Sensation of Mulberry Silkworm

The various sensory organs (reactors) of the silkworm that are distributed by nerves respond to stimuli inside and outside the body, including vision, smell, taste, touch and so on. The nerves of silkworm are divided into central nerve, peripheral nerve and sympathetic nerve.

Central nervous system

The central nerve is composed of ganglia and nerve cords. Except for the brain on the back of the digestive tract, the rest runs longitudinally through the midline of the ventral surface of the body cavity near the body wall. The larval central nervous system has 13 pairs of ganglia, including 2 pairs on the head, 3 pairs on the chest, and 8 pairs on the abdomen. The ganglia of the adult central nerves in the head and chest are similar to those of the larvae. The characteristic is that the first pair of ganglia, the brain, is particularly developed, and the pharyngeal nerve cord is shortened, so the second pair of ganglia is very close, with only a small hole in the middle. Through the esophagus and dorsal blood vessels. The first ganglion of the adult thorax is close to the second ganglion, and the second and third ganglia are closely combined into a scoop shape.

1. Brain  2. Hypopharyngeal ganglia   3～5. Thoracic ganglia   6～13. Abdominal ganglia    14. Frontal nerve cord    15. Frontal ganglia    16. Back nerve     17. Pharyngeal side body     18. Pharyngeal side sympathetic nerve    19. Lateral body

Larval nervous system

Peripheral nerve

The nerves sent from the central nervous system to various organs and tissues around the silkworm body include the efferent nerves from the ganglion to the reactor and the afferent nerves from the receptors to the central nervous system.

Peripheral nerves of larvae

Peripheral nerves of larvae head are divided into 4 nerves on each side of the brain. Among them, except for the 1 pair of frontal nerves that extend to the front of the brain and connect with the frontal ganglion, the other 3 pairs respectively extend to the upper lip, antennae and monocular, which are called the upper lip nerve, antenna nerve and optic nerve respectively. The front and left sides of the hypopharyngeal ganglion are divided into 3 nerves, which are respectively distributed in the lower palate, upper palate and lower lip; 1 nerve is divided into each side of the back of the hypopharyngeal ganglion, extending back to the first thoracic segment , Distributed on both sides of the dorsal blood vessel. The peripheral nerves of the thorax and abdomen, except for the last ganglion, each ganglion has two nerves extending from the front and back ends to the left and right, which are distributed in the back muscles, body wall and the inner layer of the abdominal muscles; the nerves branching out from the back end are distributed to The outer layer of the abdominal muscles and the tarsal limbs. The last ganglion is a compound ganglion, and 3 pairs of nerves are emitted from both sides at the back.

Adult Peripheral Nerve

The adult worm is separated from the brain with 3 peripheral nerves, which are distributed to the compound eyes, antennae and upper lip respectively. The 3 pairs of nerves from the second ganglion forward are distributed to the upper palate, lower palate and lower lip of the mouthparts. Each ganglion of the chest sends out a pair of nerves at the front and back ends, which are distributed to the feet and wings of their respective links. The nerve cord between the first ganglion of the abdomen and the third ganglion of the chest sends out two pairs of nerves, which are distributed in the first and second links of the abdomen. Two pairs of nerves are sent out from the first, second, and third ganglia of the abdomen, which are distributed on the back muscles and abdominal muscles of each link. The nerves from the fourth ganglion of the abdomen are distributed on the muscles and genitals of the sixth and subsequent segments of the abdomen.

1. Brain   2. Antennae nerve   3. Compound eye   4. First ganglion of thorax   5. Abdominal ganglion   6. Forewing nerve   7. Hindwing nerve

