<p>In reality, insects (and other groups of arthropods, molluscs, etc.) do not exactly have blood, but a similar liquid called "Hemolymph" ("Hemo" means blood).</p>
<p>This liquid has very variable colors, from orange to greenish, there being cases in which it is colorless. Its color is due to different pigments, most of which depend on the insect's food.</p>
<p>Haemolymph is a fluid that invertebrates possess. It transports the nutrients that feed the tissues and participates in the moulting of skin, among other important functions.</p>
<p>All animals have a circulating liquid in charge of transporting, with the help of the circulatory system, substances with respiratory pigments or organic molecules, formed by a protein and a particle that has affinity with oxygen (darkbiologist, 2017).</p>
<p>In the different groups of animals, besides hemolymph, there are other transport liquids; these are blood, lymph and hydrolymph.</p>
<p>Blood is a fluid that has respiratory pigments, such as hemoglobin, which has iron ions that give it its characteristic red color. It is characteristic of annelids, such as leeches and earthworms, and of vertebrates.</p>
<p>Lymph is a fluid that is found only in vertebrates and that allows liquids to circulate between cells.</p>
<p>On the other hand, hydrolympha is a colourless fluid, similar in composition to seawater, typical of echinoderms such as sea urchins and starfish (López, 2017).</p>
<p>Normally, the haemolymph represents between 5 and 40 % of the weight of the individual, depending on the species.</p>
<p>There are many differences in the way fluids circulate in vertebrates and invertebrates. One of the most significant is that the haemolymph does not carry oxygen to the organs from the lungs and carries carbon dioxide (Contreras, 2016).</p>
<p>This is because insects do not breathe through the lungs but, because of their small size, can make a passive exchange of gases through the skin and tracheas, a system of channels open to the outside that run through the body.</p>
<p>The haemolymph does not directly irrigate all the cells and organs of the insects' body, but the integument that covers them has a basal membrane of connective tissue, whose properties control the exchange of materials between the cells and the haemolymph.</p>
<p>In the blood, the pigment that transports oxygen is hemoglobin, but as in insects the transport of oxygen is not of vital importance, hemolymph does not possess hemoglobin; therefore it is of other colors, or even transparent.</p>
<p>However, in both molluscs and arthropods, haemolymph has haemocyanin, an oxygen-carrying molecule that contains copper.</p>
<p>Due to the presence of hemocyanin, the circulatory fluid of these organisms turns blue green when oxygenated; otherwise, it is gray or colorless.</p>
<p>In contrast, vertebrate hemoglobin has iron, which makes it bright red when oxygen is present, or dark red (brown) when oxygen is absent (McCarthy, 2017).</p>
<p>Certain insects and some mollusks that live in environments with low oxygen content also have circulatory fluid that contains hemoglobin, which gives it the red appearance of vertebrate blood.</p>
<p>How is hemolymph transported?</p>
<p>In arthropods, the circulatory system is open, there are no tubes or channels through which the hemolymph is distributed, but rather it exits through the anterior orifice of the circulatory system and is distributed throughout the body more or less freely. The organs are then bathed directly by it.</p>
<p>Circulation is normally induced by one or more tubular hearts. These are endowed with various lateral orifices, called ostioles, which help haemolymph enter them. The front part of the vessel is called the aorta and is a straight tube without valves.</p>
<p>Body movements return fluid within the circulatory system to a cavity surrounding the heart(s).</p>
<p>The heart sucks the hemolymph from the abdominal cavity and expels it to the head, through the aorta, from where it filters back through the tissues into the abdominal cavity. In some insects there are pumps attached to irrigate to the extremities and antennas.</p>
<p>Functions</p>
<p>The main functions of the haemolymph are:</p>
<p>- To transport nutrients to feed the tissues and collect waste materials, which are taken to the excretory organs.</p>
<p>- Thanks to hemocytes, it helps coagulation to close wounds.</p>
<p>- Prevents microbial invasion, helping the defenses.</p>
<p>- Transports oxygen, mainly in water insects because, in general, oxygen is carried directly through the tracheal system, without intervention of the circulatory system.</p>
<p>- It conducts hormones, performing important functions in metabolism.</p>