Then there’s Cope’s rule, also much argued, that newer groups in any given lineage tend to be larger, while the more ancient tend to be smaller.

But the mountain beaver is considered the most ancient of rodents, and therefore is likely to carry among the most ancient of rodent fleas. So why are these presumably ancient parasites the giants among fleas? Hystrichopsylla schefferi, it seems, avoids easy answers as adeptly as it has avoided the camera.

Enlisting a Mountain Beaver

So how to find the world’s largest flea? First, find mountain beavers. Though ubiquitous, they are not easily pinpointed, it turns out. They make their presence felt during the wee hours, when they emerge from their burrows to eat, especially ferns and seedlings like newly planted firs. But they are so secretive — spending most of their lives digging long, winding tunnels — that people often don’t know it’s mountain beavers that have done the damage.

Luck came in the form of a good friend and biologist, Peter Wimberger, whose colleague Bob Peaslee, the science support engineer at the University of Puget Sound, lives on land in Gig Harbor overlooking the sound. Its steep hillside is plagued with mountain beavers.

The original plan did not involve any contact with a mountain beaver. Unlike many parasites, this flea spends time off its host and can sometimes be found in the nest material. Merrill and Peter set out with a group of undergraduates excited to spend a Saturday digging out mountain beaver tunnels on a nearly vertical bank. But many hours, many shovelfuls, and not a few beers and chips later, there was no sign of nest or flea.

Resigned to having to get a flea directly off a mountain beaver, Merrill sought the advice of Wendy Arjo, a wildlife biologist with Ageiss, an environmental consulting firm based in Evergreen, Colo., who has worked often with mountain beavers.

It’s simple, she said. Trap a mountain beaver (perfectly legal). Coax it into a burlap bag, face first. Hold it just behind the jawline — not too far up, or you will be bitten; not too far back or it will get loose and then you will be bitten. Peel open the bag to reveal the hind end. Comb for fleas. Easy peasy!