Respiration of Silkworm

The process of gas exchange between silkworms and the outside world. The respiratory organs of silkworm larvae or adults are composed of the air valve and tracheal system. The trachea is a tube formed by the recession of the ectoderm, which is the passage for gas exchange in the silkworm body; the trachea is the opening of the tracheal system on the surface of the body wall and the gateway for gas exchange with the outside world. According to the gas diffusion law and the ventilation effect of the trachea, the silkworm sends oxygen directly to the tissues through the trachea, trachea and microtrachea, and discharges the generated carbon dioxide out of the body through the microtrachea, trachea, and trachea.

Air valve. The larva has 9 pairs of air valve. Located on both sides of the first thoracic segment and the first to eighth abdominal segment of the body, one pair for each segment. The valve is oval in shape, surrounded by a black-brown bony valve piece. The inner edge of the valve plate has a sieve plate, and a longitudinal gap is formed at the junction of the front and rear sieve plates. The sieve plate has a filtering effect on foreign matter in the air. The inner surface of the valve has a valve chamber and a valve opening and closing device. The silkworm valve is internally closed. The opening and closing device is between the valve chamber and the inner cavity of the tracheal plexus. It consists of valves, closed arches, closed rods, closed belts, open and closed muscles, etc. constitute. In addition, there is a pair of trigeminal pores in the intersegment membrane on the body side of the second and third thoracic segments, which are called degenerative valves. The inner surface is only connected to the opening by a non-spiraled trachea, which has no respiratory effect. During molting, the endotracheal membrane of the 2nd and 3rd thoracic segments protruded from the degenerative valve (Figure 1).

Figure 1. Larval valve appearance

The pupa has 8 pairs of valves. They are located on both sides of the body of the first thoracic segment and the first to seventh abdominal segments. Oblong. The sieve plate is degraded.

Adults also have 8 pairs of air valves, which are also on both sides of the body of the first thoracic segment and the first to seventh abdominal segments, one pair for each segment. Crescent shape, no sieve plate. The abdominal valve is bent backward, the upper half of the anterior and posterior edges of the valve is ossified, and the lower half of the posterior edge is membranous. The front thoracic valve is bent forward, and the structure of the front and rear edges of the valve is opposite to that of the abdominal valve. The valve opening and closing device consists of only a closing rod, a closing belt, and opening and closing muscles. In addition, at the junction of the bases of the front and rear wings, there is also a pair of degenerative valves with boneless outer edges and only composed of front and rear flaps. There is no opening and closing device. The opening and closing movement is mainly realized by the contraction of the middle and back chest and back abdominal muscles.

Trachea. The larval trachea includes tracheal plexus, longitudinal trachea, transverse trachea, tracheal branch and microtrachea, etc., forming a tracheal system in the silkworm. The tracheal plexus is the part of the trachea within each valve that emits radial branches, a total of 10 pairs, the thicker tracheal plexus before and after the connection is the longitudinal trachea; the tracheal plexus extends horizontally from each tracheal plexus to the ventral surface to the abdominal midline is the transverse trachea; 1. The transverse trachea from the second and third pairs of tracheal plexus is still crossed and converged at the abdominal midline of the second and third thoracic segments; the longitudinal trachea between the front and rear tracheal plexus and the left and right transverse trachea are in the abdominal midline. The junctions are all connected with the gray part of the trachea. The tracheal plexus and the transverse trachea send out tracheal branches forward and backward, which are distributed in the back of the silkworm body, ventral surface, internal organs and head cavity; the tracheal branches are divided and divided, becoming smaller and thinner. When the diameter reaches 2-5 microns, Entering a palm-shaped terminal cell, it branches in the cytoplasm of the terminal cell, forming a number of micro-trachea with a diameter of less than 1 micron, extending into the tissues or cells of the organ (Figure 2).

The adult trachea is similar to the larva. The difference is: There are only 9 pairs of tracheal plexus; the head and thorax trachea are more developed than the larva; the trachea from the first, second, and third pairs of tracheal plexus form two at the base of the forewing and hindwing. At the meeting, its branches are also distributed in the wings and the second and third thoracic feet; there is no gray and white part at the trachea meeting. In addition, the local trachea that is distributed between the abdominal links and close to the body wall is expanded into a balloon shape to expand the volume of inhalation and exhaust.

