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I found these little (3-4mm) insects in my indoor greenhouse where I propagate Begonia from leaf cuttings.

They are slowly walking around on the soil and don't seem to be particularly interested in the plants. When I disturb them they start running around quite fast.

The (unfortunately low quality) pictures show some individuals that I managed to move into a test tube.

location: Switzerland.

the insects immediately make themselves drop when you try to lift them up with a piece of paper.

enter image description here enter image description here

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Short answer

I think this could possibly be the nymph of some sort of cockroach.

Long answer

I initially thought these were aphids of some type due to their pear-like body shapes and the small pair of upright projections coming off their backsides (possibly cornicles?).

enter image description here

However, zooming into some of your more-detailed specimens in the 2nd photo immediately made me questions this:

Your specimen appears to lack a prominent cauda sticking off the center of their backside; further, they appear to have some sort of palps around the mouth and a fairly hypognathus head -- all of these characteristics are inconsistent with any aphids I know.

Lack of a furcula (among other details) rules out springtails.

Your specimens appear to have filiform antennae, which doesn't preclude aphids or many other insect orders including beetles, roaches, and orthopterans (e.g., grasshoppers).

Your one specimen appears to have a prominent posterior femur (upper part of leg), suggesting these insects may be good jumpers. However, it could be that the enlarged region is in fact a coxa or that other enlarged femurs are simply less apparent from your photo. Regardless, given the enlarged leg and your observed running behavior, I'd guess these are cursorial legs adapted for -- in fact -- running! [See here for more on legs].

  • Running vs jumping rules out orthoptera nymphs of any kind.

Given the head type, body shape, mouth parts, rear projections (likely cerci not cornicles), and rear leg anatomy, I suppose this is some sort of cockroach (Blattodea) nymph.

Here is an example:

enter image description here

Ectobius pallidus - young nymph (<3mm) ; Source: JungleDragon

Due to the poor quality of the photo, I think I'll stop here. You'll have to explore possible species based on your ability to ascertain in-person detail.

  • Alternatively, someone else might know a more specific group/species and/or may propose another order of insect all together. We'll see..

I'll consider updating if you provide additional detail.


Also, I'm going to pitch the informative Insects information page from NC State University: https://content.ces.ncsu.edu/extension-gardener-handbook/4-insects

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  • $\begingroup$ It's just great to read such an analytical answer to my question. Thank you! $\endgroup$
    – ert22
    Dec 27, 2021 at 19:14
  • $\begingroup$ @ert22 thanks! Glad I could help. Would you say your specimens looked anything like the photo I include? (i.e., am I close?? :p -- the photos really make it hard to tell!) $\endgroup$ Dec 27, 2021 at 20:13

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