Oil applied to the surface of water kills the larvae of mosquitoes through

Analyze the question: Oil is spread on stagnant water to control mosquitoes. We need to know how it kills the larvae.

Explain the concept: Mosquito larvae (wrigglers) live in water but they are not gill breathers. They breathe air through a breathing tube (siphon) or spiracles at the tip of the abdomen. To breathe, they hang just below the water surface and push this tube through the surface film into the air.

Step-by-step solution:

1. Oil is less dense than water, so it spreads as a thin film over the surface.
2. This oil film blocks the larvae's breathing tubes from reaching the air. It also lowers the surface tension, so the larvae cannot hang from the surface.
3. Oxygen cannot enter their tracheal system, so they cannot get the oxygen they need for respiration.
4. They die from lack of oxygen, which is suffocation (asphyxiation).

Checking the other options:

• A. Dehydration: The larvae are in water, so they cannot dry out.
• B. Poisoning: Oil mainly acts by physical blockage, not by chemical toxicity. (Some insecticides do poison, but that is not the main action here.)
• C. Starvation: Death from lack of food takes much longer. The oil kills quickly by cutting off air.

Correct answer: D. Suffocation

Exam tip: Remember "larvae breathe air at the surface, so oil blocks the air." Any question on oil and mosquito larvae or pupae points to suffocation. Pupae also breathe through tubes (respiratory trumpets) and are killed the same way.