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Ask The Entomologist Question About Mayflies

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Question:

About Shad flies (may flies).  The water in our bay is covered with the nymphal casing at this time of year.  I know the adult emerge and leave the nymphal cases on the surface.  But, I also have some casing on the side of my house, doors and windows.  The house is about 100 feet from the water and up a hill.  Do the nymphs crawl all that way before emerging?   I guess they must, but why do most emerge on the water and a few crawl great distances up the shore

Answer:

Mayflies have an interesting and unique life cycle.  The nymphs come to the surface of the water or crawl a short distance from the water and molt into a winged form called the sub-imago.  The sub-imago resembles the winged adult but is not capable of reproduction.  The mayfly then flies a short distance and molts again into a fully functioning adult.  This is the only insect that molts again after it develops wings.  The adult mayflies are very short lived, lasting only a few hours or a couple of days.  They have no mouthparts and do not feed.  Their only function is to mate, lay eggs, and die.  The caste skins you are seeing on your house may be from the sub-imago forms that have flown a short distance from the water.

The presence of mass numbers of mayflies is actually a good thing and is a sign that the water system is healthy because they are easily effected by pollution.