2021 September 19

Continuing the series “tiny insects crawling on my daughter’s arm”, on August 28, 2022 Sam came in the house and wanted to know what the practically invisible speck crawling on her arm was. We thought at first that it might be a springtail, but it was only about 2 mm long and impossible to see details with the naked eye. So I fired up the macro camera, ran the lens up to 3X, and got a few shots. And here it is:


First off, it really was that black. It wasn’t underexposed, it just reflected almost zero light, which makes seeing details a bit difficult.


Second, it has a set of wings folded over its back. Right away, this tells us it isn’t a springtail, because springtails don’t have wings. Rather, it is a thrips[1], probably one of the Tube-Tailed Thrips in the family Phlaeothripidae. This only narrows it down to one of about 350 species in North America. I thought it might be a Clover Thrips, but they are supposed to have 8 antenna segments and this one only has 6 as far as I can see. And very few people put pictures of thrips up on BugGuide, so I don’t exactly have a lot of things I can compare it with to narrow it down further.

About the wings: Unfortunately it was too tiny to make it open its wings without harming it, because their wings are, well, “different”. Instead of a membrane stiffened by veins like other insects have, a thrips wing is basically just a stick with a fringe of hairs around it. Thrips are just so small that they don’t see air the way larger creatures do. As far as they are concerned, the air is just a viscous fluid that they kind of row through, more like swimming than flying. This is referred to as Clap and Fling flight, although to be honest the description on Wikipedia is needlessly complex and the diagrams are weirdly cryptic.

Anyway, some thrips are garden and agricultural pests, but a lot of them just eat flower pollen and the like and are pretty much harmless. And I think this one is one of those.

[1] “Thrips” is the correct singular word, and is also the plural form. The name came from a greek word that happened to end with an “s” sound, and greek words don’t make plurals by adding an “s”. Although, if people wanted to suddenly start calling a single specimen a “thrip”, it would be OK by me. Just watch out for grammar pedants who also happen to be entomologists.

5 Responses
  1. Tim permalink
    September 20, 2021

    Great description of the wings and how they navigate this viscous fluid called air!

  2. September 21, 2021

    Great post as always, amigo.

    The flight mechanics are wild. Is that because they’re so light relative to the air?

  3. September 26, 2021

    KT: They aren’t any lower density relative to the air, but they are so tiny that their Reynolds number is way, way, way down in the pure viscous flow regime. Other, larger insects are well into the laminar-to-turbulent transition region, where the basic behavior of fluids is markedly different.

  4. October 7, 2021

    I had to look up the definition of the Reynolds number. 🙂

    It’s been a while since I took fluid dynamics.

  5. October 7, 2021

    Yeah, that’s the thing about college classes: you probably only will use 10% of the things you learn in college, but you don’t have any way of knowing in advance which 10% it is going to be.

    (Reynolds Number is one of the things I do use, I work a lot with particles settling through fluids and fluids percolating through porous beds. I’ve forgotten almost everything I learned about, say, Fortran 77, though).

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