Fleas are parasitic insects mentioned in the How to Train Your Dragon Book series.
Fleas are parasitic, flightless insects adapted specifically for living off of the blood of mammals and birds. They have very long hind legs for jumping through hair or feathers, hooks on their feet to remain lodged on the skin, a laterally thin body shape to weave between hair and feathers, and a life cycle based on the presence of their hosts. Fleas lay about 100 eggs per one adult. After hatching, the larva survive by eating detritus, even feces from the adult flea. Later the larva pupate. They can remain in a cocoon for a long time and are the most difficult to kill at this stage. Emergence as an adult flea and take a few days in optimum conditions, but can remain as a pupa until they sense the presence of a blood host. Fleas may occur only on certain host species, but many flea species can move readily to unideal hosts if their preferred hosts are not available.
Fleas are extremely problematic parasites both in the Books and in real life. Additionally, fleas are used as metaphors in showcasing large size differences between objects or animals.
"Dragonfleas" are also mentioned in the Book series. In real life, reptiles do not have fleas as an ectoparasite, but are instead parasitized by a variety of mites and ticks.
Fleas are mentioned a few times when comparing the size of humans and Hunting dragons to the gargantuan Green Death.
|“|| Then Fireworm bit him [Green Death] on the nose.|
It must have felt like a flea bite, but the Monster was outraged.
|— Book 1|
|“||The Creature stuck his great talon up his nose in a disgusting fashion and tried to winkle out the tickling flea that was irritating him.||”|
|— Book 1|
|“||S-s-suit yourself," shrugged Toothless, checking out his wings for dragonfleas. "Toothless thinks he's an O-O-Outcast.||”|
|— Toothless speaking in Dragonese|
Dragonfleas are mentioned when Toothless is introduced in Book 3, climbing atop Hiccup's head and "had a quick rummage for dragonfleas".
Fleas are mentioned in Book 4 to describe the frenetic movements of Semi-Spotted Snowpeckers.
|“||Semi-Spotted Snowpeckers were jumping all around them like fleas on a cow's back ...||”|
|— Book 4|