FridayAFM - Rose chafer beetle

Héctor here, your AFM expert at Nanosurf calling out for people to share their Friday afternoon experiments. Today I do some photonics with the help of a bug.

Beetles are amazing. (Full stop).

First of all, do you know that some beetles use the stars for navigation? Not joking, there is serious research about it (Dung Beetles Use the Milky Way for Orientation). 

But that is not all about beetles and light - did you ever wonder where the metallic-rainbow colors on their surface come from?

1 Rose chafer beetle. Purchase, unboxing, mounting on AFM, and how the surface looks like under the optical microscope.

According to the literature, the main optical properties of the cuticle (the hard exoskeleton), are due to chitin fibers ordered in layers where the fibers in each layer are aligned in a slightly different direction creating a short of photonic crystal.

For us, it will be hard to see the buried chitin layers, but can we see something interesting at the surface level?

1 Topography images of epicuticle surface. Using Static force mode and ContAL-G probes from Budget Sensors.

We can see periodic cracks in the surface. Period (see last figure) is about 10 μm, and step height varies between 32 and 60 nm (likely to be 30 nm and sometimes two steps merge together). This value is similar to the 27.7 nm reported as chitin layers' thickness (by Hans Arwin et al. "Cuticle structure of the scarab beetle Cetonia aurata analyzed by regression analysis of Mueller-matrix ellipsometric data," Opt. Express 21, 22645-22656 (2013)), hence it is likely that the blazed grating structure on the surface forms from the chitin layers not being parallel to the surface. Cool, we already learned something.

What else can we see?

Well, the cement channels coming from the dermal glands are kind of fairly easy to see, what about the wax channels?

1 Topography images of cement and wax channels. Using Phase contrast mode and Tap300AL-G probes from Budget Sensors.

It turns out we can see them also, but the probes we were using before where too soft and the adhesion in the surface was making it difficult to see fine details (the wax+cement mix is sticky), so we had to change to stiffer probes and dynamic mode to see the channels with better detail. Better than that, we can measure the channel aperture width, which I haven't seen reported before (please share a publication if you find it). It is about 70 nm in diameter.

Coming back to the periodic cracks in the surface... as it turns out, they can be considered as a blazed grating structure. Blazed gratings are very cool gratings due to their directionality, causing light scattering to vary based on wavelength. By measuring certain parameters, we can identify the magic angle and magic wavelength, where light coming at that angle achieves perfect reflection (check the Wikipedia link for the formula). These wavelengths are 307 nm, 153 nm, 76 nm .... But only 307 nm matters, because while in the UV range, it is still emitted by the sun and thus the only likely to appear in the beetle's day to day life. Could this function as a type of navigation mechanism? Perhaps these gratings are used to sense the sun's direction through absorption or reflection changes relative to their orientation. I'll leave this intriguing possibility for speculation or the potential discovery of a relevant reference.

1 Topography images of epicuticle surface and cross sections of different areas. Using Static force mode and ContAL-G probes from Budget Sensors. 

Some researchers suggest that the main objective of the gratings is to reduce friction when the beetle is moving between fibrous objects. 

Since the rose chafer beetle is poisonous and feeds on plants, could this also be a warning mechanism for other insects? Bees are able to see up to 300 nm wavelength, so maybe these beetles shine in the bee's eyes and they can avoid them.

The interesting part is that there is a grating, and is bound to affect the light, and the magnitude of its effect in relation to the photonic crystal underneath... is something to be studied elsewhere.

Hope you liked it!

By the way, if you made it this far, here are some discarded images.

1 Just to get a better statistic on hole distribution.

2 Actually I ordered a surprise package, I didn't plan for this beetle in particular.

3 The plastic allows for a soft grip, just need to make sure I don't rip it completely when opening it.

4 Have enough clearance to place it under the AFM and be able to move it around with the automated stage.

5 Unfortunately, the optical bench microscope I have here has a not diffused light source and creates nasty reflections.

Hopefully this captures a bit the color pattern.

Stay in touch, and if you have more info about these beetles, please share.