The four growth stages of silkworm babies: how many days for eggs to hatch, how many times they molt, and how they become silkworm moths
The silkworm, also known as the mulberry silkworm or Bombyx mori, is an economically important insect commonly found in southwestern China, the Jiangsu-Zhejiang region, Anhui and other places. Because it can spin silk and be made into various silk fabrics, it is widely raised. So how does a silkworm become a silkworm moth? How many days does it take for eggs to hatch? How many times does it molt in its lifetime? The four growth stages of silkworm babies.
1. The four growth stages of a silkworm becoming a silkworm moth:
First stage: silkworm eggs
A single female moth can lay about 400 to 500 silkworm eggs at a time. When first laid, they are yellow or pale yellow; after 1 to 2 days they turn red-bean color or pale red-bean color; after another 3 to 4 days they turn purple or grayish green, and then no longer change. The outer layer is a hard eggshell, and inside are the serosa and yolk. The embryo in the fertilized egg continuously absorbs nutrients during development. Silkworm eggs look very much like tiny sesame seeds, generally about 1 mm wide and about 0.5 mm thick. After reaching a certain temperature, they gradually develop into ant silkworms.
Second stage: ant silkworm
The ant silkworm is what hatches from the silkworm egg. Its body is extremely small, black or brown in color, and covered with fine hairs, resembling an ant, which is why it is called an ant silkworm. The ant silkworm is about 2 mm long and about 0.5 mm wide. After crawling out, it will feed on tender mulberry leaves within 2 to 3 hours.
Under normal circumstances, when the incubation temperature reaches 20 to 30°C, silkworm eggs will hatch within 9 to 11 days, and especially at 22 to 25.5°C they hatch faster. For those that have undergone incubation acceleration, it takes 2 to 9 days. In spring, when the natural temperature is above 12°C, they can hatch in 22 to 27 days. If in diapause, when the temperature reaches 25 to 30°C, it takes 30 to 100 days. If it is higher than the maximum temperature, it will affect the hatching rate.
Third stage: mature silkworm
By the end of the fifth instar, the silkworm baby gradually shows signs of maturity and begins to spin silk threads from its mouth. First, the excrement it discharges changes from dark green to leaf green and from hard to soft, and it shows obvious symptoms such as reduced mulberry consumption, decreased appetite, a transparent chest, and an empty front part of the digestive tract. Then it stops eating completely, swings left and right and up and down searching for a place to spin its cocoon, and raises its head and chest. Such a silkworm is called a mature silkworm.
Fourth stage: silkworm cocoon
After a certain time, the mature silkworm begins to spin silk and stop eating. At this point, silkworm farmers place it in specially made containers or on a cocooning mount so that it can spin silk and form a cocoon. Silkworm cocoons come in colors such as white, yellow, pale green, and flesh red. They are about 3 to 4 cm long and 1.7 to 2.1 cm in diameter, with a white surface, irregular wrinkles, and attached silk in a fuzzy form. The silk pattern on the inner wall is very regular, and the cocoon is light but tough and not easy to tear. An unemerged silkworm cocoon contains one yellow-brown silkworm pupa and the pale brown, shriveled silkworm skin shed before pupation.
The four stages of silkworm cocoon spinning:
(1) The mature silkworm first spits out silk and sticks it to the cocooning mount, then spins silk to connect the surrounding mount branches, forming a cocoon-spinning support, namely the cocoon net. The cocoon net does not have a cocoon shape; it is just a loose and messy layer of silk threads used as support for spinning the cocoon.
(2) After forming the cocoon net, the silkworm continues to spit out messy silk loops, thickening the inner layer of the cocoon net, then spins silk in an S shape, and the outline of the cocoon begins to appear. This is called spinning the cocoon coat. The silk fibers of the cocoon coat are fine and brittle, arranged very irregularly, and contain a large amount of sericin.
(3) After the cocoon coat is formed, the cocoon cavity gradually becomes smaller, and the front and rear ends of the silkworm body bend toward the dorsal side, forming a “C” shape. The silkworm continues to spit out cocoon silk, and the spinning method changes from an S shape to an ∞ shape. This begins the process of forming the cocoon layer.
(4) When the silkworm's body shrinks greatly due to spinning a large amount of silk, the swinging speed of the head and chest slows down and loses a certain rhythm. The spinning begins to appear messy, forming a loose and soft layer of silk, called the pupa lining.
Fifth stage: silkworm moth
The silkworm moth is shaped like a butterfly and covered with white scaly hairs all over. However, because its two pairs of wings are relatively small, it has lost the ability to fly. Generally, after mating for 3 to 4 hours, the female moth can lay fertilized eggs. After mating, the male dies. The female moth takes about one night to lay about 500 eggs, and then slowly dies as well.
2. Why do they sleep and molt during growth?
1. Why do they molt?
During the growth and development of the silkworm, each time it goes through a stage, it molts once. During the molting period, it does not eat anything and safely molts in the form of sleeping. After one molt, it becomes a second-instar larva. Each molt counts as one year of age. The larva molts four times in total to become a fifth-instar larva, then eats mulberry leaves for 8 days to become a mature silkworm and begins to spin silk and form a cocoon.
2. Why do they sleep?
During growth, the silkworm undergoes four molts, and before each molt it must enter a sleep state in order to prepare for molting. Sleep management includes three processes: pre-sleep treatment, sleep-period protection, and post-sleep treatment.
