The largest species of flea
Hystrichopsylla schefferi is a very specialized animal, able to obtain nourishment from a single source only: the blood of the mountain beaver (which isn't really even a beaver!), found on the Pacific coast of North America from northern California to British Columbia. It was discovered in Washington State in 1913.
As is typical for fleas, female Hystrichopsylla schefferi are larger than males, and can grow to a length of greater than a centimeter.
The species is considered one of the most primitive fleas, and its host (the mountain beaver) may be the most primitive of all rodents. It's unknown whether this is a coincidence.