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Euarsta bella and E. festiva

E. bella

Look closely in the vegetation near ragweed and you might see tiny flies doing a little dance.  You would see flies with dark wings that have clear spots, holding one wing out horizontally but with the edges pointed up and down.  It is an obvious visual display.  The flies will be a few inches apart and each displays first one wing, and slowly turn towards the  other fly, displaying its outstretched wing.  It then moves that wing straight back and displays the other wing.  The fly slowly rotates on its feet, pushing the outstretched wing forward. Mostly it just displays one wing at a time, but sometimes will have both outstretched.

The flies strut around like tiny peacocks, showing off their the colorful patterns on their wings.

Sometimes these displays happen when other flies are quite a ways away from each other.  Sometimes you won't even see the target of the display.

These flies are quite small--half to a third the size of a common housefly.

There are a number of similar flies in the same genera--Bugguide lists seven species.  The adults lay eggs on ragweeds or cockleburs.  The larva develop in the seed heads of those plants.  The two that are shown here are the only ones recorded in Iowa so far.

Euarsa bella lays eggs on common ragweed, Ambrosia artemisiifolia.  E. bella is shown to the left.  E. festiva (shown below) lays its eggs on great ragweed, A. trifida.  Both species damage their host plants by reducing seed production, but in both cases the host plants are considered to be pest weeds.

Although small, both species can be easily found at certain times of the year--typically late in the summer.  They can be quite entertaining when you look for them.

So how do these nearly identical flies maintain their genetic identities?

How do they even find each other, being so small?  The vision that these flies have must be fairly acute.

Some basic evolutionary questions can be investigated when looking at these flies.  The flies in this genus look very similar but do have differences.  They use larval host plants that are similar but are of a different species.  Can the mechanism that causes speciation be investigated using this type of fly?