CHAPTER 14
INVERTEBRATES
Those animals which do not have a backbone in their body are called invertebrates.
For example, amoeba, paramecium, housefly, mosquito, silkworm, honeybee, etc.
SILKWORM
Kingdom- Animal
Sub-kingdom- Invertebrate
Phylum- Arthropoda
Class- Insecta
Type- Silkworm
There are two types of silk moths commonly reared in Nepal. They are Eri and Seri. Bombyx mori or Seri feeds on mulberry leaves and Eri or Attacus ricinii feeds on caster leaves. The cultivation of silkworms is known as sericulture.
EXTERNAL MORPHOLOGY
The silkworm is a medium sized insect. It is creamy white in colour. The length of a silkworm is from two to three cm.
The body is divided into three parts i.e. head, thorax and abdomen. The head bears a pair of compound eyes, a pair of antennae and a sucking type of mouth. The thorax
bears two pairs of wings and three pairs of legs. The abdomen consists of ten segments containing seven pairs of spiracles. The
abdomen of the female is larger than that of the male silkworm.
LIFE CYCLE
The life cycle of silk moth demonstrates the most advanced form of metamorphosis. Silk moths are unisexual, i.e. sexes are separate. Silk moth passes all the stages of an insect in its life cycle. Silk moth lives on mulberry trees. The adult moths seldom eat and are primarily concerned with reproduction. The male dies after copulation and the female is oviparous and dies after laying eggs. The serial progressions of four distinct stages of development complete one generation of the species. The four stages in the life cycle of a Silk moth are as follows:
(i) Eggs:
A female silk moth lays about 300-400 eggs in clusters on a mulberry leaf. The eggs are whitish, small and have a round structure, which turn grey later. On getting a favourable condition and temperature of about 18°C to 25°C, the eggs hatch into larvae in about 10-12 days.
When there is a scarcity of mulberry leaves, the eggs are kept in cold places to prevent from hatching.
(ii) Larva:
The larva of the silkworm is grey in colour and it is a very active stage of the lifecycle of the silk moth. The larva of the silk moth or the caterpillar is the voracious feeder on mulberry leaves. It is only considered for eating and growing. The larva moults for four times in sixth, twelfth, eighteenth and twenty sixth days respectively. While moulting, the caterpillar stops feeding for some hours.
The stage of the larva between two successive moltings is called instar. The fifth instar is about 100 times bigger than the first instar larva. After the last moulting, it develops a pair of salivary glands which secrete liquid silk on the lateral side of the body. When this liquid silk comes in contact with air, it becomes hard with fine thread, inside which the caterpillar is enclosed. This stage is known as the cocoon or pupal case.
(iii) Pupa:
It is the inactive stage in the lifecycle of the silkworm, which doesn’t eat and move at all. However the histogenesis and histolysis are talking place inside the silk cocoon. The silk thread is obtained in the pupal stage of the silkworm.
For obtaining silk thread, the cocoons are put in hot water or oven to destroy the glue of the silk thread so they can be obtained and unwounded easily. About one thousand metres of silk thread can be yielded from a single cocoon.
(iv) Adult:
The pupa changes into an adult from about twelve to fourteen days. The imago breaks the cocoon by the secretion of alkaline fluid, which softens the thread. The imago cannot fly immediately but as its wings get dry, it flies and the female lays eggs on maturity. Hence, its lifecycle continues.
ADVANTAGES FROM SILKWORM
1. Silkworms are very useful insects. About 1000 to 1500 meter long thread can be extracted from a single cocoon.
2. Silk extracted from silkworm is natural silk which is many times shiny, light, soft and durable as compared to artificial silk.
3. Intestines of silkworms are useful in various surgical finishing purposes.
FEATURES OF NATURAL SILK FIBRE PRODUCED FROM COCOON
1. Silk fibre is very long, attractive, strong and shiny.
2. Clothes made from silk are valuable and that can be used in all seasons.
3. It has high elasticity so it can be stretched easily.
4. It is used in making very fine and expensive clothes.
5. It absorbs water rapidly and can be dried easily.
6. It can be coloured easily.
7. It has high resistant to deformation.
8. The fibre is more comfortable to wear.
USES OF SILK FIBRE
1. Silk fibre is used to make clothes and insulation coil of telephone.
2. The dead pupae obtained during extraction of silk fibre are used to feed livestock and fishes.
HONEYBEE
Kingdom : Animalia
Phylum : Arthropoda
Class : Insecta
Type : Honeybee
Honeybees are the most familiar members of the phylum Arthropoda and class insecta.
The small sized honeybee, employed for commercial bee-keeping (apiculture) in Nepal are Apis cerana, Apis dorsata, the giant honeybee and Apis florae, the little honeybee.
HABIT AND HABITAT
Honeybees are found all over the world and are known for their art of manufacturing the honey and bee-wax.
BEE COLONY
A colony of honeybees consists of three kinds of individuals or castes: (i) workers which are sterile females, (ii) drones which are fertile males, and (iii) a queen which is a fertile female. The workers are assigned the job of producing royal jelly, building the beehives, caring the larvae and disposing of the debris and dead bees. The drones and queen are for reproductive purposes. An average-sized colony consists of one queen, about 100 drones and 60,000 workers.
