Arsenic Adderwings are small, worm-like dragons with a red spot on the spade at the end of their tails, likely as a warning for their toxicity. They look extremely similar to Glow-worms, except for that red spot. They possess very tiny wings. Despite their small size, they are still classified as a Bog Dragon rather than a Nanodragon.
This dragon is poisonous, but little is detailed about the poison itself, how it is delivered, or what the symptoms are. Though specific information about the Arsenic Adderwing's poison is not given, it might be extrapolated that the poison contains arsenic, since it is specified in their name.
The author mentions two different methods of avoiding or deterring these poisonous dragons. In A Hero's Guide to Deadly Dragons, when Hiccup is trying to recall a way to avoid Poisonous Piffleworms, he instead remembers that Arsenic Adderwings can be tickled behind their ears to subdue them. This is in direct opposition to the account given in The Incomplete Book of Dragons, where the reader is told "DO NOT TOUCH THEM". Also, the image does not show an ears at all. Instead The Incomplete Book of Dragons indicates that Arsenic Adderwings avoid the color yellow, possibly because their enemy or predator is the yellow Toxic Nightshade.
According to the stats given in The Incomplete Book of Dragons, Arsenic Adderwings rate an "8" on Disobedience. This means they are very difficult to train, if it can be done at all.
Arsenic Adderwings remained unaligned during the Dragon Rebellion.
The Arsenic Adderwing is not seen in this book, but is referred to by Hiccup, when trying to think how to subdue some Poisonous Piffleworms.
The Arsenic Adderwing has an entry in this reference book, as well as a chart in the back of the book.