Trachea structure. The larval trachea is purple-brown, and the basic structure is similar to the body wall, but the inner and outer layers are opposite. The outermost layer is a non-cellular base membrane. Polygonal flat cells form the middle tube wall cell layer, and the inside is the epicortical intima. There is a spherical nucleus in the wall cells, and the cytoplasm contains light brown pigment particles. During the molting process, the tube wall cells secreted lively. Figure 2 The distribution state of the larval ventral trachea. The intima is locally thickened and formed. 1-9 are the first to ninth tracheal plexus, dark brown and elastic spirals (1) are the parts of the degenerated valve. Keep the tracheal cavity in an expanded state. The structure of the microtrachea is the same as that of the trachea. There is no spiral wire in the gray part of the trachea. There are many hairy protrusions on the inner membrane. When the larva molts, the old tracheal endometrium breaks in the gray part and protrudes from the air valve where it is located. The new endometrium is secreted by the cells of the tube wall during the dormant period.

Figure 2. Microtrachea of Silkworm

The structure of the adult trachea is the same as that of the larva, but the spiral filaments of the inner membrane of the trachea are very thin and light in color, so the trachea appears silvery white and shiny. No gray parts.

Gas exchange. It is mainly realized by gas diffusion and ventilation in the trachea. The main factor that determines the direction of gas diffusion is the partial pressure difference of the gas. The silkworm obtains oxygen or eliminates carbon dioxide by the partial pressure difference of oxygen and carbon dioxide between the atmosphere and the trachea, between the trachea and the microtrachea, and between the microtrachea and the tissue. The greater the partial pressure difference between oxygen and carbon dioxide in the inhaled or exhaled gas, the easier it is to ventilate in a diffuse manner. Oxygen enters tissue cells by diffusion through the tracheal system through the air valve, and the carbon dioxide produced by the metabolism in the tissue can be smoothly discharged through the air valve through diffusion. Gas diffusion is also related to the thickness and length of the trachea. It is easy to diffuse in a thick trachea, and if the trachea is extended, it is not conducive to diffusion. Therefore, in the young silkworm stage, the body is small, the body diameter is small, and the trachea is short, which is easy to spread. Large silkworms are large in size, their body diameters are thicker, and the tracheal system is correspondingly extended. Diffusion and ventilation have become less convenient than small silkworms. Intratracheal ventilation is mainly caused by the airflow in the tracheal system. The greater the intensity of the airflow, the higher the ventilation effect. Although the silkworm’s tracheal system can meet the need for ventilation only by diffusion, the telescopic movement of the body changes the length and volume of the trachea, which causes ventilation in the trachea. The pulsation of the dorsal blood vessel and the peristalsis of the digestive tube also have a ventilation effect. Influence. The opening and closing of the valve controls the intake and exhaust. During ventilation, due to a change in tracheal length and cross-sectional shape, approximately 20-30% of tracheal capacity can be increased or decreased to enhance ventilation, so that air diffuses into the tissues faster, ensuring rapid oxygen supply and as soon as possible Groundly remove carbon dioxide and a part of moisture. Silkworm exhales or inhales about 1/3 of its total volume. The ventilation intensity of silkworm trachea varies with its developmental period and activity state. The ventilation intensity of male moths with vibrating wings is greater than that of larvae, and the ventilation intensity of larvae is greater than that of inactive pupae. The air velocity in the larval trachea is about 0.0949 mm/sec at 20°C.

