Excretion of Mulberry Silkworm

In the life activities of silkworms, the end products of nitrogen metabolism produced by alienation are discharged into the physiological process outside the body. Its function is to remove nitrogenous wastes from the body, adjust the balance of salt and water, and keep the internal environment, namely the composition and physical and chemical properties of blood, relatively constant.

The metabolic wastes of silkworms often accumulate in the blood, in which water as a solvent can be discharged from the body through various channels, and carbon dioxide is removed by diffusion through the body wall and tracheal system (the body wall excludes about 1/4 to 1/3 of the total carbon dioxide capacity) , Excess amino acids and part of salts are discharged from the silk through the silk gland synthetic silk material. The elimination of the metabolic end products of protein and other nitrogen-containing substances, as well as the balance adjustment of salt and water, are carried out through the Markov tube that is exclusively excreted.

Markov’s tube originates from the ectoderm and is an organ excreted exclusively by silkworms. Its function is equivalent to that of vertebrate kidneys, so it is also called renal tube and urinary tube. The Markov tube of the larva is a light yellow curved slender tube, which is arranged on both sides of the middle and rear part of the digestive tube. The common duct at the opening is enlarged and shaped like a bladder. The bladder is divided into 2 branches, and the back side is divided into two, so each side is divided into 3, a total of 6 branches. These 6 Markov tubes all extend parallel to the front of the midgut, from 2 on the back to the anterior edge of the fourth abdominal segment, 2 on the side to the middle of the third abdominal segment, and 2 on the ventral surface reaching the front of the third abdominal segment. The parts are folded backward, each tube to the small intestine and colon forms many flexures, and finally inserted into the rectal wall from both sides of the front end of the rectum. The section from the bladder that goes forward to the fold back is called the base tube, and the section from the fold back to the rectal insertion site is called the end tube, and the part inserted into the rectal wall is called the hidden kidney tube. After the hidden kidney tube penetrates into the wall of the rectum, the tube wall becomes extremely thin and separates many interconnected cystic branches, and finally ends in a blind tube. In the process of metamorphosis in the pupal stage, the larval martensian tube tissue dissociates, and the adult martensian tube develops from the adult bud. Adult Markov tubes are also three on the left and right. Common tubes on each side open on both sides of the border between the mid-intestine and the small intestine. The ends are blind tubes. Each tube is flexed and freed in the abdominal cavity (see figure).

Markov tube of silkworm larvae

In addition to the bladder part, the Markov tube is composed of three layers of connective tissue layer, base membrane, and tube wall cell layer from the outside to the inside. The wall cells are relatively large, and two cells are relatively held together into a tube. The number of cells in the Markov tube has been determined during embryonic division and proliferation. After incubation, the cells no longer proliferate, only the cells increase. The shape of the nucleus changes due to development. It is spherical at 1st instar and dendritic at 4 to 5th instar. There are many granular or short filamentous mitochondria scattered in the cell, which is a component of the energy transformation in the cell. In addition to the connective tissue layer, base membrane, and cell layer, the structure of the bladder is composed of 3 to 5 cells in the lumen. There is a chitin inner membrane on the inner side of the lumen, and the bladder is surrounded by a powerful muscle layer and nerve distribution.

The excrement of Markov tube is usually called silkworm urine. The larvae’s silkworm urine is often mixed with silkworm feces, and the urine is acidic, and the main components are uric acid and urate. Both of them are hard to dissolve in water, and easy to precipitate crystals. Uric acid nitrogen accounts for 85.8% of the total nitrogen in silkworm urine. Uric acid salt is mainly sodium and ammonium salt. In addition, it also contains a large amount of calcium oxalate crystals, as well as a small amount of sodium chloride, sodium carbonate, calcium carbonate, and magnesium phosphate. , Inorganic salts such as ammonium phosphate. The yellow-green liquid discharged by the mature silkworm after defecation is called the mature silkworm urine, which is alkaline with chlorophyll particles and a small amount of uric acid. This indicates that the mature silkworm urine is mainly digestive juice. Mixed with urine. Moth urine is a metabolite in the pupal stage, which is stored in the rectal sac through Markov’s duct, and is excreted from the body after the moth is transformed. Moth urine contains riboflavin pigments, which are generally reddish brown with a special odor. A moth urinates about 0.2 to 0.5 ml, and the pH value of moth urine is 5.8 to 6.3. Fresh moth urine water accounted for 96.66%, dry matter accounted for 3.34%. Ash content in the dry matter accounts for 2.18%. Among nitrogen-containing compounds, uric acid nitrogen accounts for 76.4%, and other nitrogen accounts for 23.6%.

Excreting uric acid is an effective way to reduce water loss in silkworm bodies. The percentage of hydrogen atoms in uric acid molecules is less than that of other nitrogen-containing metabolites. Therefore, less water is consumed when uric acid is formed in the metabolic process, and uric acid is insoluble in water and is not required for excretion. Take away a lot of water, which is conducive to the water retention of silkworm body.

Uric acid in silkworms is mainly produced by the decomposition of nucleic acid, and it may also be formed by two molecules of urea and one molecule of tricarbonic acid. The synthesis part is mainly fat body. The content of various components in larval excrement varies with the feed and the development period of the silkworm. In the young silkworm stage, the young leaves containing more organic acids are eaten. There are more calcium oxalate crystals in the urine than uric acid and its salts. In the silkworm stage, a large amount of mulberry is consumed, resulting in an excess of amino acids in the body. In particular, the uric acid accumulated in the midgut epithelium and body wall cells of the mature silkworm stage is transferred to the blood and discharged through the Martensian tube, so the uric acid crystals increase significantly. When the ratio of carbon to nitrogen in the feed is large, calcium oxalate crystals are more, and when the ratio is small, uric acid crystals are more. In addition, the urine of healthy silkworms contains more calcium oxalate, and there are more urates in fasting silkworms and softening diseased silkworms. Urea was also found in the silk glands of silkworm reared with artificial feed.

