COMPLETE METAMORPHOSIS IN MOSQUITOES

Mating and laying of eggs

The adult male and female mate. The female lays fertilized eggs on the surface of a stagnant water. Eggs are laid in partially sunny but shaded places. Female Anopheles and Aedes mosquitoes lay eggs singly while Culex mosquito lays eggs in batches.

 

Anopheles mosquito

Egg Laying

About 300 eggs are laid singly. The eggs are boat shaped, with air-floats to keep them floating. After about two days, the eggs hatch into larvae.

 

Larva (Wriggler)

The larva has a distinct head, thorax and abdomen. The head has a pair of antennae, and mouth brushes for collecting food particles in the water. A pair of eyes is present. Movement is by wriggling.

The thorax is the largest portion of the body. The abdomen is the longest and is divided into nine segments. The eighth abdominal segment bears a pair of breathing spiracles.

At first, the young larva absorbs oxygen from water and feeds on suspended particles. Later, it breathes the atmospheric air through a pair of breathing spiracles. The larva moults four times. After the fourth moulting, it changes to a pupa.

 

Pupa

The body of the pupa consists of cephalothorax (head and thorax fused together) and a narrow and segmented abdomen. There are two breathing tubes (breathing trumpets) projecting from the cephalothorax which hang on water surface for breathing the atmospheric air. The pupa does not feed. When disturbed, it carries out wriggling movements downwards into the water. The pupa stage lasts for about 2 days after which it changes into imago (adult).

 

Imago

It rests on the surface of water for some hours for the wings to expand and harden. Mating takes place and eggs are laid. The cycle starts again.

The adult rests in a sloping position with head downward forming an angle of 30-40° with the resting surface.

 

CONTROL

Control measures could be achieved in three ways:

• Against the vectors ( mosquitoes).
• Measures to reduce contact with the vectors.
• Against the parasites in man.

 

A) Against the vectors

These measures are against adult mosquitoes, larvae and pupae.

Adult mosquito control: Spray all living places with insecticides such as pyrethrum to kill the mosquitos. Kill any mosquito physically. If seeing

Against larvae and pupae:

1)All breeding places should be filled with soil.

2) All broken vessels which can retain water must be turned upside-down.

3) Where drainage is not applicable, kerosene or oil mixed with pyrethrum should be poured on the water surface to kill both the larvae and pupae since the oil layer will prevent respiration.

4) Paris green is also used in the control of the larvae. Pouring Paris green on the water surface and subsequent consumption of the chemical by larvae and pupae kills them.

5) Introduce fish such as Alestes and Tilapia to feed on the larvae and pupae. This is a biological control.

 

B) Reduction of contact with mosquitoes

1. When siting living houses, avoid marshy areas that are breeding grounds of mosquitoes.
2. Screen living houses with wire gauze to prevent mosquitoes from entering.
3. Use of mosquito nets. Sleep under mosquito net to prevent mosquitoes from biting you.
4. Wear anti-mosquito clothes that repel mosquitoes.
5. Use mosquito repellant such as Dimethyl phthalate to rub the body. This chemical repels mosquitoes.

 

C) Against parasites in man

Use preventive drugs such as Daraprin or Paludrine. Taking one tablet every week will help to prevent malaria attack.

 

Mode of transmission of malaria parasites

By the bite of an infected female anopheles mosquito.

 

Symptoms

Fever, headache, vomiting, body pains, high body temperature, profuse sweating, bitter mouth and loss of appetite.

 

Treatment

Curative drugs such as nivaquine, chloroquine, resochin, etc will help to destroy the parasites in the blood.

 

Economic importance

1) Vector of parasites

Mosquitoes are of great economic importance as they are vectors carriers of parasites that cause malaria fever, yellow fever, filariasis (elephantiasis) and dengue fever. Malaria fever alone kills over three million people world-wide every year. These blood sucking mosquitoes transmit the parasites by sucking blood from an infected person and transmitting the parasite to another uninfected person through their bites. The female mosquitoes need blood meal to nourish their eggs before laying are the vectors (carriers) of malaria parasites.

 

2) Cost of treatment and prevention

A lot of money is spent in buying drugs for the treatment and prevention of malaria. The purchase of mosquito nets and insecticides require money. Many people especially the poor can hardly afford money to buy drugs leading to frequent malaria attack.

 

3) Low productivity

Mosquito diseases cause ill-heath. Sick people especially workers are less efficient in their working places resulting in low economic productivity.

The financial cost of destroying the breeding grounds of mosquitoes is high.

 

4) Sleep disturbance

The biting and buzzing noise of mosquitoes at night disturb sleep and may result to an ill-heath and lack of mental concentration.