Lacewing flies are mundane flying insects whose lace-like, transparent wings are used as an ingredient in Polyjuice Potion (CS10, CS11, CS12, Pm).
The author wrote that she chose this ingredient because the lace-like wings represent the "binding together of two identities" (Pm).
From the Web
Polyjuice Potion on Pottermore
Lacewing fly on Harry Potter Wikia
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