Let's take a look at the insect circulatory system. As we've previously learned, insects and other arthropods have an open circulatory system. This means that the insect equivalent of blood, the hemolymph, is contained within the insect body cavity known as the hemocoel and not restricted to blood vessels like in humans. The hemolymph, however, does not come into direct contact with cells or tissues. Instead, basement membranes lie in the internal organs and the inside of the exoskeleton which creates a selective barrier between the hemolymph and the tissues. The hemolymph is a watery fluid that contains cells, inorganic ions and organic molecules such as lipids, sugars, amino acids and proteins. It is typically colorless unlike the blood of vertebrates, though in some cases, pigments provide colors such as yellow, blue and rarely even red. The hemolymph is a major component of an insect's mass. In soft body larvae, this fluid can make up 20 to 40 percent of the insect's body weight and usually makes up about 20 percent of the body weight in nymphs and adults. Hemolymph has two main components: the liquid plasma and the cells referred to as hemocytes. Plasma is the main constituent of the hemolymph and mediates all chemical exchanges including hormones, nutrients and wastes between insect tissues. For example, plasma distributes nutrients such as trehalose, the main blood sugar in many insects. Insect hormones including those that regulate growth and development are also distributed by the plasma. Maybe surprisingly, hemolymph does not transport oxygen to insect tissues. This is because it lacks oxygen carrying pigments like hemoglobin used by vertebrates such as ourselves. However, some insects that live in low oxygen environments such as stone flies and some [inaudible] larvae have specialized proteins that help store oxygen to allow them to survive in these difficult conditions. These proteins can even include hemoglobin. Although in these insects, it is primarily used for oxygen storage rather than transportation. For the most part, the circulatory and respiratory systems in insects are separate. Hemocytes are nucleated cells that serve many functions and are the main component of the insect's immune system. Some hemocytes ingest foreign particles, microorganisms or dead and dying cells while others work together to encapsulate parasites. Hemocytes are also involved in hemolymph coagulation in the event of a wound to the integument. The coagulation helps to heal the injury and inhibit the entry of harmful pathogens and foreign substances into the insect's body. In many insects, distasteful chemicals are incorporated into the hemolymph to deter hungry predators. The insect's circulatory system has a single tubular muscular vessel, the dorsal vessel, which is supported by the muscular dorsal diaphragm. Hemolymph moves into the dorsal vessel through one-way valves located along the length of the organ known as ostia. Waves of muscular contractions along the dorsal vessel push hemolymph anteriorly and out of the vessel into the insect's head. Hemolymph is then gradually directed back to the rear end of the body by the rhythmic contractions of a muscular structure known as the ventral diaphragm. Hemolymph movement into major appendages such as the antennae, wings and legs is managed by additional pulsatile pumps located at the base of each appendage. There's a regular pattern of hemolymph flow between body segments and appendages, thanks to impressive coordination between the dorsal vessel, accessory pumps, and diaphragms. The insect nervous system helps to mediate such coordination. We'll learn more about the nervous system in the next module. Gas exchange refers to the intake of oxygen and release of carbon dioxide between organic tissues and the environment. As we have discussed previously, gas exchange in insects is achieved by a network of tubes lined with cuticle known as trachea located throughout the body. The cuticular lining is reinforced along most areas by spiral thickenings that prevent it from collapsing while still retaining flexibility. Because they are cuticular in nature, these linings are shed with the rest of the exoskeleton and replaced every time the insect molts. To transport oxygen to the tissues that need it, the trachea branch into smaller tubes called tracheoles that measure less than one micrometer in diameter. Tracheoles are in close contact with tissues or even embedded into organs. Tracheoles generally occur in higher numbers in tissues that have high demand for oxygen such as the muscles that power flight. The trachea connect to the outside environment through openings in the insect body wall called spiracles. Spiracles are generally found laterally along an insect's thorax and abdomen. They can have filters or valves that open only when gas exchange is taking place. This helps prevent entry of harmful particles and microorganisms and reduces water loss from evaporation. If the spiracles are held closed, insects can quite literally hold their breath and minimize exposure to