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This specimen was discovered in the West Holland River in central Ontario, Canada. I am wasn't sure what it was. I guessed sciomyzidae but that is because I am only familiar with aquatic diptera.

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Here is the posterior end.

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A close up of the anterior.

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Cecidomyidae. At the ventral side of the head you'll see a Y-shaped chitinized plate, which is typical for this family. I think it is not aquatic but fell of some other plant along the riverbank, or swept of from it.

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Thank you Atylotus! We likely get more terrestrial diptera in our collection nets more than I would like. They are there just to confuse us.