The excretion of Markov tube varies with the developmental period of the silkworm. The uric acid excretion of a silkworm increases with the increase of silkworm age, but the uric acid excretion per unit weight per unit time decreases with the increase of age. It is due to the strong nitrogen metabolism in the small silkworm stage, while the silk glands of the large silkworms, especially the 5th instar silkworms, increase rapidly and synthesize a lot of silk protein. In an instar, the excretion of uric acid from silkworms is the least. It increases with the increase of nibbling mulberry, and decreases after the full feeding period. A large amount of excrement is stored in the martensian tube and passes through the bladder and the old and new inner parts of the hindgut at the time of molting. The membrane is excreted between the new and old epidermis of the silkworm body, and after molting, it dries into powdery crystals and remains on the body surface.

The Markov tube is immersed in the blood, and its function varies from part to part. The content of the end tube is liquid, which is the part that absorbs urate and low-molecular compounds from the blood; the base tube contains uric acid and oxalate-based pasty metabolic end products, which is the concentrated and dehydrated part of excrement; hidden The kidney tube is the part that penetrates the wall of the rectum to recover water and salt from the rectal contents. That is, the end tube cells absorb metabolites from the blood, secrete them into the lumen, and gradually move into the base tube. During the movement, while freeing uric acid, they generate carbonate or phosphate, these easily soluble salts and water. When put back into the blood together, the insoluble uric acid crystallizes and precipitates. It is discharged into the hindgut through the bladder and mixed with silkworm feces. When it reaches the rectum, when the rectal muscles contract the silkworm feces into a hexagonal short column, the hidden kidney tube is removed from the rectum. Part of the water and ions are recovered in the cavity. During this excretion process, water and inorganic salts return from the blood to discharge nitrogenous waste through the Markov tube, forming a continuous circulation.

The contents of the Martens tube, the flow direction from the end tube to the base tube and into the hind intestine through the opening of the bladder, is due to the pressure generated by the elastic tube wall on the lumen, the suction force formed by the contraction of the bladder muscle layer, and the hidden The kidney tube absorbs water from the rectum, causing a flow of fluid, which flushes the contents toward the mouth of the tube.

Markov tube is the balance regulating mechanism of water and salt in silkworm. Moisture and inorganic salts are important components of silkworm bodies. In addition to the cells, water exists in a large amount in the blood, and the exchange of substances between the organs and the blood is carried out by the blood circulation. In addition to constituting tissues, inorganic salts mainly exist in the blood in an ionic state, adjusting osmotic pressure, pH, etc. to maintain a relatively constant physiological environment in the body. Silkworm larvae obtain water and salt by feeding, and the absorption amount is proportional to the content of the feed. However, due to defecation, respiration and evapotranspiration, water is continuously lost, and ions are only discharged through the Markov tube. This excretion is affected by changes in the meteorological environment and nutritional conditions, and it is difficult to maintain normal life activities without the necessary water and salt regulation mechanisms. This kind of regulation mechanism of silkworm is Markov tube. The Markov tube is bathed in the blood, and the hidden kidney tube penetrates into the rectal wall to balance the water and ion concentration in the blood and rectal fluid.

When the water content in the blood drops, the rate at which the Markov tube absorbs water from the rectal fluid to the blood is relatively faster than the rate at which it absorbs inorganic salts, making the rectal fluid hypertonic, reducing the water content of silkworm manure, and silkworm manure becoming dry; On the contrary, when the water content of blood is too high, the rate of absorption of inorganic salts into the rectal fluid by the Markov tube is greater than the rate of absorption of water, and the rectal fluid becomes hypotonic, which increases the water content of silkworm feces, and the fecal particles become thin and soft. The maximum concentration of rectal fluid is determined by the maximum osmotic gradient of water absorption; the minimum concentration of rectal fluid is determined by the maximum concentration gradient of salt absorption. Silkworm larvae use Markov’s tube to absorb water or inorganic salts to adjust the concentration of rectal fluid, so as to exchange substances in the blood through Markov’s tube to make blood composition, osmotic pressure and pH The physical and chemical properties are adjusted to maintain the normal and relatively constant physiological environment in the body.

In the process of sericulture, when feed with high moisture content is given, the moisture content of manure pellets is also high. In the young silkworm stage, because the body of the silkworm is small, the body surface area is relatively large, and the waxy layer of the body wall is thin, the water of the silkworm body is easy to lose. At the same time, because the silkworm body grows fast, it needs a lot of water. Fecal pellets are still relatively dry: On the contrary, when the 5th instar silkworm body grows extremely, the body surface area is relatively reduced, the wax layer on the body wall is thickened, and the water is not easily lost. Although the water content of the mulberry leaf is lower than that of the young silkworm, the manure pellets The moisture content of the feces is higher than that of the small silkworm stage, which is also the result of the balance adjustment of the Markov tube.

The function of the Markov tube, in addition to eliminating the end products of nitrogen metabolism and regulating the balance of water and ions in the body, during molting, the excrement of the Markov tube is discharged into the new and old epidermis as a large amount of liquid, which affects the old epidermis. It has lubricating effect when taken off.