Mimicry is a defense strategy employed by many species of Lepidoptera. Some adults can easily be mistaken for bees or wasps; others resemble foul-tasting cousins. Bird-dropping imitators abound among both larvae and adults. In all this variety, however, one would be hard-pressed to find a case of mimicry as impressive as that of the Gaudy Sphinx (Eumorpha labruscae) caterpillar. Late instars of this caterpillar bear an uncanny resemblance to a snake. Furthermore, it is not just any snake: an unwary bird peering around a leaf and spotting this creature would surely fly off in fright, thinking it had just escaped being devoured by a rattlesnake.
When the caterpillar featured in this essay was found feeding on Cissus incisa, it was about 1 cm in length and looked like a typical hornworm: it had a green body and a (hypertrophied) black horn. Its light brown head and a brown horn base distinguished it from the more common Vine Sphinx, E. vitis. It is probable the caterpillar was still in its first instar.
When it entered the second instar two days later, the caterpillar underwent a transformation. The body was now purplish; the spiracles were surrounded by large, odd-shaped greenish patches, and there was a notable swelling of the last thoracic and first abdominal segments. Another two days and a new instar brought further transformation. The swelling gained eye-spots and dark markings. The dorsum of the caterpillar was now a different color than the sides, and it bore markings that gave it the appearance of being scaled. In this third instar, the eyespots and the scale-like pattern along the dorsum of the caterpillar made it obvious that this was a snake-mimic. The caterpillar would even vibrate its horn rapidly in a manner reminiscent of rattlesnakes.
The transformation continued as the caterpillar grew. It was about 5 cm long when it entered its penultimate instar. The purple shading had completely faded, so that the body was brown and tan. The green spiracular patches were the only part of the body that did not resemble a rattlesnake.
The green patches disappeared in the fifth and final instar, as did the horn. In place of the horn, there was a shiny and reflective eyespot. When alarmed, the caterpillar would sometimes rapidly vibrate its “eye” (caudal end). The entire caterpillar now bore an incredibly detailed resemblance to that of a Western Diamondback Rattlesnake (Crotalus atrox). The caterpillar's swollen thorax made for a convincing snake head; it even had dark spots where the snake's nostrils would be seen. The body pattern eerily mimicked the dark and light "diamonds" (which can be very blocky on Crotalus atrox). The skin appeared to bear scales. However, the skin was actually very smooth and very soft, with the feel of supple leather.
At rest, the caterpillar measured about 12.5 cm shortly before it pupated. Stretched out, it could easily reach 14 or 15 cm in length.
The adult emerged about 4 weeks after it pupated. When I gently handled the fresh moth, it made quiet chirping noises that reminded me of a low-pitched version of bats squeaking. I believed the moth was a female, so I kept her for a couple of days to see if a male could be attracted. When none appeared, she was released in hopes she would find a mate and leave lots of eggs for a next generation.
A picture comparing the face of a Gaudy Sphinx caterpillar to that of a Western Diamondback Rattlensake may be seen at http://leps.thenalls.net/Species/zSphingidae/labruscae/life/mimicry.htm
 Dr. David Wagner, personal communication