This is one of my favorite insects of all time; I wrote about it several times for different websites including Listverse. Can´t have enough of it
The Hemeroplanes triptolemus caterpillar lives in Mexico, Central and South America, in rainforests. The adult moth is plain-looking although quite large, but the caterpillar is one of the most impressive mimics in the world. When threatened, it inflates the front part of its body, which transforms to resemble a tree viper's head, complete will shiny eyes, scales and even what resemble the viper's heat-sensing pits.
Needless to say most animals (including some humans) prefer to run away as fast as they can instead of staying and finding if it's a real viper.
The caterpillar goes as far as to imitate the movements of a snake that is about to strike.
Here's the moth:
And here's the larva: