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External Morphology of Bombyx Mori

  • Zoology

In this external morphology of Bombyx Mori post we have briefly explained about bombyx mori moth external morphology eggs, larva, head, thorax, abdomen, pupa and adult.

Bombyx Mori moth pass through a complete metamorphosis (Holometabola) from egg to the adult stage through two intermediate stages of larva (caterpillar) and pupa (cocoon). External morphology of bombyx mori moth (silkworm) in its different life-cycle stages are discussed below:

External Morphology of Bombyx Mori Moth

Egg

Bombyx Mori moth eggs are tiny and weigh around 2000 eggs to a gram. They measure 1 to 1.3 mm in length and 0.9 to 1.2 mm in width. The eggs of the European races are comparatively larger and heavier. They may be ovoid, flat, ellipsoid or oval and flat with a micropyle at the anterior pole and slightly off-centre. Races producing white cocoons lay pale yellow eggs while races producing yellow cocoons lay deep yellow eggs. In both cases the Japanese races lay slightly darker eggs than the Chinese races.

External Morphology of Bombyx Mori

Structure of bombyx mori moth (Silkworm) Egg

In the case of the hibernating eggs laid by bivoltine and univoltine races, the egg colour changes and becomes dark brown or purple with the deepening of the colour of the serosal pigments.

Larva

Bombyx Mori moth larva when newly hatched is black or dark brown. It has a large head and the body is densely covered with bristles so that it looks like a hairy caterpillar. There are four pairs of tubercles; the subdorsal, supra spiracular, infra spiracular and basal tubercle each of which carries three to six setae. As the larva grows, it becomes smoother and lighter in colour due to the rapid stretching of the cuticular skin during the different instars of the larval stage.

External Morphology of Bombyx Mori

Structure of Bombyx Mori moth (Silkworm) Larva

The larval body is composed of the same segments as any insect: head, thorax and abdomen. The entire body is covered with a thin and elastic chitinous cuticle which is capable of being extended considerably to permit rapid growth of the larva during any instar. The integument consists of the epidermis and the cuticle over it.

Head

The head consist of six body segments fused together with a cranium. The second, fourth, fifth and sixth segments carry appendages which are modified into antennae, mandibles, maxillae and labium respectively. There are six pairs of ocelli or larval eyes which are located behind and a little above the base of the antennae. There is a pair of antennae formed of five jointed segments and these are used as sensory organs (feelers).

Structure of bombyx mori moth (Silkworm) Head

The mandibles are well developed and powerful and are adapted for mastication. In the mouth region the maxillary lobe and maxillary palpi discriminate the taste of food. Also, a median process or spinneret is present through which silk is expelled from the silk gland to form the silk bave or thread. The sensory labial palpi are found on both sides of the spinneret.

Thorax

The thorax consists of three body segments called the pro-meso-and meta-thorax. Each of the three thoracic segments carries ventrally a pair of legs each comprising in turn three jointed segments. These are the true legs which are conical in shape and carry sharp distal claws. These claws are not used for crawling but for holding mulberry leaves while feeding. All the Bombyx Mori moth larvae that show body larval markings carry the so-called eye spot on the dorsal side of the meso-thorax.

Abdomen

The abdomen is comprised of eleven body segments although only nine can be distinguished as the last three are fused together to form the apparent ninth segment, the anal plate and the caudal legs. The third to sixth and the last abdominal segments each bear a pair of abdominal legs which are fleshy, unjointed muscular protuberances. 

At the extremity they form a sort of disc with a series of hooks inwardly curved and arranged in a semicircle. On the dorsal side of the eighth abdominal segment, the larva carries the caudal horn. The abdominal segments carry the sexual markings which develop distinctly in the fourth and fifth instars in the eighth and ninth segments on the ventral side. 

In the female the sexual markings appear as a pair of milky white spots in each of the eighth and ninth segments. The pair of spots on the eight segments is known as Ishiwata’s fore glands and the pair on the ninth segment is referred as Ishiwata’s hind glands. In the male a small milky white body (Herold’s gland) appears at the centre of the ventral side between the eighth and ninth segments. 

There are nine pairs of spiracles placed laterally on either side of the body. They are found on the first thoracic segment and the first eight abdominal segments. These are the breathing or respiration pores. The larval skin or integument consists of the cuticle and the hypodermis. The cuticle is made of chitin as well as protein and is covered with a thin layer of wax. 

Nodules are found all over the body surface of the Bombyx Mori moth larva. The nodules are found in larger numbers over parts of the body where normally the larval markings are found. The larval markings in Bombyx Mori moth are caused by skin pigment.

Pupa

The pupal stage is generally called the resting, inactive stage of the Bombyx Mori moth when it is incapable of feeding and appears quiescent. The pupal stage is a transitional phase during which definite changes take place. During this period of biological activity, the larval body and its internal organs undergo a complete change and assume the new form of the adult moth. 

The mature Bombyx Mori moth larva passes through a short transitory stage of pre-pupa before becoming a pupa. During the pre-pupal stage, the dissolution of the larval organs takes place and this is followed by the formation of adult organs during the pupal stage. Soon after pupation the pupa is white in color and soft but gradually turns brown to dark brown and the pupal skin becomes harder.

Male and Female bombyx mori moth Pupa

The prominent morphological parts visible are a pair of large compound eyes, a pair of large antennae, fore-and hind-wings and the legs. Ten of the abdominal segments can be seen on the ventral side, but only nine of them are visible on the dorsal side. 

Seven pairs of spiracles can also be discerned in the abdomen; the last pair is non-functional. Sex markings are prominent and it is much easier to determine sex in the pupal stage than in the larval stage. The female has a fine longitudinal line on the eighth abdominal segment, whereas such a marking is absent in the case of the male.

Adult

The adult moth emerging from the pupa is incapable of flight as a result of its domestication for more than four thousand years. It does not feed during its short adult life. The body of the moth like that of 8 the larva is composed of three distinct segments: head, thorax and abdomen. 

Another important characteristic in common with other moths and butterflies is that the body surface is covered with scales. The compound eyes are situated on either side of the head. The ocelli are absent. The antennae are conspicuous, large and bi-pectinate.

External Morphology of Bombyx Mori

Adult Male and Female bombyx mori moth.

The thorax consists of three segments namely meso and meta-thorax as in the larva. The meso thorax is the largest and is pentagonal. There are three pairs of thoracic legs, one pair on each of the three thorasic segments. Each of the thorasic legs is composed of five segments. 

The meso and meta-thorax bear two pairs of wings, the front pair overlapping the hind pair when the moth is in the resting position. In the male eight abdominal segments are visible; in the female seven. There are six pairs of spiracles present laterally on either side of the body.

 Morphologically the female and male can be easily distinguished in the adult stage. The female has comparatively smaller antennae, its body and the abdomen are fatter and larger and it is generally less active than the male moth. 

At the caudal end, the male moth has a pair of hooks known as harpes whereas the female moth has a knob-like projection with sensory hairs. These differences help to a large extent in separating the sexes for preparation of hybrid eggs.

Further Readings