The answer:

As black swallowtail larvae develop, they pass through two distinct morphs.  From the first through third larval stage, they look like bird droppings (below), that is, black with a white spot in the middle.  Uric acid, a waste product in birds is a white compound, and it turns out that the white patch in the larva is twice as high in uric acid as the dark patches.   Thus, not only do the larvae look like bird droppings (visual mimics), they also contain some of the same compounds (chemical mimic).

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The presence of uric acid may be important in more than mimicry; it may also protect the larvae from furanocoumarins.   You'll notice that uric acid is present in the dark patches of the larva above.  It also occurs in the white spots of the wings of the adult and in the light stripes of the later instars that do not  mimic bird droppings (see below).  Uric acid happens to be a good antioxidant, and furanocoumarins, in the presence of UV, generate toxic species of oxygen.

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So there you have it.  A mimic that not only looks like its model (the bird dropping) but also is made of similar stuff.   The question is whether they taste like their model. No one in this lab has volunteered to test that hypothesis!