It often happens that all the mayflies in a population mature at once (called "the hatch"). For a day or two in the Spring or Autumn (Fall), mayflies will be everywhere, dancing around each other in large groups, or resting on every available surface. The adults have various special features. Their rear wings are small or almost vestigial; the males have two long front legs for holding females, the rest have no function. Mating is usually in mid-air. The mayfly are quite a successful group. About 2,500 species are known worldwide, including about 630 species in North America. The hatch at La Crosse, Wisconsin has been a news event for several years. The hatch is so large it appears on weather radar. The emerging mayflies land on everything and cover some areas like snow.