Blood Circulation of Mulberry Silkworm

Through the blood flowing between the various tissues and organs in the body cavity, nutrients are transported to each part of the silkworm body, and the metabolic waste of each part is transported to the excretory organs to build the silkworm body and maintain the physiological process of life. The blood circulatory system of silkworm is composed of three parts: circulating blood, back blood vessel of power mechanism, and body cavity of circulation place. It is an open circulatory system.

The chemical composition of blood

Mainly include water, protein, carbohydrates, inorganic and organic acids and a small amount of lipids, vitamins, pigments, etc. The blood volume of silkworms accounts for 21-25% of silkworm body weight, the water content is 90-95%, and the water content of silkworms is about 40%. The organic matter in the dry matter accounts for 5 to 9.1%, and the inorganic matter accounts for 0.5 to 0.9%. The dry matter composition of 100 ml blood of the 5th instar silkworm is as follows. In addition to protein and amino acids, nitrogen in the blood also contains uric acid. Its content is about 10% mg during the larval feeding period, up to 16% mg for mature silkworms, and about 5% mg during other periods. The carbohydrates in the blood are mainly trehalose, which is a non-reducing disaccharide, which occupies an important part of the sugar storage. In addition, it also contains a small amount of glucose, fructose and sucrose. In 100 milliliters of larvae blood contains 400 to 500 mg of trehalose, and 202 mg in the pupal stage. Trehalose is not only used as an energy source for silkworm material metabolism, but also as a raw material for the synthesis of chitin during molting. There are particularly high amounts of organic acids in the blood, including citric acid, malic acid, succinic acid and several α-keto acids. The maximum amount of citric acid reaches 32 milligrams. Inorganic substances in the blood have the highest content of potassium, phosphorus, and iron, followed by calcium and sodium. In addition, there are a small amount of chlorine, sulfur, iron, silicon, manganese, copper, zinc and so on.

The composition of the 5th instar silkworm in 100 ml of blood

Ingredients	Content (g)	Ingredients	Content (g)
Protein amino acid Other non-protein nitride trehalose	1.9 1.2 1.9 0.4	Phosphate Citrate Sodium, Potassium, Calcium, Magnesium, Chlorine	1.1 0.6 0.4

The volume of blood cells in the blood of silkworms is only 0.034 to 0.605%, and there are 1,000 to 8,000 blood cells per 1 cubic millimeter of blood, but they vary depending on the silkworm species, development period, male and female, and body position. In the process of development, it increases with the instar of silkworms. In the same instar, the dormant period is the most, and the silkworms start to full feeding period gradually decrease. There are more female silkworms than male silkworms. The blood cells of silkworms can be divided into five types: primordial white blood cells, plasma cells, granular cells, small spherical cells and giant cells. The main function of blood cells is phagocytosis, swallowing foreign bodies in the blood and dissociated tissues produced in metamorphosis.

Physical and chemical properties of blood

Silkworm blood is colorless or light yellow, and its specific gravity is slightly larger than that of water, ranging from 1.01 to 1.05 (average 1.073). The increase or decrease of blood proportion is closely related to the larvae eating mulberry, and it usually increases with the increase of the larvae eating mulberry. The specific gravity of blood cells is slightly larger than that of blood, and the average specific gravity in each period is about 1.732. The changes in the specific gravity of blood cells are consistent with the tendency of changes in the specific gravity of blood. The osmotic pressure of the blood of the 5th instar silkworm is represented by the freezing point drop of 0.5 to 0.6°C.

The blood of silkworms is slightly acidic, with a pH of 6.2 to 6.8, which varies with silkworm species, breeding environment and developmental period. During an instar period, the pH value of the blood is relatively small when silkworms are raised, and gradually increases after eating mulberries, reaching the highest before and after the full feeding period, and then gradually decreasing. Since the blood contains a certain buffer system, such as carbonate, phosphate, protein, amino acid, etc., these substances have the effect of regulating the pH in the blood, so the range of blood pH is not large.

Enzymes in the blood

Silkworm blood contains oxidase, carbohydrase, lipase and so on. Oxidase enzymes include catalase, tyrosinase, dihydroxyphenylalanase and polyphenol oxidase. There are two types of proteases: acid proteases acting on acidic substrates and alkaline proteases acting on alkaline substrates. Acid proteases have a greater ability to decompose proteins. The carbohydrases in the blood are trehalase, amylase, maltase and sucrase. The activity of trehalase appears in every sleep and cocooning period, and is in an inactive state during mulberry eating.

Larval dorsal vessel

The dorsal blood vessel of the larva is a simple long tube running from the head to the 9th abdominal segment, thin in the front and thick in the back, slightly flattened, located below the dorsal midline body wall and above the digestive tube. The dorsal blood vessels can be divided into two parts, the front part is distributed in the head and the first thoracic segment. It enters the head along the back of the throat next to the esophagus and opens slightly in front of the brain. This part is relatively short and round. Shape, called large blood vessels (or aorta). The posterior part occupies most of the dorsal blood vessels, distributed from the second thoracic segment to the ninth abdominal segment, and gradually becomes flat. When the eighth abdominal segment expands into the caudal horn, the end ends in a blind tube. The thick part is the pulsating part of the dorsal blood vessel, called the heart.

On both sides of the back of the heart, between the second thoracic segment and the ninth abdominal segment, each link has a pair of small holes, which are the holes for blood to enter the dorsal blood vessel, called the heart gate, there are a total of 11 pairs. The ventral valve is an opening from the rear to the outside of the tube wall to the front and the inside. The valve between the inner and outer holes has a valve function. When the tube wall contracts, the blood pressure in the tube rises and the ventral valve closes, allowing blood to flow forward.

