the species to stick to predators and subsequently attack the predator; it was found that with larger predators, more aphids were stuck after the predator was defeated.