Adult nervous system

Sympathetic nerve

Nerve issued by the central nervous system, mainly in the digestive system. The larval head sympathetic nerve has a small nerve from the frontal ganglion forward and its branch called the prefrontal nerve, which is distributed on the back wall of the throat and the back of the oral cavity; a thicker nerve called the back nerve is branched back and passes through the underside of the brain It stretches along the back of the esophagus and is divided into 2 near the esophagus: one is distributed in the esophagus and the front part of the midgut; the other is distributed to the dorsal blood vessel. The gyri nerves dominate the contraction of the throat. A nerve is sent out from the rear edge of both sides of the brain, and then it is divided into two, and it is connected to the star-shaped side of the heart along the side wall of the throat, which is called the throat sympathetic nerve. The nerves sent from the inner side of the heart are distributed to the dorsal blood vessels and throat, and the nerves on the outside are distributed to the adductor palatine muscle, one of the branches is connected to the hypopharyngeal ganglion; and a nerve that branches off from the posterior edge of the lateral heart , Bypassing the peripheral wall of the pharyngeal body, distributed to the salivary glands and esophagus. The thoracic and abdomen sympathetic nerve is a nerve extending from the back of each ganglion of the thorax and abdomen, going longitudinally backward along the nerve cord, and in front of the next ganglion, branching horizontally to the left and right, and distributed to the longitudinal abdominal muscles. On the valve opening and closing device, it is connected to the nerve branching from the front end of the next ganglion, and finally enters the dorsal longitudinal muscle. Between the previous sympathetic nerve and the next ganglion, there are nerves that intersect with the trachea to form a network, called the sympathetic nerve network. There are 10 pairs of sympathetic nerve networks from the thoracic ganglion to the seventh ganglion of the abdomen. Adult sympathetic nerves, there are also prefrontal nerves and gyrus nerves sent from the frontal ganglion forward and backward, and the pharyngeal sympathetic nerves sent from the back edge of the brain, and are distributed to the pharyngeal body and salivary glands through the lateral body of the heart. The sympathetic nerves in the thorax and abdomen of adults are similar to those of larvae. The nerves sent out from the rear end of each ganglion of the thorax and abdomen travel along the nerve cord to the left and right branches in front of the next ganglion, and are distributed to the muscles of each valve opening and closing device. However, the adult worm does not have a sympathetic nerve network. The two sides of the nerve cord send out most nerves to form a triangle. The top is attached to the longitudinal muscle at the link level, and the bottom is on the nerve cord.

Tissue structure of silkworm nerve

The ganglion is pentagonal or hexagonal, and the left and the right are merged into a pair, and the front and back ends are connected in series with the adjacent ganglia to form the abdominal nerve cord. Each ganglion is composed of many nerve cells and their fibrils, mainly the cell body of motor neuron and its axons and side branches, the axon of sensory neuron and its end plexus, and the cell body and its axis of connecting neuron. Processes, lateral branches and end plexuses. The cell body is mostly located at the periphery of the ganglion. The center of the ganglion is filled with nerve fibers to form the medulla of the ganglion. The surface of the ganglion is covered with a nerve sheath. The outer layer is acellular tissue, called the epineurium, the inner layer It is the cell layer, called perineurium. The inner side of the nerve sheath is the peripheral cortex, which is mainly distributed with neuron cell bodies.

The nerve originates from the medulla of the ganglion and is composed of nerve fibers. The outer periphery is covered with a nerve sheath composed of epineurium and perineurium. The central part of the nerve in the nerve sheath is collectively called axon. Nerve fibers are connected into a bundle with glial cells. The surface of each nerve fiber is surrounded by a thin axonal membrane, which contains axoplasm, which contains mitochondria. Peripheral nerves reach the end points of various tissues and organs, which are called nerve terminals.

Silkworm’s sensory organs

The visual organs of silkworms are organs that are stimulated by light waves, including monocular and compound eyes. Silkworm larvae have only one eye, while adults have compound eyes. The basic structure of monocular and compound eyes are similar, and both consist of a refractor and a photoreceptor. The refractor includes the cornea and the crystal, which can transmit and condense light; the photoreceptor is composed of omental cells. The compound eyes of the silkworm moth consist of hexagonal eyelets arranged in a honeycomb shape. A compound eye is made up of more than 3000 eyelets. The structure of the eyelets is very similar to the single eye of a larva. The visual focal length of larvae’s single eye and adult’s compound eye is extremely short, and the ability to adjust the distance and light refraction of objects is extremely poor, and can only myopia within the range of 1.4 cm. The visual organs of silkworms have different responses to light sources. The larvae have the strongest tropism to yellow-green light, followed by orange, green, and cyan, and ant silkworms and mature silkworms are the most prominent. The larvae of each instar show positive phototaxis when the contrast is below 100 lux, and the low illuminance of 15 lux is stronger, and the mature silkworm is more prominent. In the same instar period, the phototaxis of silkworms is the strongest, and the phagocytic period and hypnotic period are weak.

The olfactory organ of the silkworm is the response organ to the stimulation of gaseous substances. It is mainly a kind of bell-shaped sensilla. Its shape is similar to the base of the bristles. There are olfactory holes on the head shell, head appendages, body and appendages of larvae. , So the silkworm’s sense of smell is very sensitive.

The taste organs of silkworms are organs that sense the stimulation of liquid substances, including cone sensilla and plug cone sensilla. Mainly distributed on the mouthparts. There are two small nodular protrusions on the lower palate tumor-like body of the silkworm, which can sense the sweetness and bitterness of food, salt and water. The 3 pairs of protrusions on the inner surface of the upper lip are also taste organs.

The tactile organs of silkworms are also called mechanoreceptors, which mainly sense the stimulation of external solid objects and can sense temperature, airflow, and air pressure. The sensilla is spread all over the surface of the silkworm, especially the antennae and mouthparts.