Often , all the mayflies in a population mature at once ( a hatch ) , and for a day or two in the spring or autumn , mayflies are everywhere , dancing around each other in large groups , or resting on every available surface .
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 .