Simplify the following text for a Grade 7 native English-speaking person.
Original: Mayflies are unique among the winged insects in that they moult one more time after acquiring functional wings ( this is also known as the alate stage ) ; this second-to-last winged instar is usually very short , often a matter of hours , and is known as a subimago or to fly fishermen as a dun .
Simplified: Mayflies are unique in that they moult one more time after getting functional wings ( this is also known as the alate stage ) . This second-to-last winged stage is usually very short , a matter of hours .