On both sides of the dorsal blood vessel, between the second abdominal segment and the ninth abdominal segment, each link has a pair of muscles attached. This type of muscle is called pterygoid muscle, and there are 8 pairs in total. The pterygoid muscle is composed of many filamentous muscles, which are triangular in shape. The bottom side is attached to the tube wall, and the top side is attached to the body wall above and in front of the valve. The pterygoid muscle has the function of fixing the position of the dorsal blood vessel and assisting the heartbeat.

Ⅰ, Ⅱ, Ⅲ, the first to third thoracic segments

1～9. The 1st～9th abdominal segment

10. Pterygoid muscle

11. Heart

12. Adipose tissue

The wall of the dorsal blood vessel is mainly composed of the muscle layer. There are membranes formed by connective tissue inside and outside the muscle layer, which are called adventitia and intima. There are many cells attached to the inner and outer walls of the dorsal blood vessels, called pericardial cells. Nerves are distributed on the dorsal blood vessels, and tracheal branches are also distributed on the back end of the dorsal blood vessels.

Adult dorsal blood vessel

The dorsal blood vessel of the adult worm starts from the head to the seventh abdominal segment, and ends in a blind tube at the end. The part on the abdomen is slightly thicker, next to the body wall, and is called the heart; on the head and chest, it has a thin tube, called the great blood vessel. Due to the muscular head of the adult worm, when the dorsal blood vessel enters the posterior chest from the abdomen, it leaves the body wall, turns to the ventral surface and extends near the foregut. After entering the mid-thorax, it passes through the longitudinal muscles and turns sharply upward until it is close to the body wall and then flexes. Go down along the back of the esophagus into the head and pass the underside of the brain, opening slightly in front of the brain.

There are a total of 10 pairs of the heart ports of the adult dorsal blood vessels, and the last pair of the heart ports of the larval stage disappears. There are also 8 pairs of pterygoid muscles on both sides of the dorsal blood vessel.

Blood circulation

Silkworm blood circulates in the body, mainly relying on the pulsation of the heart of the dorsal blood vessel and the activity of the pterygoid muscles. There are certain ways to circulate in the body, but sometimes reverse pulse phenomenon may also occur.

The beating of the heart. The heart part of the dorsal blood vessel of the larva is the power organ for the blood of the larva. Due to the contraction of the muscles of the tube wall, the dorsal blood vessels continue to expand and contract rhythmically from back to front, which is called pulse. When the pterygoid muscle contracts and the tube wall relaxes, the blood pressure in the tube decreases, the heart door opens, and blood enters the tube; when the pterygoid muscle relaxes and the tube wall contracts, the dorsal blood vessel returns to its original shape, the blood pressure in the tube increases, and the heart door closes; therefore, the dorsal blood vessel keeps showing The wave-like pulsation causes the blood to flow forward. The pulse rate of the dorsal blood vessel varies greatly depending on the silkworm species, development period, activity status and feeding conditions, and generally decreases with development. At a temperature of 21 to 22 ℃, the first to fourth instar is 50 per minute. ~ 70 times, the 5th instar is 40-50 times; in the same age, the number of silkworm raising is less, the full feeding period is the most, and then it will be reduced later. In feeding, mulberry is the most, followed by moving, and least at rest. It also decreases when the nutritional status is poor. Within a certain temperature range, the number of times increases as the temperature rises, and the humidity and temperature tend to be the same, but not as significant as the temperature. There are also differences in the pulse frequency between the varieties. The Chinese variety is the most, the European variety is the least, and the Japanese variety is between the two. The frequency of the 2 chemical varieties is more than the 1 chemical variety.

Blood circulation route. When the wall of the dorsal blood vessel dilates and the volume of the lumen increases, the blood pressure in the tube is lower than that in the body cavity, so blood enters the dorsal blood vessel from the last two pairs of hilums, and only a small amount of blood flows into the middle hilum. When the tube wall contracts, the heart gate closes, the volume of the tube becomes smaller, and the blood pressure in the tube increases. Because the pulsation energy of the posterior dorsal blood vessel is greater than that of the front, the end of the dorsal blood vessel is a blind tube, so the blood entering the tube is forced forward. Advance; until the opening at the front end and the portal of the second and third thoracic segments flow out and enter the body cavity. At this time, the blood pressure in the front body cavity increases, and the blood in the back of the body cavity, because it enters the heart portal, the blood pressure decreases, so the blood flows to the back of the body cavity , And then enter the tube from the last two pairs of hearts, and continue to circulate. The blood of the 5th instar silkworm circulates in the body once, and it takes about 30 to 60 seconds when the temperature is below 25°C.

Reverse pulse. The dorsal blood vessel in the larval stage generally pulsates from the posterior direction, but before and after the pupal stage, the pulsation direction of the dorsal blood vessel often reverses, that is, the blood temporarily flows back from the head to the back, which is called the reversal pulse. This kind of reversal phenomenon, in addition to the back flow from the head to the back, sometimes the pupa’s 3rd, 5th abdominal segment or the 3rd, 6th abdominal segment as the center of the pupa to flow forward and backward, there are also forward and reverse flow. The appearance of the inverse pulse firstly begins with the part of the compound pulse in the front and back of the pupa body, and then it presents the complete inverse pulse, which returns to the normal pulse after a short period of time. Before the occurrence of inverse pulse, the pulse frequency gradually decreases, and the frequency of inverse pulse is about 1/2 of normal. The reason for the prone to reversal pulse before and after pupation is mainly due to the reduced distribution of the trachea at the back end of the dorsal blood vessel at this time, the lack of oxygen supply, the weakened pulsatility, and the loss of the pulsatile energy advantage that exists in the larval stage, and the reversal pulse occurs.