Class 10 Science Invertebrates

Class 10 Science Biology Notes on Invertebrates

Those animals which do not have a backbone in their body are called invertebrates. For example amoeba, paramecium, housefly, mosquito, silkworm, honey bees
etc. are invertebrates.

Silkworm:

A silkworm is a useful insect as it produces high-quality silk fibre. The silk fibre is used to produce silk clothes, shawls and other attractive as well as expensive clothes. Silk is obtained from the cocoons of silkworms.

Types of Silkworm:

There are two types of silk moths commonly reared in Nepal. They are:

1. Eri silkworm )Attacus ricini)
2. Seri silkworm )Bombyx mori)

Here, Seri silkworm feeds on mulberry leaves and Eri silkworm feeds on castor leaves

The external structure of silk moth:

A silk moth is two to three centimetres long (in average 2.5 cm) with a creamy white feathery body. 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. The last segment has external genitalia (reproductive organ). The abdomen of the female is larger than a male silkworm.

The life cycle of silkworm:

 

Fig: Life cycle of silkworm

The lifecycle of silkworm completes in four stages i.e. egg, Iarva, pupa and adult. The male silkworm dies after copulation and the female moth dies after laying eggs.

1. Eggs

A female moth lays about 300 — 400 eggs in clusters on a mulberry leaf. The eggs are whitish, small and have a round structure, which turns grey later. During suitable conditions and temperatures of about 18C to 25C, the egg hatch into larvae in about 10-12 days.

2. Larva

The larva of the silkworm is grey in colour and it is the very active stage of the lifecycle of the silk moth. The larva of the silk moth or caterpillar is the voracious feeder on mulberry leaves. It is only the stage of eating and growing. The larva moults four times in the sixth, twelfth, eighteenth, and twenty-sixth days respectively. While moulting, the caterpillar stops feeding for some hours.

Note: The stage of the larva between two successive moultings is called an instar. After the last moulting, it develops a pair of salivary glands which secrete liquid silk on the lateral side of the body and when this liquid silk comes in contact with air, it becomes hard with fine thread, inside which the caterpillar is enclosed which is also known as the cocoon or pupa case.

3. Pupa

It is the inactive stage in the lifecycle of the silkworm, which doe5 not eat and move at all. However, histogenesis and histolysis are taking place inside the silk cocoon. The silk thread is obtained in the pupa stage of the silkworm.

[Note: For obtaining silk thread for commercial purposes, the cocoons are put in hot water or oven to destroy the glue of the silk thread and to kill the pupa so they can obtain very long silk thread from a single cocoon.]

4. Adult

The pupa develops inside the cocoon into the adult silkworm. It emerges out by breaking th» cocoon with the secretion of alkaline fluid, which softens the thread. The adult silkworm has 3 pairs of legs and 2 pairs of wings. It lives for Sto 6 days. An adult silkworm does not feed at all. This stage is just for reproduction and to continue the lifecycle.

Features of silk thread:

1. Silk is shiny, attractive, light and durable fibre.
2. It is strong, long and elastic fibre.
3. It can be coloured easily.
4. It can be dried easily.
5.  It does not decay easily.
6. The clothes made from silk fibre are attractive, durable and expensive.

Economic importance of silkworm

1. Silk is used to make clothes, nets, surgical stitching thread, curtains, etc.
2. It is used for fishing.
3. It can be blended with other fabrics to make more attractive clothes.
4. It increases the economic condition of a person thereby uplifting the economic status of the nation.
5. A maximum number of women can be employed in sericulture.
6. It can be reared in low land areas.

Honey bee

Honey bees are very useful social insects. They belong to the phylum Arthropoda and class Insecta of invertebrates. They live in a big colony/hive feed on pollen grains and nectar or juices of flowers. Honey bees being social insects; live in a big colony, produce honey and have a strong discipline, leadership, higher understandings, unity and division of labour.

Anatomy of Honeybee

Honey bees are usually oval shape creatures with golden-yellow colours and brown brands. The queen bee is the largest bed with a large elongated abdomen and it is up to 2cm long in length. Drone bees are smaller than the queen bee but larger than workers. Worker bees are the smallest in size and they measure up to 1.5cm.

