If you saw one, it may be an Asian Tiger Mosquito
black and white stripes on its body and legs.
If you saw a mosquito that has black and white stripes on its body and legs, you may have seen one of the Asian tiger mosquitoes that are invading the United States. Originally from Asia, they first came to this country in the 1980s as eggs which stowed away in used tires and other cargo. The eggs hatched and found the new habitat to their liking. Asian tiger mosquitoes have now spread over most of the southeastern United states. Their Latin name is Aedes albopictus, though some scientists also identify them as Stegomyia albopicta.
To see close-up photos of the larva, pupa, and adult stages, with identifying points, visit this website, produced by the University of Florida. You'd need a powerful magnifying glass or microscope to see some of the details, but the bold black and white stripes are easier to notice.
The adults are average in size for mosquitoes, ranging from 2 to 10 millimeters long, but only a few reach the upper end of that range. The males are about 20 percent smaller than the females.
You can identify them not only by their stripes, but by the way they behave. Most mosquitoes bite between dusk and dawn, but fierce tiger mosquitoes live up to their name, biting during the day, especially in the afternoon. For this reason, they're sometimes also called "forest day mosquitoes." Both males and females have white stripes along the thorax, or body, though you're more apt to notice the females, because they're the ones who bite. Like other mosquitoes, they sense carbon dioxide and typical chemicals and smells which humans give off, so fortunately they can be caught in the same traps that give off similar vapors to attract and kill other kinds of mosquitoes.
Tiger mosquitoes bite birds and mammals as well as people, but they're easily scared away while biting, so they may draw a little blood from several different humans or animals before they get one full meal. This characteristic makes them more than just an annoyance, because they can spread dangerous diseases from one host to another, such as dengue fever, encephalitis and yellow fever, as well as heartworm in cats and dogs. In 2005-2006, they caused an epidemic of Chikungunya fever on La Reunion island, off the southeast coast of Africa, that sickened a quarter million people and killed 248.
According to the Centers for Disease Control, Eastern equine encephalomyelitis and Cache Valley viruses have actually been found in Asian tiger mosquitoes in the United States, though they're potential carriers of additional diseases, if people or animals who have yellow fever or other diseases come to the United States and get bitten. Right now, that's not a danger yet.
In North America, some other mosquitoes have a similar pattern of black and white stripes on their legs, but the Asian tiger is unique in having the same striking stripes on its body as well. You're most apt to encounter the Asian tiger in the areas in orange on the map below. Based on the CDC's county-by-county map, it shows where this pest is naturalized, but new populations are being discovered and, hopefully, eradicated, so people are occasionally finding them outside this area.
Though they first permanently settled here in the 1980s, a few eggs arrived in shipments even before then. Some were found in California in 1971, but officials worked quickly to kill them off. This article describes California's efforts to keep them from naturalizing in that state. Because their eggs are so small and can survive long periods of drying, keeping them out of the United States is almost impossible, as long as shipments of cargo continued to arrive from areas of Asia where they live. Many eggs rode in used tires that had contained standing water, and California also received some eggs that hitched a ride with "Lucky Bamboo" from south China, sold as an ornamental plant in the United States.
For more information and resources about this invasive species, what diseases it can carry and how it's being controlled, see the USDA species profile.
Public domain photo of Aedes albopictus from the CDC.