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Describe reproduction in:
Differentiate between complete and incomplete metamorphosis.


Metamorphosis is defined as the series of gradual changes of formed and shape of an insect from the fertilized egg (immature stage) to adult (mature stage of adulthood).

Certain organisms when they are hatched from eggs do not exactly resemble the adult form. They therefore undergo series of changes in shape or form until the adult stage is reached. This process is what is called metamorphosis.


There are two types of metamorphosis. These are complete and incomplete metamorphosis.

Complete metamorphosis

Complete metamorphosis is the series of gradual changes which take place in insect from fertilized eggs to lava, then to pupa and finally to the adult stage.
Complete metamorphosis involves four stages. Which are: eggs==larva==pupa==adult
Examples of insects which exhibit complete metamorphosis include housefly, mosquito, butterfly, bees, wasps and beetles

Incomplete metamorphosis

Incomplete metamorphosis is defined as the series of gradual changes in insects which take place from fertilized eggs to nymph and finally to the adult stage. Incomplete metamorphosis involves only three stages. They are: egg=nymph==adult.
Examples of insects which exhibit incomplete metamorphosis include: grasshopper, cockroach, dragonfly, locust, aphids, termites and praying mantis.

Differences between complete and incomplete metamorphosis

1. Four stages of development are involved.
2. There is presence of pupa (resting stage)
3. There is no resemblance between pupa and adult
4. There is absence of nymph
5. Examples of complete metamorphosis are housefly and mosquito.

1. Three stages of development are involved
2. There is absence of pupa (no resting stage)
3. There is resemblance between nymph and adult.
4. Nymph is present
5. Examples of incomplete metamorphosis are cockroach and grasshopper.


Life history of cockroach is an example of incomplete metamorphosis.
The cockroach exhibits incomplete metamorphosis. This starts from fertilized egg to nymph and finally to adult or imago stage i.e. Egg==Nymph==Adult
Mating begins when the male introduces sperm into the posterior part of the female’s abdomen through the genital opening. These sperm are then stored in sperm pouch in the body of the female. From here, they meet the eggs as the latter pass towards the exterior. Fertilization is internal.
i. Egg stage: the egg of the cockroach are laid in a horny egg case called ootheca (plural oothecae) formed in the body. This is a chitinous structure which looks like a purse. There are usually 10-15 eggs in one egg case and they are arranged in two rows of 5 to 8 eggs per row. The female cockroach carries this egg case for some time in its abdominal pouch. It then deposits it in a dark, warm and humid place.
ii. Nymph stage: after some time (about 30-100 days) these eggs hatch out into very small, wingless, colourless nymphs. These nymphs feed and grow large and soon begin to moult. During these series of moulting, the wings appear, the nymptial life last for 10 to 16 months.
iii. Adult stage: the appearance of the wings shows adulthood in the nymph. The adult cockroach now has wings, legs, abdomen, antenna etc. fully developed. The cockroach requires about 11 to 20 months to develop from egg to adult.


Life history of the housefly is an example of complete metamorphosis
The housefly exhibits complete metamorphosis, i.e. Egg==Larva==Pupa==Adult.
The housefly breeds on any decomposing organic materials e.g. faeces, rotten meat etc.
Housefly has a short life cycle of about 3-4 weeks.

Egg stage:

the female housefly lays about 2-7 batches of eggs, each batch consisting of 100-150 eggs. The eggs which are white are laid in moist, rotten animal, and plant remains including human faeces. They hatch out in 8 hours to 3 days into white larvae called maggots.

Larva stage:

the body of the larva called maggots is made up of twelve segments. It is pointed at the anterior end which bears the head (on the first segment) but very broad at the posterior end. The head bears a pair of hook used for tearing food and to draw the maggot along. It has small mouth which lies between two oral lobes. On the ventral surface of each of the sixth to twelveth segment lies a small spiny pad which takes the place of the legs. The maggot has two pairs of spiracles used for breathing. They are located on the 2nd and on the 12th segments of the body. It moults or sheds its skin several times and last for about 5 to 14 days after which it moves to a dry place to begin the pupa stage

Pupa stage:

the maggot shortens, its skin becomes hard and brown and functioning as the pupa case or puparium. The pupa is an oval barrel-shaped object with rings running transversely around it. It does not feed or move. It is during this stage that internal re-organisation of the body takes place.

