Acarodomatia!

You know those hairs that line the basal portion of the midrib on the lower surface of black cherry (Rosaceae: Prunus serotina) leaves?

Or the ones in the vein axils on the lower surface of many other tree leaves, like this black oak (Fagaceae: Quercus velutina)?

It turns out these are Acarodomatia! Which is to say, they are structures that evolved for the purpose of housing mites that benefit these plants, either by preying on tiny herbivores (e.g. other mites) or by feeding on pathogenic fungi. I just heard about this for the first time last night when James Trager accidentally sent me a message on Facebook, which caused me to see a link he’d sent me four years earlier that I’d missed until now. (Tip: if you want me to see something, send me an email; I often miss things on Facebook messenger. If you want to make sure I never hear something, leave me a voicemail.) That link was to this “In Defense of Plants” podcast, which I listened to last night; it features Gussie Maccracken talking about her work with fossils, including this paper that gives evidence of acarodomatia already existing around 75 million years ago. Googling “acarodomatia” turns up lots of other information, including this review paper which says (among other things) that these structures are found in about half of North American tree species. I’m a little annoyed that I spent almost 20 years in school and no one ever mentioned the existence of these things.

So, today I walked around the yard and plucked a few leaves that had acarodomatia on them, including the two pictured above, and had a look at them under the microscope. I didn’t see anything among the hairs of the black cherry leaf, but they provide an extensive hiding place and it would be easy to miss any mites that may have been there. When I looked at the ones in the black oak vein axils, I saw a little colorless mite go scrambling out of the second one I looked at, and the same thing happened with the third—I think they were disturbed by the intense light under the scope. I was unable to relocate them when I trained my camera on them, but I believe the spherical object at the center of this photo is a mite egg:

Leaves of striped maple (Sapindaceae: Acer pensylvanicum), it turns out, have nice cavernous retreats in the vein axils behind these hairs. There are several pairs of these acarodomatia along the length of the midrib.

The red maple (A. rubrum) leaves I looked at just had a single pair of tufts at the base of the midrib. This restricted real estate, it seems, improved my chances of spotting the resident mite. See it?

Okay, it’s pretty subtle, given that it’s extremely tiny (~0.2 mm long) and hiding behind a couple of hairs. Here’s a closer crop:

I got a few other photos of it as it ran around on the leaf veins; they’re certainly not good, but maybe sufficient for an acarologist to tell us approximately what kind of mite it is:

So, as with extrafloral nectaries, these acarodomatia are common structures that are often mentioned in botanical manuals (hairs in the axils of veins noted in this key, for instance), but bafflingly, no one ever bothers to point out what their purpose is.