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Size in Eustalomyia (and other commensals) is of little value as the larvae are dependent on how much food they can get, and if in short supply they will pupate without waiting and then a small adult will emerge.

Subhylemyia longula is a parasite on snails, and the adults vary in size enormously. Small ones have a wing length of only 3 mm, while eggs laid on a large snail can produce adults with wing length of 6mm.