Only female mosquitoes bite, and all mosquitoes live on the sugar found in plant nectar, not on blood. But there is a reason females seek blood.
Female mosquitoes, unlike males, have a proboscis. This is a long thin needle-like built-in syringe located at the mouth. They use this to impale their victims, in order to fill their abdomens with blood. Proteins in the blood are necessary to produce fertile eggs. Since males cannot produce eggs they have no need for blood. Females require a new blood 'meal' for every nest they lay, and produce about 250 eggs per meal.
Mosquitoes lay eggs in standing water where they hatch into squirming larva. The larva molt four times before going into the next stage of pupa. Inside the hardened case of the pupa, the adult mosquito forms and eventually hatches.
Female mosquitoes live anywhere from one to several weeks depending on the species and the environment. Some females can survive the winter to lay eggs in the spring, before dying. Males generally live 4-5 days, and die after mating.
Mosquitoes have been evolving for 30 million years. During that time they've built an impressive array of sensory receptors. They possess chemical, visual, and heat sensors, all designed to zero in on a blood source.
The chemical sensory receptors are located on the antennae, which allow them to detect carbon dioxide. All mammals give off carbon dioxide, including humans. Our skin excretes the gas, as does our breath. A mosquito can detect this scent from 100 feet (30 meters) away.
When a mosquito bites it injects a small amount of saliva that thins the blood so it doesn't coagulate in the narrow channel of the proboscis. When it's done feeding, some of this saliva remains in the wound. This causes an immune system response and itching. A welt forms, known as a wheal, and the body goes to work breaking down the proteins from the saliva. The bite will continue to itch until the body has broken down all of the proteins.
Mosquitoes can carry diseases that can be transferred to humans through their bite (malaria, encephalitis, West Nile virus, Yellow and Dengue Fever). To keep the mosquito population down, avoid allowing standing water to collect. Flowerpots, trash lids -- anything deep enough to collect a puddle can be a breeding ground. Fountainless birdbaths or any other necessary standing water should be flushed with the hose every few days. If you have a fishpond the fish will eat the mosquito larva.
There are about 3,000 species of mosquitoes worldwide.