A mosquito's life cycle

Why are we told never to let water stagnate (stand still and collect)? Female mosquitoes suck blood and spread diseases such as malaria, dengue and chikungunya. Mosquitoes lay their eggs on or near stagnant water: in desert coolers, plant pots and any open container.

A mosquito has four stages, in this order:

• egg, laid on or near water;
• larva, which lives and feeds in the water;
• pupa, a resting stage that does not eat;
• adult, which rests a short time on the water's surface and then flies away.

An adult may live 10 to 15 days. The female lays eggs on or near water, and the cycle goes on.

The stages look very different from each other: the egg is nothing like the larva, the larva nothing like the pupa, and the pupa nothing like the adult.

Larvae and pupae live in water but need air to respire, so they keep coming up to the surface. A thin layer of kerosene oil spread on stagnant water separates it from the air: the larvae and pupae cannot get air, and they die.

The silk moth also has four stages: egg, larva, pupa, adult. Its larvae give out a thread-like material and wrap it around themselves before they turn into pupae. These are the fibres used to make silk. In India, the Khadi and Village Industries Commission (KVIC) has set up several centres for silk production.