The following points highlight the four main stages involved in the life cycle of silkworm moth. The stages are: 1. Adult Moth 2. Eggs 3. Larva 4. Pupa and Imago.
Stage # 1. Adult Moth:
The adult moths are 25 mm in length and the span of wings is 40-50 mm. The female silkworm moths are larger than the males. The univoltines are larger than the multivoltines. Usually whitish in colour and in some forms specially the males have grey marks on their wings. The body is distinctly divisible into three tagmata—head, thorax and abdomen.
The head contains distinct eyes and feathery antennae, the latter being larger in males. Three pairs of legs and two pairs of wings are present in the thoracic region. Female moths do not have any mouth. They rarely move. Internally, the body contains well-developed excretory and reproductive systems.
The digestive system is poorly developed. The excretory organs are three pairs of Malpighian tubules. There are three such tubules on each side. A duct from each side unites together to form a common tube which opens into the stomach at its posterior end. In males, the paired testes are lodged within a capsule.
From each testis originates a duct, called vas deferens, which inflates immediately after its origin to form a seminal vesicle. Posteriorly the two vasa deferentia unite to form a much coiled ejaculatory duct which opens to the exterior through the genital opening. In the female, each of the paired ovaries contains four egg tubes.
From each ovary arises an oviduct; the two oviducts unite to form a common oviduct. There are two female genital apertures—through one opens the oviduct and through the other communicates a large sac-like copulatory pouch. A short tube links the pouch with the oviduct. This tube is called seminal duct.
A portion of it is dilated to act as spermatheca. Both the sexes are provided with accessory glands which open within the genital ducts. A scent gland is present at the terminal end of the female abdominal cavity. Its secretion attracts the males of the same species. During copulation, the male sits on the back of the female and grips it tightly by its chitinous hooks.
Such pairing lasts for three hours. This is immediately followed by egg lying. The sperms enter through a small opening on the egg, called the micropyle. But the actual fusion of male and female pronuclei occurs two hours after lying. Parthenogenesis, i.e., development of egg without the participation of sperm, is also common in silkworm moth.
Stage # 2. Eggs:
The colouration, size and weight vary in different species. The eggs are small, oval and usually slightly yellowish in colour. The egg contains a good amount of yolk and is covered by a smooth hard chitinous shell. Approximately 500 eggs are laid in 24 hours.
In some forms the eggs are glued on the surface of the leaf by a product secreted from a special gland. The univoltine broods hatch after one year but the multivoltine broods come out after 10-12 days. From the egg hatches out a larva, called caterpillar, which has no resemblance to the adult.
Stage # 3. Larva:
Each larva is 3 mm in length and is provided with a thick hairy covering (Fig. 18.82A). The colour is usually greyish brown but the colouration changes in course of development. The larva possesses a prominent head, distinctly segmented thorax and elongated abdomen. A conspicuous crescent spot is present on the dorsal side of the sixth segment.
The head is formed by the fusion of three segments. At the anterior end a triangular area is formed by a pair of oval lobes. The mouth parts include a pair of strong mandibles, a pair of lips, a pair of maxillae and two pairs of maxillary and labial palps. The head also bears a pair of antennae and six pairs of ocelli. A distinct hook-like structure, the spinneret, is present for the extrusion of silk from the inner silk gland.
Each of the three thoracic segments bears a pair of legs having three articulations. The tip of each leg has a recurved hook for locomotion and ingestion of leaves.
The abdomen is divisible into ten segments of which the first nine are clearly marked while the tenth one is indistinct. The third, fourth, fifth, sixth and ninth abdominal segments bear abdominal appendages, called false legs. Each leg is retractile and more or less cylindrical.
The body of the caterpillar contains following internal structures (Fig. 18.82B):
(1) Alimentary system:
The alimentary canal is prominent and includes a short oesophagus, a spacious stomach, much coiled and long intestine and a swollen rectum. A pair of salivary glands opens by a common salivary duct to the mouth cavity and serves as a digestive gland.
(2) Respiratory system:
Tracheal trunks open to the exterior by nine pairs of spiracle. First pair are present on the first thoracic segment and remaining pairs are situated on the first eight abdominal segments.
(3) Circulatory system:
The heart is present immediately above the alimentary canal along the mid-dorsal line of the body cavity. It is a transparent contractile tube, also known as the dorsal vessel. The blood is usually colourless but may be pink or green. The colour of the silk depends upon the colour of the blood.
(4) Nervous system:
The nervous system includes a pair of cerebral ganglia as brain and a paired ventral nerve cord carrying twelve ganglia in its path. The sense organs are present as six pairs of ocelli for detecting light and numerous sensory receptors on the maxillary and labial palps.
(5) Silk gland:
These unique and conspicuous glands are paired structures, which are present in the fourth to eighth segments of the larva. When fully formed each gland becomes five times the length of the larva and its weight becomes two-fifths of the body weight. Each gland is divisible into three sections—the anterior, middle and posterior (Fig. 18.83).
The middle part is broad and called the reservoir. The anterior and posterior parts are pointed at their two ends. The two anterior ends of the glands unite to form a common duct which opens through a spinneret. The posterior part produces a protein, called fibroin, around which the middle part puts an envelope, called sericin.
The silk is released in a liquid state, which soon hardens. A pair of accessory glands, called the glands of filippi or Lyononet’s glands, open into the duct of the silk gland. The secretion of this gland mixes with that of the silk gland and probably lubricates the inner and outer cores of the silk.
(6) Reproductive system:
The reproductive organs are very minute and their ducts are indistinct.
Transformation of larva:
The larva is a voracious eater. In the beginning, chopped young mulberry leaves are given as food, but with the advancement of age entire and matured leaves are provided as food. The routine of caterpillar includes only two activities—eating and sleeping. It grows fantastically and increases, 10,000 times in weight from newly hatched state.
Such growth involves the consumption of mulberry leaves which are 30,000 times more than its body weight. The growth requires the replacement of exoskeletal covering and the larva within its 30-40 days life does the same for four times. Such removal of the old exoskeleton is known as moulting.
At the time of moulting, the larva does not take any food and places its head upwards. This phase is very critical in the life of larva. At the end of fourth moult a fully formed larva with matured silk gland becomes transparent and golden brown in appearance. At this stage the larva ceases to eat and starts spinning silk around its body from outside to inside (Fig. 18.84).
In order to make a complete covering the larva rotates 60,000 to 3,00,000 times and the silk is liberated at the rate of 15 cm per minute. This covering, called the cocoon, is formed of a continuous silken thread of 400-1500 metres long.
The caterpillar takes 3-4 days to complete a cocoon within which the larva, now known as pupa, remains completely immobile. A cocoon may be of varied shapes and colours. The cocoon formed by a male silkworm moth is lighter in colour than that of a female and contains more silk.
Stage # 4. Pupa and Imago:
It is covered by a hard shell. It generally remains immobile, but can change its position by the contractile movement of the last few abdominal segments. Within the pupa, considerable activity takes place. The old structures of pupa are broken down by a process, called histolysis, and new parts of the adult are prod iced.
Gradually within the cocoon, pupa transforms into a stage, called imago. For the purpose of breaking the cocoon, the imago liberates a fluid which dissolves the covering at one end. The emergence of adult takes place after 10 days of pupal life (Fig. 18.85).