Well, friends, since you asked: let's talk about RAT SCROTUM BEETLES. This is the talk I gave at ESA 2017.

Obviously we all know about leiodids ("round fungus beetles"), the best beetles. This family includes a variety of mammal-associated species, from the humble beaver beetle (the only truly parasitic beetle found on mammals)...

...to the shrew beetle, Leptinus. Shrews are the most ravenous insectivores on the planet, definitely a cool animal to ride around on, good job everyone.

And of course good ol' Silphopsyllus, found only on the Siberian desman. Desmans are a kind of aquatic mole, because that's a thing.

You also have a pleasing variety of scarabs found on, around, and SOMETIMES IN the rectum of larger mammals. This is fine. It makes sense. Dung beetles gonna dung, gotta get that product while it's fresh.

Amblyopinine staphylinids, as elegantly described by Ashe & Timm (1987, 1988) live in harmony with their rodent hosts, hanging out behind the ears of mice and rats and acting as a specialized in-house pest control team for fleas and lice.

If you put the beetle on the wrong species of mouse, it will abandon ship or else the mouse will destroy it immediately. Also, the mice don't seem to mind that an insect the proportional size of a dachshund is SCURRYING AROUND ON THEIR FACE. Cool mutualism!

The rat scrotum question comes in here. Ambyopinines, like many beetle groups, seem to have a "southern disjunct" distribution: there are some in South America, and some in Australia. BUT ARE THE AUSSIE ONES *TRUE* AMBLYOPININES?

Some have argued they MUST be, because they love to hang out on... RAT SCROTUMS (and buttholes). If it's on a rat, it's gotta be an amblyopinine, right?

But research by @asolodovnikov1 and others indicates that they're not true mutualists of these rats, and probably not even part of a monophyletic Ambylopinini.

However, hanging out on rat scrotums is VERY consistent with another beetle, the erotylid genus Loberopsyllus. Loberopsyllus lives on the volcano mouse, specifically on The Butt Zone.

If you put them on the wrong species of mouse or remove them from the volcano mouse? They immediately run back over and hop onto the preferred host. Always on the butt.

Why? We don't really know. Barerra (1969) looked VERY hard and concluded that they were eating detritus and "skin scurf."

This gives us not one but TWO independent colonizations of the Rat Scrotum habitat, by two unrelated lineages of beetles. In addition to the sloth, shrew, wallaby, beaver, mole, mouse, mountain beaver, and desman.
BEETLES, EVERYONE!