The body of the honey bee is divided into three major parts (i.e. head thorax and abdomen) The head consists of eyes, an antennae and a feeding structure. The eye includes a compound eye and a simple eye. Compound eyes help the bees to understand colour, light and UV-ray direction while simple eyes detect the amount of light present. The antenna detects odours and measures flight speed. The mandible is the bee's jaw used in eating pollen, cutting and shaping wax, feeding larvae and the queen, cleaning the hive, grooming and fighting. Worker bees have the ability to chew and lapping.

The thorax has three segments and it consists of two pairs of wings, three pairs of legs and

movement controller muscles. Fore wings are used for flight and cooling the body while hind limbs cool the hive. The legs of worker bees are covered with hair and they contain a pollen basket for storing the pollen and nectar. The sixth segment of the abdomen includes the female reproductive organ in the queen bee and the male reproductive organ in the drone. Both the queen and worker bees have stingers at the end of their abdomen.

Three types of Honey Bee

There are three types of the honey bees. In a hive or colony, They are:

1. Queen bee (which is egg-laying female; one in a colony)
2. Drones ( a few hundred in a colony)
3. Worker (sterile females, about 20 thousand to 80 thousand in a colony) 

Life Cycle of a Honeybee

The honey bee's life cycle goes through 4 basic stages. They are egg, larva, pupa and adult stage. As a colony has three classes of bees, queens, drones and workers, each class of bees develops as an adult over varying durations.

Egg stage

A queen bee lays eggs up to 3,000 eggs in a day. The queen bee packs eggs closely together within cells. The eggs are long elongated and pink coloured. During the first stage of its development, the egg cell divides rapidly and form the digestive system, the nervous system and the outer covering. After three days, the egg hatch into the larva.

Note:

Each and every egg of the bee doesn't fertilize by sperms. Fertilized eggs are diploid with a pair of 16 chromosomes (16 from male and 16 from female) while unfertilized eggs are haploid and have only 16 chromosomes in them. Fertilized eggs are developed as queen bees and workers bees while unfertilized eggs develop as drone bees. Future queens develop into a large cells in the egg stage.

Larva Stage

During the larva stage, each larva is fed about 1,300 times a day. They are fed ‘royal jelly’, a nutritious food, by worker bees for 3 days. These workers bees have the specific task of feeding larvae and tending the brood and are often called ‘brood nurses’. After three days, the larvae of worker bees and drone bees are feed with less nutritious food called ‘bee bread’ which is made from pollen, honey and the secretion of brood nurse while the potential queen larva receives royal jelly continuously. Due to differences in intake of nutrients, bees show differences in the rate of their development. All the larvae undergo moulting and their body develop rapidly.

Pupa stage

At the end of the larva stage, the brood cell gets close and the larva becomes the pupa. In about 5.5 days, the potential queen bee's larva spins itself into a cocoon and becomes a pupa while it takes 6 days for the drone and the worker larva to become a cocoon inside it. Under the broad, inside the cocoon, the pupa gets metamorphosed into an adult honey bee.

Adult stage

A queen bee gets metamorphosed from the pupa to the adult stage in about 7.5 days while it takes 14.5 days for the drone bee and 12 days for the worker bee. For about 21 days, the adult worker bees engage themselves inside the hive as brood nurses by cutting and shaping wax, feeding larvae and the queen, cleaning and protecting the hive. After 21 days, worker bees get out of the hive for the collection of pollen and nectar from the flower of plants as well as they protect the hive from enemies.

When a worker bee sees a source of pollen and nectar, it dances and communicates the information with other bees which is known as the honey bee dance. Drone bees are lazy and do not work. Honey bee colonies survive in winter as they have enough food resources, are able to keep sufficiently warm and are free from diseases. However, in the winter, colonies are smaller than in summer because part of the colony leaves the hive and some worker bees

die due to cold. The queen bee lives up to 2 years, drones up to 4 months and the worker bees live up to 4to 6 months. When the number of bees and queens increases in the hive, a new queen and worker bees separated from the colony along with drones to form a new colony. When the existing queen dies or becomes incapable of laying eggs, worker honey bees rises a new queen from their colony. As a new queen becomes young, she attends a nuptial flight.

