Hemolymph, or haemolymph, is a fluid, similar to the blood in

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s, that circulates in the inside of the arthropod's body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which hemolymph cells called hemocytes are dispersed. In addition to hemocytes, the plasma also contains many chemicals. It is the major tissue type of the open circulatory system characteristic of

arthropods Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

(for example,

arachnid Arachnids are arthropods in the Class (biology), class Arachnida () of the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, opiliones, harvestmen, Solifugae, camel spiders, Amblypygi, wh ...

s, crustaceans and

insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

s). In addition, some non-arthropods such as mollusks possess a hemolymphatic circulatory system. Oxygen-transport systems were long thought unnecessary in insects, but ancestral and functional hemocyanin has been found in the hemolymph. Insect "blood" generally does not carry

hemoglobin Hemoglobin (haemoglobin, Hb or Hgb) is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. Hemoglobin ...

, although hemoglobin may be present in the tracheal system instead and play some role in respiration.

Method of transport

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. 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. 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. Hemolymph fills all of the interior (the hemocoel) of the animal's body and surrounds all cells. It contains hemocyanin, a

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

-based protein that turns blue when oxygenated, instead of the

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

-based

in red blood cells found in vertebrates, 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 Insects (from Latin ') are hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed ...

, is not used for oxygen transport because these animals respirate through other means, such as tracheas, but it does contain nutrients such as proteins and sugars. Muscular movements by the animal during locomotion can facilitate hemolymph movement, but diverting flow from one area to another is limited. When the

heart The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...

relaxes, hemolymph is drawn back toward the heart through open-ended pores called ostia. Note that the term " ostia" is not specific to insect circulation; it literally means "doors" or "openings", and must be understood in context.

Constituents

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), and Hymenoptera.

Inorganic

Hemolymph is composed of

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

inorganic An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemistry''. Inor ...

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

s (mostly

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

chlorine Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

, and calcium), and

organic compounds Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

(mostly

carbohydrate A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s, and

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

s). The primary oxygen transporter molecule is hemocyanin.

Amino acids

Arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

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.

Proteins

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.

Other organic constituents

Nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

metabolism end products are present in the hemolymph in low concentrations. These include

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

allantoin Allantoin is a chemical compound with formula C4H6N4O3. It is also called 5-ureidohydantoin or glyoxyldiureide. It is a diureide of glyoxylic acid. Allantoin is a major metabolic intermediate in most organisms including animals, plants and bacter ...

uric acid Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the Chemical formula, formula C5H4N4O3. It forms ions and salts known as urates and acid urates, such as ammonium acid urate. Uric acid is a product of the meta ...

, and

urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

. Arthropod

hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...

s are present, most notably the juvenile hormone. Trehalose can be present and sometimes in great amounts along with

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

. These sugar levels are maintained by the control of hormones. Other

s can be present. These include inositol, sugar alcohol, hexosamines, mannitol,

glycerol Glycerol () is a simple triol compound. It is a colorless, odorless, sweet-tasting, viscous liquid. The glycerol backbone is found in lipids known as glycerides. It is also widely used as a sweetener in the food industry and as a humectant in pha ...

and those components that are precursors to

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

. Free

s are present and are used as fuel for flight.

Hemocytes

There are free-floating cells, the hemocytes, within the hemolymph. They play a role in the arthropod

immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...

. The immune system resides in the hemolymph.

Comparisons to vertebrates

This open system might appear to be inefficient compared to the closed circulatory systems of the

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s, 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 the tracheal 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 the

-containing hemocyanin, however, for oxygen transport.

Specialist uses

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 are sclerotized. It can also be used hydraulically as a means of assisting movement, such as in arachnid locomotion. Some species of insect or arachnid are able to autohaemorrhage when they are attacked by predators. Queens of the ant genus '' Leptanilla'' are fed with hemolymph produced by the

larvae A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect developmental biology, development such as insects, some arachnids, amphibians, or cnidarians typical ...

.Genus ''Leptanilla''
Australian Ants Online 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.

References

Sources

External links

* * {{cite web , last1=Bolstad , first1=Kat , title=Blue Squid Blood - Murky Water , url=http://blog.tepapa.govt.nz/2008/05/02/murky-water , website=Te Papa Tongarewa Museum of New Zealand , date=May 2, 2008 , archive-url=https://web.archive.org/web/20150402094309/http://blog.tepapa.govt.nz/2008/05/02/murky-water/ , archive-date=April 2, 2015 Blood Arthropod anatomy Insect anatomy Crustacean anatomy Arachnid anatomy Body fluids