FridayAFM - ... and the bees.

Héctor here, your AFM expert at Nanosurf calling out for people to share their Friday afternoon experiments. Today I I'll show you why insects don't need to lubricate their eyes to keep them clean.

A few #fridayAFMs ago, we look at flowers and I showed you how to prepare pollen for AFM imaging, today, we will focus our attention onto another actor of the cycle of plant's life: the bees.

Human eyes require lubrication to stay clean. How insect eyes remain clean? (Thanks Henrik to draw my attention towards this topic).

To answer this question, the first challenge is to capture insects, and... well, let's say that they are not inclined to collaborate.

However, thanks to Christian and Jonathan we managed to secure a bee and a fly (which happened to be perfect examples of two different behaviours).

(The fly was more brittle by the way).

Next challenge is how to mount these samples for AFM imaging.

Initially I just used double-sided tape and put the insects on top in with one eye facing up. But this turned out to be too unstable, the surface was rocking sideways because the insect hairs touched the AFM probe chip. So... let's say that I took a more pragmatic approach.

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With the eyes ready for imaging, the next challenge is the hair. Yes, the hair. It turns out that bees are so hairy that they even have hair in their eyes, and... this bee as well.

How did I solved this challenge?

Patience, long cantilevers (stiff ones), and using the optical camera focus to navigate in 3D and enter with the probe in between the hair fibers. Something like this:

If you don't believe me, here is the proof it works (by the way, using a Tap-150 probe and imaging in WaveMode).

So... what is the difference between the bee's eye and the fly's eye?

Well, not much really, a part from the size of the ommatidial lens size.

Or so it looks like...

If I flatten the images to see the tiny details on the surface of the lenses something else is revealed.

The surface of the fly's eye is covered with little protuberances, while the bee's eye lenses are quite flat a part from the borders. 

What are these portubertances for? and why the bee doesn't have them?

The protuberances create a "lotus effect" to prevent things from sticking to the eyes (see references 1 and 2). That is why a simple rubbing of the legs against the eye is enough to clean them.

But what happens in the case of the bee?

The bee is very furry and the hair fibers is what prevents things like pollen to stick to the eye's surface. So a rubbing with the legs is also enough to cleanse the eyes, but for a different reason, and thus is performed in a slightly different way.

By the way. Did you noticed that in the fly's case towards the centre the protuberances tend to be circular but towards the edges they are elongated?

Let's recap. Nature is amazing but tricky. Imaging insects eyes required patience to find a suitable spot and good sample preparation to make them stable. Once stable, we saw that the fly and the bee have different eye structures, and that in the case of the fly, the eye is covered with protuberances that provide anti-sticking properties.

I hope you find this useful, entertaining, and try it yourselves. Please let me know if you use some of this, and as usual, if you have suggestions or requests, don't hesitate to contact me.

References:

[1] Henrik Peisker, Stanislav N. Gorb; Always on the bright side of life: anti-adhesive properties of insect ommatidia grating. J Exp Biol 15 October 2010; 213 (20): 3457–3462. doi: https://doi.org/10.1242/jeb.043661

[2] Kryuchkov, M., Bilousov, O., Lehmann, J. et al. Reverse and forward engineering of Drosophila corneal nanocoatings. Nature 585, 383–389 (2020). https://doi.org/10.1038/s41586-020-2707-9 

Extra:

If you like to combine AFM and insects, I remind you of the optical properties of the rose chaffer beetle (follow the link for more details about it).