The following is largely from information in the Wikipedia account of Culex quinquefasciatus, accessed on 27 June 2014. The image is courtesy of the Centers for Disease Control.
The adult Culex quinquefasciatus mosquito is a medium-sized, brown mosquito. The body is about 3.96 to 4.25 mm long. While the main body is brown, the proboscis, thorax, wings, and tarsi are darker than the rest of the body. The head is light brown with the lightest portion in the center.
Mature Culex quinquefasciatus females fly at night to nutrient-rich standing water to lay eggs. The larvae feed on organic material in the water and require between 5 to 8 days to complete their development at 30°C. The larvae pass through four larval instars, and towards the end of the fourth instar they stop eating and undergo molting to give rise to pupae. After 36 hours (at 27°C) adults emerge. The exact timing of development can vary depending on temperature. Both males and females take nectar from plants, but after mating, the females seek a blood meal from mammals or birds. Ingested blood is necessary for egg development. A single female can lay up to five rafts of eggs in a lifetime, with each raft containing thousands of eggs. The exact number varies depending on climatic conditions. It breeds profusely in dirty water collections, including stagnant drains, cesspools, septic tanks with leaks, borrow pits, and almost all organic polluted water sources. Under optimum temperature and humidity, the life cycle will be completed in 7 days, passing through the egg, larva, pupa, and adult stages.
Culex quinquefasciatus mosquitoes transmit zoonotic diseases that affect humans and other animals. These include St. Louis encephalitis, Western equine encephalitis, and West Nile fever. Infection occurs during feeding on blood. In southern U.S. it is the primary vector of St. Louis encephalitis virus.