The mosquito (Culex pipiens) lays its eggs in stagnant water. The eggs stick together in a ‘raft’ and are buoyed up by the air trapped between them. The eggs hatch into larvae which escape into the water from the bottom of the egg cases. The larvae hang from the surface film of the water by means of a breathing tube, through which they take in air. If disturbed, they swim by flicking movements to the bottom, and then return to the surface. They feed on microscopic plants which they collect by whisking movements of their mouthparts which carry a dense fringe of bristles. They shed their ‘skins’ at intervals as they grow.
By the time the last larval ‘skin’ is shed, the larva has turned into a pupa, quite different in appearance from the larva. The pupa does not feed but continues to breath air through two breathing tubes while hanging from the surface film of the water.
By this time, the pupa has developed all the features of the adult: wings, legs, compound eyes etc. but these are all crammed into the pupal skin, giving it a somewhat grotesque comma shape.
The pupal case eventually breaks open at the top and the adult mosquito works its way out and supports itself on the floating pupal case and the surface film while its wings expand and harden before it flies away.
The adult mosquito has 3 pairs of legs and one pair of wings on its thorax. Its head bears a set of mouthparts which are adapted to piercing and sucking. The female mosquito often feeds on blood before she lays her eggs. She lands on a suitable animal and pushes her sharp piercing mouthparts through the skin until they reach a capillary blood vessel. She injects saliva which contains a substance that prevents the blood from clotting, and then sucks up a meal of blood which is digested in her gut. The mouthparts of male mosquitoes cannot penetrate the skin, and males feed on plant juices such as nectar.
When humans are ‘bitten’ by a mosquito, the salivary secretions often lead to an inflamed itchy swelling. In the tropics, however, the bite of a mosquito can transmit the disease malaria.
More information, and illustrations to accompany this article, can be found on biology-resources.com
D G Mackean is the author of GCSE Biology, IGCSE Biology, and many other Biology text books. He has a site of Biology Teaching Resources at http://www.biology-resources.com which includes a bank of experiments for teachers, sample PowerPoint presentations, and many biological drawings