Adult or Imago stage:

in about 3 to 10 days later, the young adult or imago hatches out from the puparium. It emerges from the puparium by using a sac-like organ attached to its head to break it open. This organ is called the ptilinum. It inflates this organ with blood and presses it against the front end of the puparium. The pressure forces the top part of the case to crack. The imago then moves to the surface of the rotten filth and flies away when its wings are dry.



1. Takes place on land
2. Pupa or resting stage present
3. Takes shorter time
1. Takes place in water
2. No pupa or resting stage
3. Takes longer time


Snails belongs to group called Mollusca. They reproduce sexually. The adult male possesses the male cells called sperms while the female possess eggs. When the male mate with the female body leading to internal fertilization which involves the fusion of the sperms and the eggs. The female later lays the eggs in a cool, dry land. After some days the egg hatches on their own and new baby snails emerge.


In sexual reproduction, the male produces the sperm while the female produces the egg or ovum. The fusion of the sperm and the egg in the fallopian tube of the female during mating is called fertilization. The fusion of these sex cells, i.e. sperm and egg results in the formation of zygote. The zygote undergoes cell division to form a developing organism called embryo.


1. Courtship stage: reproduction in toad usually takes place during the rainy season. The male makes loud croaking noise to attract the females which are usually swollen with eggs. The male toad later climbs on the female’s back and holds her firmly with the thick pads on its thumbs. As the female lays her eggs, the male releases its sperm on them resulting in external fertilization.
2. The egg stage: the eggs laid are surrounded in strings of jelly. The jelly performs the following functions:
i. Protects the eggs from mechanical injury when water is disturbed.
ii. It separates the eggs to enable them receive adequate oxygen.
iii. Prevents bacterial and fungal attacks on the eggs.
iv. It prevents the eggs from drying up. The fertilized eggs (zygote) undergo cell division by mitosis process to form the embryo. The embryo feeds and grow around the egg yolk. Dissolved energy diffuses through the jelly into the embryo.
3. The young tadpole stage: the series of changes which takes place from the development of tadpoles inside the egg to adult stage is called metamorphosis. After 1 to 2 days, the young tadpole emerges from the jelly and grow. At this time, it attaches itself to a water weed by a sticky substance secreted by a V-shaped cement gland on the underside of the head. At this stage, the tadpole has no mouth and still feeds on the egg yolk. It obtains dissolved oxygen through the skin for respiration. Developing eyes, ears and nostrils are seen and a portion marked for the appearance of the mouth is equally noticed.
4. The external gill stage: the young tadpole now has a clear head, body and tail. It develops three external gills on either side of the head for respiration. It also develops horny jaws which are used to feed on water weeds. The intestine is long coiled. The V-shaped cement gland is still present in the tadpole.
5. Internal stage: about 6-10 days after hatching, the external gills disintegrate while the internal gills develop. A gill cover (operculum) grows over the gills leaving an opening called spout on the left side only through which water flows out of the gill chamber. The tadpole now breathes like a fish. During this stage, the tail elongates and muscles then develop.
6. The limb stage: the hind limbs grow first and develop. The fore limbs then starts to grow out. The left fore limbs appears first through the opercula opening, followed by the right which disintegrates while the lungs start developing. The mouth develops and replaces the horny jaws. The intestine shortens, the tadpole starts eating small small animals in the water. The eyes become big and prominent.
7. Young toad stage: the tadpole changes into a small toad by reabsorbing the tail. It comes out of the water to the land where it grows into a full adult toad. It usually takes about 40-45 days for metamorphosis in toad to take place, i.e. from egg to adult toad.

Effect or Role of Hormone in the Development of Toad

In toad and other amphibians metamorphosis is controlled by the hormone called thyroxin. It is produced by thyroid gland in the region at the junction of the head and the trunk. For thyroxin to be made, there must be iodine in the water. The tadpole cannot change into adult form in the absence of this hormone. However, an increased level of thyroxine hastens metamorphosis.

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