Worker bee
The worker bee is the smallest member of the colony and makes up the largest number of the colony individuals. It is black or brownish in colour with the body densely covered with hairs. For 10 days, it acts as a foster mother, feeding younger brothers and sisters. For more 10 days it cleans the hive and builds combs. As the fourth week approaches, it leaves the hive to forage for pollen and nectar till old age. Worker bees can visit 20-30 flowers in a minute to collect nectar and honey. Like all other insects, the body of the worker bee is divisible into three regions. They are head, thorax and abdomen.
Duties of worker bees
1. Making honey combs.
2. Collecting pollen and nectar to make honey.
3. Protecting the hive from enemies.
4. Looking after eggs, larvae and pupae.
5. Feeding the larvae and the queen.
Queen
The queen is the only fertile female in a beehive, having immensely developed ovaries. The queen arises from a fertilized egg and larva especially fed on royal jelly. She alone lays eggs and is the mother of almost all the members of the hive. She lives for several successive years laying about 2,000 or more eggs a day and up to about 1,500,000 eggs during her lifetime.
Duties of a Queen
1. To lay eggs.
2. To co-ordinate the worker and drone bees according to their necessity.
Drones
Drones are the male honeybees. They fly near the Fig. hive in the sunshine. There are usually 100 drones in a typical colony, depending upon the season of the year.
They develop parthenogenetically from the unfertilized eggs laid by the queen and exist only to mate with the queen of their own or some other colony.
Duties of Drones
1. To fertilize the queen bee
2. To keep the hive warm.
Parthenogenesis
In parthenogenesis, reproduction occurs asexually when a female egg cell develops into a new individual without fertilization. The formation of drones, worker bees and queen is shown below:
Swarming
The behaviour of the honeybee to come out of the hive in large numbers is called swarming. It takes place during the spring or early summer.
Nuptial or marriage flight
About a week after emergence, the new queen takes her first aerial flight followed by a swarm of drones. Thequeenfliesveryhighandthedronesgradually drop out of the race. The last drone left in the race, mates with her.
LIFE CYCLE
The life cycle of honeybee shows complete metamorphosis. There are four stages in the life cycle of honeybee. They are:
(i) Eggs
An adult queen of 3-5 days comes out of hive and mates with drones in air. Two to three days after mating, the young queen begins to lay eggs. The queen lays about 1500-3000 eggs per day. However, the number of eggs may vary according to the species of honeybee. The eggs are white and elongated. In the first day of laying, the egg is erect, in the second day, it is slanted and in third day, it is horizontal in the cell. The queen lays eggs in drone cells to make males, queen cells to make queens and in worker cells to make worker honeybees. The stage of egg lasts for three days. A fertilized egg is laid in a worker or queen cell while an unfertilized egg in a drone cell. All eggs laid by the queen bee are not fertilized. The unfertilized egg contains 16 chromosomes whereas a fertilized egg contains 32 chromosomes.
(ii) Larva
It is the second stage of the life cycle. After three days small larvae or grubs hatch out from the eggs. The formation of a queen or workers depends on the diet which the larvae receive. All larvae are fed by the workers for the first three days on the royal jelly, which is composed of digested honey and pollen mixed with a glandular secretion. The larvae destined to become queens receive this food throughout their life while, after the third day, the worker and drone larvae are fed upon increasing proportions of honey. If a worker larva is transferred to a queen cell before third day, it develops into a queen, and vice versa. The fully grown larvae are sealed with wax in the cells by worker bees. The larva sheds its skin for 5 times before changing into pupa.
(iii) Pupa
Pupa is the third stage of the life cycle of honeybee. it is non-motile stage. A pupal stage is passed within the sealed chamber during which the larva secretes a thin, silken cocoon around it. It takes the queen on an average 7.5 days, the worker 12 days and the drone 14.5 days to complete metamorphosis and emerge out as adults.
(iv) Adult
It is the last stage of the life cycle of honeybee. All the organs develop and the pupa changes into an adult after completing the metamorphosis. After laying eggs, a queen bee develops within 15-16 days, a worker bee develops within 20-21 days and a drone bee develops within 22-24 days. The body structure and functions of queen, workers and drones are different.
The division of labour according to the age of the worker bee is given below:
1. The worker bees of 1-3 days are very weak. They are also called baby bees. They move on bee hive, keep eggs and larvae warm and eat the food spread around them. Similarly, they clean the cells around them.
2. The worker bees of 4-6 days provide food to matured larvae. They also take a lot of food.
3. The worker bees of 7-11 days develop special gland in their head to secrete royal jelly. Worker bees feed this royal jelly to the queen and young larvae of 1-3 days. They also take a lot of food.
4. The gland that secretes royal jelly get dried in the workers of 12-17 days. They develop 4 pairs of wax glands in their abdomen which secrete wax. The wax is used to construct the beehive and to close the larvae cells and honey cells. The workers of 12-17 days are also called "constructors".
5. The worker bees of 18-20 days develop sting and poison glands. They protect their hive sitting at the entry point.
6. After 21 days, the worker bees come out of hive and they fly away to collect nectar, pollen, sap, water, etc. from crop fields and forests.
USES OF HONEYBEE
1. Honey bees produce honey which is used as a nutritious food. Honey is also used to prepare jam, honey cakes, biscuits and breads.
2. Bee wax is used for making face packs, creams, lotions, candle, etc.
3. Honey is used as a medicine to cure weakness, fatigue, nervous disorder and mental disorder.
4. Honey is also used to cure hyperacidity, gastric ulcer and to kill bacteria.
5. Honeybees increase crop productivity by helping in pollination.
