A detailed explanation of the silkworm growth process: changes at each stage at a glance
The silkworm is a holometabolous insect, the most common being the mulberry silkworm, also called the domesticated silkworm, one of the economic insects that feeds on mulberry leaves and spins silk to form a cocoon. The mulberry silkworm originated in China, its development temperature is 7-40°C, and the suitable rearing temperature is 20-30°C. Silkworm babies live on mulberry leaves; after continuously eating mulberry leaves their bodies turn white, and after a period of time they begin to molt. Molting takes about one day, during which they neither eat nor move, as if asleep; this is called “dormancy”. After one molt, it is 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.
Silkworm instar, also called stadium, indicates that the silkworm baby is at a certain developmental stage. From the ant silkworm to the first molt is the first instar; after waking from dormancy it enters the second instar; after molting again it enters the third instar; after the third molt it enters the fourth instar, and the fourth molt is also called the big dormancy. After the big dormancy it enters the fifth instar; fifth-instar silkworm babies grow extremely fast, with body length reaching 6-7cm and body weight reaching about 10,000 times the weight of the ant silkworm.
Silkworm instars are divided as follows: first instar four to five days; second instar three to four days; third instar four days; fourth instar six days; fifth instar seven to nine days; pupal stage fourteen to eighteen days; moth stage three to five days.
Silkworm eggs
Silkworms reproduce by eggs. Silkworm eggs look very much like fine grains of sesame, about 1 mm wide and about 0.5 mm thick. One female moth can lay 400-500 silkworm eggs. 1700-2000 silkworm eggs weigh about 1 gram. As for the color of silkworm eggs, when just laid they are pale yellow or yellow, after 1-2 days they become pale adzuki bean color or adzuki bean color, and after another 3-4 days they become grayish green or purple, and then no longer change; this is called the fixed color. The outer layer of the silkworm egg is a hard eggshell, and inside are the yolk and serosa. The embryo in the fertilized egg continuously takes in nutrients during development and gradually develops into an ant silkworm; it crawls out of the eggshell, and after the eggshell is empty it becomes white or pale yellow.
Ant silkworm
When the silkworm hatches from the silkworm egg, its body color is brown or black, it is extremely small, and it has many fine hairs, looking somewhat like an ant, so it is called an ant silkworm. The ant silkworm is about 2 mm long and about 0.5mm wide. After it crawls out of the eggshell, it will feed on mulberry leaves after 2-3 hours.
Mature silkworm
When the silkworm baby reaches the end of the fifth instar, it gradually shows the characteristics of being mature: first, the excrement it discharges changes from hard to soft, and from dark green to leaf green; appetite decreases, and the amount of mulberry eaten declines; the front part of the digestive tract is empty, and the chest becomes transparent; then it stops eating completely, the body shortens, and the abdomen also tends to become transparent; the head and chest of the silkworm body are raised, it spits out threads from its mouth, and sways left and right, up and down, looking for a place to make a cocoon. Such a silkworm is called a mature silkworm. People place the mature silkworm in a specially made container or on a mounting frame, and the silkworm then spins silk and forms a cocoon. Cocoon formation can be divided into four processes:
1. The mature silkworm first spits out silk and bonds it to the mounting frame, then spits silk to connect the surrounding mounting branches, forming a cocoon support, namely the cocoon net. The cocoon net does not have a cocoon shape; it is only some loose and messy layers of silk, serving as the support for cocoon formation.
2. After the silkworm makes the cocoon net, it continues to spit out messy silk loops, thickening the inner layer of the cocoon net, and then spins silk in an S-shaped manner, and the outline of the cocoon begins to appear; this is called forming the cocoon floss. The silk fibers of the cocoon floss are fine and brittle, arranged very irregularly, and also contain much sericin.
3. After the cocoon floss 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 S-shaped to ∞-shaped; this begins the process of forming the cocoon layer.
4. When the silkworm, due to spinning a large amount of silk, has its body greatly shrunk, the swinging speed of the head and chest slows down, and there is no certain rhythm; the spinning begins to appear messy, forming a loose and soft layer of cocoon silk, called the pupal lining.
Silkworm pupa
About 4 days after the silkworm mounts the frame and forms a cocoon, it turns into a pupa. The silkworm pupa is shaped like a spindle, divided into three body segments: head, chest, and abdomen. The head is very small, with compound eyes and antennae; the chest has thoracic legs and wings; the bulging abdomen has 9 body segments. Professionals can distinguish male from female by the line patterns and small brown dots on the abdomen of the silkworm pupa. When the silkworm has just pupated, the body color is pale yellow, and the pupal body is tender and soft; gradually it becomes yellow, yellowish brown, or brown, and the pupal skin also hardens. After about 12-15 days, when the pupal body begins to soften again, and the pupal skin is somewhat wrinkled and earth-brown in color, it is about to become a moth.
Silkworm moth (adult)
The silkworm moth is shaped like a butterfly, with its whole body covered in white scaly hairs, but because its two pairs of wings are relatively small, it has lost the ability to fly. The head of the silkworm moth is small and round, with bulging compound eyes and antennae; the chest has three pairs of thoracic legs and two pairs of wings; the abdomen no longer has abdominal legs, and the terminal body segments have evolved into external genitalia. The female moth has a large body and crawls slowly; the male moth has a small body and crawls relatively fast, vibrating its wings rapidly as it seeks a mate. Generally after mating for 3-4 hours, the female moth can lay fertilized eggs. After mating, the male dies; the female moth spends about one night and can lay about 500 eggs, and then also slowly dies.
The eggs laid by the silkworm moth → hatching into silkworms → turning into pupae → transforming into moths, will again complete a new generation's cycle and enter the next generation's growth process.