In wet years, especially if you're in Florida, you may notice some mosquitoes that are twice as large as ordinary ones--maybe a little over half an inch long, with hairy legs and yellow-colored bands. Fortunately, they can be controlled like any other mosquitoes, so they're no tougher, despite their giant size.
They're sometimes called gallinippers or mega-mosquitoes, and though they're not very common, they can be found over most of the U.S. east of the rockies, as far north as southern Wisconsin, Michigan or Minnesota, and northeast except for Maine, though some have even recently been found even there. This map shows their range in the U.S. However, the long, warm, wet summers of Florida are perfect for them. Their scientific name is Psorophora ciliata. Because their eggs lie dormant until flooding occurs, you may not notice the few that hatch in dry years, but the adults can appear in sudden large quantities when weather conditions are right. In those years, they're not new to the area, but people may start noticing them for the first time.
One way to control them is to kill the larvae before they turn into adults. The adults lay eggs on low-lying ground, but they may not hatch for weeks or months. When rains come, raising the water level and soaking the eggs, the dormant eggs come to life, hatching into larvae who spend less than a week in the water before emerging as adults. If you can kill the larvae during those few days, though, you can stop them from turning into adults, preventing not only those adults but all the future mosquitoes they would have produced, which might be several generations in a long, warm, wet summer. Dunks or bits, or similar treatment of standing water, will control mega-mosquito larvae in addition to other mosquito larvae.
If the larvae survive their time in the water, they can emerge as adults in as few as four days, in ideal conditions, so you may start noticing the adults flying around only a few days after a flood. Once the females mate, they start looking for blood to nourish another generation of eggs. If a cycle of dryness and flooding continues during the summer, there can be several generations, before the last eggs laid in the fall lie dormant over the winter.
Gallinippers primarily bite mammals, especially cattle, but also smaller wild animals like armadillos, raccoons and rabbits, but of course they'll also bite humans. They fly and bite in the daytime as well as night, and their large size makes the pain and itch of their bites bigger than normal too.
Though mega-mosquitoes have the potential to carry diseases, scientists don't believe people and animals are currently at risk from them. Some gallinippers have tested positive for Eastern equine encephalitis virus, Venezuelan equine encephalitis virus, Western equine encephalitis virus, Tensaw viris, and West Nile virus, but there is no confirmation that they're a "bridge vector," or in other words, there's no current concern that they will pass diseases on to humans or animals. 
The larvae are oversize, just like the adults. They're bullies of the underwater world, eating not only microorganisms but other smaller mosquito larvae and sometimes even each other. Scientists have considered the possibility of using them to kill the larvae of more dangerous, disease-carrying mosquitoes, but concluded "the point is moot because of the pestiferous nature of adult females effectively undermining any potential benefits" --or in other words, nobody wants more mega-mosquitoes around, even if their larvae might do some good.
 "Psorophora ciliata," Ephraim V. Ragasa and Phillip E. Kaufman, University of Florida Entomology and Nematology Department
Photo Credit: University of Florida Entomology and Nematology News, March 14, 2013