(1) Pre-sleep treatment: when the silkworm enters the low-feeding period, it should be given additional leaves for sleep, feeding 1 to 2 rounds of leaves, that is, carrying out sleep removal before going to sleep, so that the sleeping silkworms have a dry and refreshing silkworm seat. The amount of mulberry given before sleep should be gradually reduced, which neither wastes mulberry leaves nor helps make the silkworm seat flat and facilitate netting. After the vast majority of silkworms have gone to sleep, for 1st to 3rd instar silkworms use 7 parts fresh lime powder and 3 parts scorched chaff; for 4th to 5th instar silkworms use fresh lime powder and sprinkle it on the silkworm seat to stop mulberry feeding, then use a lifting net to move them to another silkworm tray and feed high-quality mulberry leaves to let them grow and develop. After 3 to 4 days they can catch up with the development progress of the large group of silkworms. Pay attention to observing green-head silkworms and promptly eliminate the weak and diseased ones among them. Green-head silkworms should be raised separately and not mixed with the large group of silkworms, to prevent the spread of silkworm diseases.
(2) Sleep-period protection: the length of the silkworm sleep period is obviously affected by temperature. Under normal temperature protection, the 1st-instar sleep period is about 22 hours, the 2nd-instar sleep period is 18 to 20 hours, the 3rd-instar sleep period is 24 hours, and the 4th-instar sleep period is 44 to 46 hours. The temperature during the sleep period should be about 1°C lower than usual rearing temperature. In the early sleep period (from the start of molting to before feeding after molting), observe indoor humidity at all times. Generally, a wet-dry difference of about 2°C is appropriate, and slightly higher humidity is helpful for the silkworm to molt. The environment during the sleep period should be quiet; light should be dim; keep the air fresh; and prevent strong wind from blowing directly.
(3) Post-sleep treatment: after the silkworm molts, the head is light brown, which is the suitable time for feeding after molting. Appropriately delaying feeding after molting is beneficial for uniform silkworm development and has no adverse effect on silkworm constitution or cocoon yield. For spring silkworms and mid-to-late autumn silkworms, feed after 90% to 95% have molted; for summer silkworms and early autumn silkworms, because the temperature is higher, feed when 85% to 90% of the silkworms have molted. Before feeding after molting, use silkworm seat disinfectant or disease prevention No. 1 or anti-stiffening agent to disinfect the silkworm body and silkworm seat, so as to kill pathogens. After feeding after molting, feed two rounds of leaves and carry out removal after sleep. If the silkworm heads are crowded and dense, were not divided into batches before sleep, and the post-sleep silkworms are not uniform, then they can be fed after molting in two batches. That is, after disinfecting the silkworm body and silkworm seat, add a net and give mulberry leaves. Observe carefully, and when half the silkworms have crawled onto the net, lift off the silkworm net. After several hours, when all the late-sleeping silkworms have molted, carry out the second feeding after molting.
There are two methods for adding nets and dividing batches for feeding after molting: one is to place two silkworm nets horizontally in each silkworm tray (each occupying half the silkworm seat area), and during removal after sleep put one silkworm tray on each net, that is, divide one tray of silkworms into two trays. The other method: first place one silkworm net vertically in each silkworm tray, with the net area slightly larger than the silkworm seat. When about half the silkworms have crawled onto the silkworm net, lift off the silkworm net, then add a net again to the other half of the silkworms and give mulberry leaves to carry out removal after sleep. This can save time in expanding the seat and prevents injury to the silkworm body. Leaves used for feeding after molting should be suitably mature but on the tender side, and the amount of leaves fed should be small to prevent waste. As the amount of mulberry eaten by the silkworms increases, the amount of leaves fed also increases.
3. Why do they spin cocoons?
1. Why can they spin cocoons?
The silkworm's silk spinning and cocoon formation are related to the silk gland organs in the silkworm body. From the perspective of the silkworm's growth process, during the larval stage, after eating a large amount of mulberry leaves, the silkworm digests and absorbs the nutrients in the mulberry leaves. In particular, various amino acids in mulberry leaves are absorbed and stored by the silk glands in the silkworm body. As the silkworm age increases, the silk glands also increase, and their expansion speed is especially rapid during the fifth instar. Before spinning silk and forming a cocoon, it can be seen from the appearance that the chest of the silkworm body is slightly transparent, which is precisely due to the accumulation of a large amount of protein in the silk glands.
2. Why do they spin cocoons?
Physiologically speaking, because the proteins that make up the silk glands are composed of amino acids, too many amino acids in the silkworm body will poison the silkworm, so the silkworm needs to eliminate amino acids by spinning silk to achieve detoxification. Under natural conditions, as an insect with complete metamorphosis, the silkworm is very fragile at every stage of life, especially at the pupal stage, when it stops feeding, loses mobility, and is very vulnerable to natural enemies. The silkworm evolved the behavior of spinning silk and forming a cocoon, enclosing itself in a dense cocoon shell. In this way, it can both avoid natural enemies and protect itself, and safely transform into a pupa inside the cocoon, allowing the organs in the pupa body to transform into the form of a silkworm moth and finally emerge as a silkworm moth.
3. What to do after cocoon spinning?
After the silkworm finishes spinning its cocoon, it molts and pupates inside the cocoon, emerges after 8 to 10 days, mates and lays eggs, and then dies naturally after about one week. Therefore, after the silkworm spins its cocoon, you can place a sheet of white paper at the bottom of a clean cardboard box with no peculiar smell, and also hang a layer of white paper around the sides (do not glue it to death). Put the cocoons in the box, and the emerged silkworm moths will mate, while the female moth lays eggs on the paper. In silk production, if the pupa is allowed to metamorphose into an adult and naturally dissolve the cocoon and drill out, the silk threads will become shorter because the cocoon is broken open with a hole, and cannot be used for spinning and weaving silk. Therefore, before it breaks out of the cocoon, the cocoon must be placed in boiling water to kill the silkworm pupa and make the cocoon easier to unravel.