parasites or airborne insecticides. In highly active insects, the tracheal system may make up as much as 50 percent of the insect's body volume, while in relatively inactive insects, it can take up as little as five percent of the insect's body volume. One fascinating thing about tracheal cells is that they are particularly sensitive to oxygen levels. If areas with low oxygen levels are detected within the insect's body, additional tracheal tissues will grow to supply these oxygen-deprived tissues. In some insects, parts of the tracheal network maybe enlarged to form air sacs which are flexible structures with multiple functions. For instance, the air sacs act as a reservoir and their expansion and contraction can help increase the volume of air flowing through the trachea and allow more efficient distribution of oxygen. Air sacs are also involved in sound production by some insects such as cicadas. Finally, they may be compressed to provide room for the growth of internal tissues before the insect's next molt. While the insect's gas exchange system is relatively consistent across species, like most things we've discussed so far, it can be adopted for specific functions by insects in different habitats. Let's compare the gas exchange system between terrestrial and aquatic insects. Most terrestrial insects have what is called an open tracheal system, whereby spiracles connect the tracheal system to the external environment. In most aquatic insects and some parasitic larvae, the spiracles are absent which produces a closed tracheal system. In these insects, a network of trachea is formed near the insect's integument. This allows gas exchange to occur through the cuticle which may be thinner in these areas. Some aquatic insects have special respiratory structures such as gills, leaf-like extensions of the body covered by a thin layer of cuticle with a network of tracheoles located immediately underneath. So how does the tracheal system work? Airflow in the tracheal system is driven by the diffusion of gases from high to low concentration which is often supplemented by active ventilation. Oxygen enters through the spiracles and diffuses through the trachea to the tracheoles and into tissues and cells. This produces a net movement of oxygen inwards into the insect's body. At the same time, carbon dioxide moves outward through the tracheal network and is released from the spiracles. Another way in which carbon dioxide may travel through an insect's body is diffusion from the tissues through the hemolymph into trachea near the spiracles, where it is released to the external environment. This is because carbon dioxide tends to be more soluble than oxygen, and so its movement is not limited only to the tracheal network. Other than carbon dioxide, water vapor is also released from an insect during gas exchange. In terrestrial insects, the loss of water from gas exchange can be costly especially in arid habitats. Therefore gas exchange in terrestrial insects is a delicate balance between oxygen intake and water loss. Depending on the insect and its habitat, the spiracles may be left open continuously, cyclically or discontinuously. Discontinuous gas exchange is often practiced by insects during periods of inactivity or diapause. The spiracles are kept closed and opened only periodically. In insects without air sacs, gas exchange is achieved through diffusion although some insects may facilitate airflow by pumping the thorax and abdomen. Some insects also coordinate the opening and closing of the spiracles to help create unidirectional airflow in the main trachea. Specifically, anterior spiracles are open during air intake and posterior ones are opened when the air is expelled. The rate of gas exchange is typically dependent on two factors. The first is the diameter of the trachea, which can be enlarged in some species in low oxygen environments to allow greater airflow. The second factor, the distance of diffusion, is typically more important. Generally, the shorter the distance of diffusion, the greater the rate of airflow. As such, the diffusion-based nature of gas exchange in insects is a limiting factor on insect body size as oxygen diffusion can only occur effectively over short distances. Typically, the larger the insect, the greater the proportion of body mass dedicated to a tracheal network because diffusion is not efficient over large distances. To compensate for this, many large insects have long thin body plans like we see in dragonflies that result in short distances between the spiracles and the body tissues. As you can see, the circulatory and gas exchange systems in insects are completely separate. Instead of having a closed circulatory system that carries oxygen, like vertebrate blood, arthropod hemolymph is contained in an open circulatory system and does not carry oxygen. Gas exchange is instead facilitated by the tracheal network. A network of tubes lined with cuticular tissue that is shed at each molt. These systems can be variously modified to help insects survive in their variable environments.