The air exchange between microtrachea and tissue is closely related to the physiological condition of silkworm tissue cells. The thin wall at the end of the microtrachea can not only permeate gas but also liquid. When the end of the microtrachea is filled with liquid, the air in the tracheal system cannot reach the distal part of the trachea. Only when the liquid in the microtracheal leaks out, the air can enter and diffuse through the tube wall to the tissue cells. This diffusion process is affected by the microtrachea. Control of the osmotic pressure of tissue fluid around the ends of the trachea. When the tissue is in a state of insufficient oxygen or activity, the osmotic pressure of the tissue fluid increases due to the metabolites produced by the surrounding tissues that cannot penetrate the microtrachea. The liquid in the microtrachea quickly leaks out through the tube wall, and at the same time, the fluid in the tube The air in the upper part of the column also diffuses to the ends of the microtrachea and enters the tissues, and directly contacts the cells undergoing oxidation; when the metabolic intensity is reduced, or in the case of sufficient oxygen, the metabolites in the tissue fluid are oxidized and the osmotic pressure drops. , The end of the microtracheal tube is filled with liquid again, and the air column in the tube recedes back again. The rise and fall of the liquid column in the microtrachea is directly affected by the osmotic pressure of the tissue fluid. When the diameter of the microtrachea is 0.3 microns, the capillary pressure is 10 atmospheres, and the diameter of the larval microtrachea is greater than 0.3 microns. Therefore, the tissue fluid must be at a pressure greater than 10 atmospheres to lower the liquid column in the microtrachea.

The breathing movement of silkworms is controlled by nerves. Each ganglion in the thoracic and abdomen of the silkworm has a “respiratory center” that controls the opening and closing movement of the valve. When environmental conditions change, the nerve center is first stimulated and excited, and then it spreads to the nerves distributed in the valve opening and closing muscles. Cause the valve to open and close. Any external stimulus, such as temperature, humidity, light, airflow, etc., can have a certain impact on the nervous system, thereby changing the breathing rate. For example, undesirable gases in the air cause the valve opening to increase, the number of opening and closing cycles increases, or the valve is often opened; in the larval stage, when the carbon dioxide concentration in the air reaches 4% and the ammonia content reaches 0.05%, the valve often opens.

Silkworm blood does not have the ability to carry or release oxygen, but it can dissolve a small amount of gas. The dissolved amount of oxygen in 100 ml of blood is 0.44 ml, and the amount of carbon dioxide is 4-12 ml. It can supplement the supply of oxygen in the body; carbon dioxide is in the blood. And the diffusion rate in the tissue is faster, therefore, in addition to the trachea, it can also be discharged through the body wall, which is a kind of regulation for tracheal breathing.

Respiratory volume. Respiratory volume indicates the breathing intensity of silkworms. It is the amount of silkworm that inhales oxygen or emits carbon dioxide in a certain period of time. The breathing volume of the yolk egg. The eggs rise linearly after laying, and reach the maximum respiratory volume (100 g silkworm eggs emit about 0.31 grams of carbon dioxide a day and night) on the second day after spawning. After that, they decline and have been maintained at a very low level. After the diapause is lifted, the respiratory volume can be reached. Gradually, the amount of carbon dioxide exhaled by the first weekend of the greening rose to the equivalent of the second day after spawning. After that, it continued to rise until it reached the highest amount before the ant silkworm hatched. During the nearly 10 months from egg laying to hatching, One kilogram of silkworm eggs emit a total of 201.9 to 208.9 grams of carbon dioxide. The respiration of non-yielding eggs continues to increase from the laying of silkworm eggs to the hatching of ant silkworms. In just 11 days, one kilogram of silkworm eggs emits 153.6 to 160.0 grams of carbon dioxide. The average daily exhaled amount of carbon dioxide, non-yielding eggs Significantly more eggs than Yuenian.

The respiration of larvae increases for individuals. The carbon dioxide exhalation of 1,000 first-instar silkworms per hour is 0.0154 g, while that of fifth-instar silkworms is 3.4930 g. However, the respiration per unit body weight decreases, one kilogram 1 The instar silkworms emit 6.664 grams of carbon dioxide in one hour, and the fifth instar silkworms emit 2.6859 grams. The smaller the silkworm age, the greater the breathing intensity. In the same age, the breathing volume is the smallest when the silkworm is raised, and the largest is the silkworm, and it becomes smaller during hypnosis and sleep.

The pupa has a smaller breathing volume than the silkworm. Adult respiration tends to increase before mating, and decrease significantly after mating.

The respiration of silkworms is affected by the environment. Within a suitable temperature range, the respiration of silkworms increases with the rise of temperature; no matter in high temperature or low temperature conditions, it is usually larger when it is humid than when it is dry; within a certain range of temperature, humidity and airflow intensity, strong airflow is weaker The airflow breathing volume is large.