Why Do Silkworms Produce Cocoons

It is an instinct of silkworms to spin silk and form cocoons in mature silkworms. It is generally believed that it has been preserved as a genetic trait for generations because of its beneficial effects on the protection of the pupae. The behavior of making cocoons is not caused by external environmental stimuli, but by the internal environment (physiological conditions) of the silkworm itself. The direct cause is the increase in the level of ecdysone in the blood. When the silkworm reaches the 5th instar stage, the function of secreting juvenile hormone in the pharyngeal body is weakened. On the other hand, the prothoracic gland is still secreting ecdysone. At the end of 5th instar, the amount of ecdysone rises to a certain level, which induces silkworms to stop eating mulberry and enter the cocooning stage.

Find a place to form a cocoon

One of the characteristics of mature silkworms is that they are constantly walking and crawling. It is generally believed that they are looking for cocooning sites, but the physiological reasons why silkworms become so restless at this time are still unclear. Another characteristic of mature silkworms is that they exhibit negative geotaxis and climb to high places to camp cocoons. Bombyx mori’s gastropod grip is not as good as that of wild insects, so its ability to climb is limited. This negative geotaxis of mature silkworms may be related to the rapid increase of calcium ions in the blood.

The body of the silkworm can be large or small. As long as you find certain gaps, corners, twigs, etc., the silkworm can make a cocoon. However, if the place is too small, the shape of the cocoons barely formed is not correct, which will reduce the commodity value of silkworm cocoons. Therefore, silkworm rearing provides suitable cocooning places for silkworms in order to obtain high-quality silkworm cocoons.

Make Cocoon

After the mature silkworm finds a suitable cocoon site, it spins and attaches to form a cocoon support. Then empty the contents of the digestive tube, and continue to spin silk on the stent to make a cocoon net. This is the preparation period for cocooning, and the outline of the cocoon has not yet been formed. On the inner surface of the cocoon net, a messy and loose silk loop is spit out to thicken the cocoon net, and then the spinning movement is changed to a “~” shape, so as to form a layer of loose cocoon clothes. With the completion of the cocoon garment, the outline of the silkworm cocoon began to appear. After the cocoon is formed, continue to spit out the silk circle from the “~” shape to the “∞” shape around the inner space (cocoon cavity) of the cocoon garment. At this time, the front and back ends of the silkworm body are bent back to the back, fixed on the inner wall of the cocoon cavity with their stomach and feet, and the head swings left and right to spin silk. The long axis of each “∞”-shaped wire loop is 1~2mm, and every 15-25  “∞”-shaped wire loops are spit out as a group, which is called a cocoon piece. After each cocoon piece is completed, the second cocoon piece is formed by transferring the position, and the cocoon layer is gradually formed in this way. Due to the large amount of physical energy consumption, the silkworm body shrinks significantly, the head swings slower, the amplitude shrinks, and it loses rhythm. It spits out a thin layer of pupa lining in the innermost layer of the cocoon. Finally, the head of the larva moves toward the upper end of the cocoon, and the tail goes down, spitting out the silk material that remains in the body. A soft cocoon top is formed, which completes the process of spinning and forming a cocoon.

Spinning speed varies with silkworm species and temperature, generally 0.7-0.8 cm per second, spinning fast at high temperatures, and slow spinning at low temperatures. The current bichemical silkworm species each spin about 0.5 g silk, which can reel about 85% of the cocoon silk, and the length is about 1000 ~ 1400m .

Three-dimensional space requirements for cocooning

Place the mature silkworms on a flat plate. No matter whether the plate is placed horizontally or tilted, the silkworms cannot form cocoons on it. So what is the minimum three-dimensional space condition for silkworm cocoons? Use the following experiment to answer.

• Put one thin plate on the other, when the angle between the two plates is 90°, the silkworm can easily cocoon at the angle.
• When the angle is extended to 110°, there is almost no difficulty in cocooning.
• When the angle is extended to 130°, the situation is different. Generally, it takes more than 20 hours to form a cocoon in a few hours.
• When the angle extends to 150°, there are few individuals capable of cocooning.
• When the included angle is 170°, it is almost close to the plane, and it is impossible for silkworms to cocoon. Because the cocoon cannot be erected and there is no room for drilling, the cocoon cannot be formed normally. The mature silkworms kept crawling to find the cocoon site but failed. As time passed, the “maturity” of the silkworm became deeper and deeper, and finally the silkworm gave up the instinctive procedure of erecting the cocoon and spun silk on the plane around the silkworm body. . This kind of silkworm that spins on a flat surface finishes spinning in about 2 days, but it cannot form a cocoon. This kind of flat cocoon silk layer formed by occasionally spinning silk on a flat surface or deliberately spinning silkworms on a flat surface is not a cocoon, but is often called a flat cocoon. Flat spinning is not the original spinning behavior of silkworms. Therefore, the spinning speed is slow, and the spinning volume is only about 70% of the normal spinning and cocooning, and a large amount of silk material remains in the pupa. The mortality rate is also higher than that of normal spinners. Flat spinning is absolutely detrimental to the physiology of silkworms. The flat cocoon is mainly used to make works of art.