This is one of the best mimics known.  The caterpillar, in its early stages, resembles a bird dropping (brown caterpillar in image above and to left) and is found exposed during the day on the upper side of the leaves.  After molting to the 4^th instar (I believe there are 5 total instars in this species) the caterpillar turns green (large caterpillar).  Both the early instars and the later ones display the snake’s head mimicry, with large spots on the swollen thorax giving the appearance of a snake’s head.  The spots look like eyes, even to the point of having a white “highlight” to simulate moisture.  The later instars roll up leaves, holding them together with silk.  They stay in these retreats by day; a bird investigating the rolled up leaf will be confronted by a “snake” peering back at it.  

A second line of defense peculiar to this family of butterflies is the presence of an osmeterium, an eversible structure normally folded up inside the body.  When threatened, the caterpillar will extend the structure, which is branched and covered with an aromatic chemical which serves to repel many insect and even vertebrate attackers.  Apparently, in this species the osmeterium has the added benefit of looking like the forked tongue of a snake, although on our specimen the “tongue” is yellow instead of black, and I had to press on the thorax to get it to evert the osmeterium at all.  Thus the picture with the everted osmeterium is obviously posed (pressing on the thorax also causes the head to protrude).  The aroma of the deterrent chemical persisted in my lab for quite some time.  It was actually a pleasant smell, probably derived from some of the defensive chemicals which give the spicebush its fragrance and its name.

As they prepare to pupate, they turn yellow.  We brought back two specimens; the larger one was green yesterday but is yellow today, a sign it is ready to pupate – we will try to rear both out.  I have attached 5  images.  One shows the 2^nd or 3^rd instar larvae (bird-dropping mimic) next to the 5^th instar.  The other 3 show the snake mimicry in the 5^th instar.

Above:  Adult Spicebush Swallowtail.