On a recent crawl through the woods near my house, a white and thoroughly nondescript bracket fungus caught my eye. It was so plain-looking that I didn’t imagine I would ever know its name, but looking in George Barron’s Mushrooms of Northeast North America, it seems like a pretty good match for Tyromyces chioneus. In conducting a Google search for some more photos to compare mine with, I found that I’m not the first to be struck by this species’ blandness. “Oh, sure, the world probably needs Tyromyces chioneus,” writes Michael Kuo, “but that doesn’t mean I have to get excited about it.”
My mycologist friends tell me that this identification is most likely correct, but that examination under a microscope would be required to distinguish this from other, much less common species of Tyromyces. Anyway, I took on the challenge of finding something interesting on this unassuming, seemingly immaculate fungus. I rolled over the dead branch it was growing on, and I saw a dozen or so little dark specks moving around on the white pores. The photo below shows them as they looked with my lens set to 1x–the field of view is about 22 mm.
I extended the lens all the way to 5x to see what sort of springtails they were, and was shocked to discover that they were beetles.
Based on how much of the 4 mm field of view these beetles filled, they were 0.35 to 0.4 mm long. I hadn’t realized beetles could be quite this small. They were just the right size to stick their front ends into the fungus’ pores, which they did repeatedly, presumably munching on the spores inside.
That night I went to BugGuide.net to see if I could figure out to which of the 131 North American beetle families these might belong. I looked at the ten families with “fungus” in their common names, and none seemed right. Then I thought maybe the extreme tininess would be the key. My mind jumped to fairyflies (Mymaridae), the wasp family that includes the smallest known insect (down to 0.1 mm, according to this site), and I thought of the feathery wings that some mymarids have–like this one I found at the Berkshire BioBlitz this spring (at ~0.6 mm, apparently a gargantuan by mymarid standards):
In the same instant, I remembered this amazing image I had seen of a beetle with feathery hind wings. Those feathery wings probably meant that beetle was small too, I thought… what was it called? Featherwinged beetle? Sure enough, it was, and according to BugGuide’s guide page for featherwinged beetles (Ptiliidae), they are “the smallest known beetles, most species being under 1 mm, and the smallest known being 0.35 mm.” So by stopping to inspect that unassuming bracket fungus, I had stumbled on the very smallest beetles in the world. (Added 10/13/11: Ptiliid specialist Gene Hall says these belong to the genus Nanosella. Determining exactly which species would require dissecting a female. According to his page about the genus on the Tree of Life website, there are species even smaller than the one I found–down to 0.25 mm.)
The next morning I woke up thinking about all the tiny things with frilly wings. The smallest moths are in the family Nepticulidae–not quite as tiny as beetles or wasps can be; the smallest have 1.5 mm wings. Here’s one I found on my bedroom window this summer:
It’s got decent fringes on its forewings, but check out the pinned specimens here (the nepticulids are #0037-0114) and you’ll see the hind wings are almost all fringe. And how about this furry little moth fly (Psychodidae) I saw in Kentucky–at ~0.5 mm body length, it’s about as small as a fly can get:
Fringed wings are the rule for thrips–which are usually around 1 mm long–giving their order its name: Thysanoptera, from the Greek thysanos, fringe, and pteron, wing.
There must be some simple physical law that explains why these fringes are a good idea on tiny wings, but I can’t quite put my finger on it. I brought this up with a friend who is into model airplanes, and he said something about the relative size of (and space between?) molecules in the air becoming more of an issue for tiny flying things, and something about turbulence, and how the frills somehow help with this. I’m obviously still fuzzy on the details. If you think you can explain it, I’d love to hear it!
BugTracks 
This is a fantastic post of discoveries! I found a similar fungus near my house, but it’s firmly attached to a buried piece of wood and I can’t get a good look at it.
Great fun reading this–I’m always captivated by the way you use words. : )
I wonder if the frilly wings are somehow protection from things like spider webs? I remember reading about moth scales and I believe lacewings’ wings in For Love of Insects and how they allow the insects to sort of brush by a spider web without becoming stuck in it. I’m not sure how much of an issue that would be for these very tiny insects, but it’s a thought. Great photos of these tiny beetles!
Interesting thought, but I think these fringes are firmly attached to the wings. The deal with moth scales is that they flake off easily, so a moth just loses a few scales in the web and is able to fly away.
Right, I know the moth scales flake off, but in the case of lacewings, my understanding is that the fringes on the wings don’t allow the web to adhere tightly to the wing itself. This website:
http://www.cirrusimage.com/neuroptera_green_lacewing.htm
summarizes what Tom Eisner wrote about – when lacewings are caught in the web, they chew their legs and antennae free, then hang freely from their wings in the web. Because the web makes contact with the hairs, rather than with the wing surface itself, the lacewing can slide out of the web.
Anyway, just speculation as to the hairs on the wings of these little guys. The lacewing paper from Masters and Eisner is at:
http://www.springerlink.com/content/l55848463th204j0/
Thanks for the follow-up. I had thought about the fringes on the wings of green lacewings, winter crane flies, and various other non-tiny insects, and wondered if maybe it’s just advantageous to have a fringe of a certain length, and this becomes proportionately larger on smaller wings. I had been thinking only in terms of how the fringe affects flight, though, so I appreciate your thinking outside my box.
As I understand it, at very small sizes air is highly viscous to an insect, more like water is to us, and the fringed wings act to increase drag and reduce their rate of sinking and to ‘row’ through the air. Analogous structures are fringe hairs on swimming legs of aquatic insects etc. and perhaps the fluff on a dandelion seed. However, I was never very good at physics and even thinking about Reynolds numbers gives me a headache, so I may have it garbled. Could also be a weight-saving device, since the boundary area captured by the fringe would act as a single surface, but presumably weigh less than a normal wing with the same area.
Ptiliids are great beetles, almost as interesting as mites.
Interesting connection between different groups of fringe-winged insects! I just started work in a thrips lab, and we’ve been finding lots of ptiliid beetles in the leaf litter samples as well.
Maybe I’m making an awfully distant connection by saying this, but my first thought is of bird wings. But, feathers tend to overlap quite a lot, whereas these fringes don’t.
It may be thinking of things the wrong way around to ask “how does that help it fly?”. As a creature gets smaller and lighter, flight is easier and easier (well, requires less wing area, anyway). It also starts to make more sense to build wings out of hairs because at larger scales, the hairs would have to be pretty stiff and heavy, but at this size they stay straight easily.
A new paper in Nature about Ptiliidae flight:
https://www.nature.com/articles/s41586-021-04303-7