A nuptial flight is a mode of flying of the queen bee for mating with several drones. After mating drones die due to the exhausting of energy released in the form of sperm. When the sperms are stored in its abdomen, the queen bees begin to lay eggs in the hive.

Different types of honey bees and their work division.

Bees	Work
Drone bees	To fertilize female.
Queen	Laying egg and controlling the colony.
Workers of 1- 3 days	They are very small and weak. They warm the eggs, larva and pupa.
Workers of 4- 6 days	They feed larva and eat themselves.
Workers of 7- 11 days	They produce royal jelly. They feed royal jelly to larvae and queens and also eat themselves.
Workers of 12- 17 days	They produce wax and use wax to make honeycomb. They close the mouth of the honeycomb by using wax.
Workers of 18 - 20 days	They produce poison and protect the whole comb.
Workers of 21 days	They become mature and go out of the comb for searching nectar, pollen, water, etc.

                                               

Importance of honey bees:

1. Honey bees help in pollination while moving from flowers to flowers of plants. Without pollination from bees, some of the plants can not fertilize and fruit/food production decreases.
2. Honey is packed with a number of nutrients that are necessary for human health benefits.
3. Honey bees are important for apiculture, through which honey is manufactured on a large scale.
4. Honey is used in bakeries, confectioneries, food industries, pharmaceutical industries, etc.
5. Natural waxes which are secretion of honey bees are used in quality candles, cosmetics and in polishing goods.

Important Question Answers related to Silkworm and Honeybee

Q.1) What is sericulture?

Ans. The rearing of silkworms for the production of silk thread in a commercial scale is known as sericulture.

Q. 2) Classify silkworm.

Kingdom  — Animal

Sub-kingdom — Invertebrate

Phylum — Arthropoda  

Class      — Insecta

Type      — Silkworm

Q. 3) In which phase does the larva produce the silk thread?

Ans. In the 5th instar of the larva, the silk thread is produced by the larva.

Q. 4) Why silk moth is called the queen of fibres?

Ans. Silkmoth produces shiny, strong, long and attractive quality fibre. So, it is also called the queen fibres.

Q. 5) Why do farmers keep the silkworm's eggs in cold places?

Ans. The cold environment (i.e. below 180C ) for the egg of the silk moth prevents the hatching the larva from the eggs. As larvae of the silkworm is a voracious feeder on mulberry leaves, it is done when there is a shortage of the mulberry leaves as the food for the newly hatched larvae.

Q. 6) What is swarming?

Ans. The process by which a new honey bee colony is formed when the queen bee leaves the colony with a large group of worker bees is called swarming.

Q. 7) Show the differences between:

a) Silkworm and honeybee

Ans: The  differences between silkworm and honeybee are as follows:

Silkworm	Honeybee
It has five stages called instars.	It has no such instar.
Different forms of larvae are not differentiated on the basis of food intake.	Different forms of larvae like queens, workers and drones are differentiated on the basis of food intake.

b) workers bees and drones

Answer: The differences between worker bees and drones are as follows:

Worker bees	Drone Bees
They feed the newly formed larva.	They fertilize the queen.
These are sterile female bees.	These are fertile male bees.
They are smaller than drones.	They are larger than the workers.
They are the busiest and hardworking bees.	They are not busy and hardworking bees.

                                                        

Q. 8. Define the following terms:

a. Brood nurse

Ans: Workers bees have the specific task of feeding larvae and tending the brood and often called ‘brood nurse’

b. Bee bread

Ans: Low nutritious food fed to all the larvas for 3 days and only fed to the worker and drone bees afterwards is called bee bread.

c. Nuptial flight

Ans: A nuptial flight is a mode of flight for the queen bees to mate with drones.

d. Drone bee

Ans: Drone bees are basically male bees.

 9. Give reasons:

a. Royal jelly is fed continuously to the queen bee for six days. Why?

Ans: Royal jelly is a nutritious food that helps in the biological and physical development of queen bee as queen bee is larger in size than other. Hence, for the proper development of the queen bee royal jelly is fed continuously to the queen bee for six days.

b. Why don't worker bees have reproductive organs in their abdomen?

Ans: Worker bees don't have reproductive organs in their abdomen because worker bees are sterile female bees and their reproduction organs do not function.