Poisonous Piffleworms are diminutive in size: about the "size of a largish newt." However, they have large venomous fans to make up for it. They are depicted as being solid purple or purple and green, have one pair of wings, a spaded tail and sometimes are shown with a since horn sprouting from the top of their head.
Poisonous Piffleworm venom is so deadly that a victim of its bite will die in about a quarter of a second. It is purple in color and acidic enough to burn stone. They have poison glands on their necks and inject the toxin via pin-point sharp fangs.
Piffleworms despise the sound of whistling. Doing so will essentially stun them for a time, as stiff as a statue.
Evidently Piffleworms like chowing down on paper and have earned the nickname "bookworm."
Hiccup and friends encounter a large group of sleeping Piffleworms lining the floors of a secret cave-like exit from the Meathead Public Library. Hiccup temporarily forgets how to deal with Piffleworms, but finally remembers about whistling. Stormfly obliges them, and the group are able to sneak past the frozen Piffleworms to safety from Driller-Dragons.
The Poisonous Piffleworm is depicted and described in this reference book.