Megacephala sp

[Gérard-64]

40 pics with Zerene

[rjlittlefield]

Fascinating! What is the structure of the eyes, that we can only see ommatidia in what appear to be cracks in some covering?

--Rik

[Gérard-64]

Rik,I can't define what it is.The insect is quite small(12mm)I am not sure if the "cracks" on the eyes are natural or resulting of a too quick drying..
I am going to order an other one to make a comparison.'Could it be that the insect have been varnished?..)

[rjlittlefield]

Could it be that the insect have been varnished?

I have never encountered that, but cracked varnish is exactly what this looks like. It's like somebody painted over the eyes and the paint is cracked and shrinking.

--Rik

[Gérard-64]

I am going to put the question to the seller!..

[yeatzee]

You can buy one of these?

[Mitch640]

You can buy one of these?

HAHA, there is a whole cottage industry built around specimen bugs and living microbes.

Interesting coating on the eyes though.

[Planapo]

What is the structure of the eyes, that we can only see ommatidia in what appear to be cracks in some covering?

Not necessarily varnish, what's going on here with the eyes looks more to be likely the following to me:
It's a specimen that has been dried to preserve it. Often one can see this (even without Gerard having told us now) when looking at the eyes: In some specimens, when dehydrating, the outer layer(s) of tissue that cover the omatidia oftentimes delaminate from the underlying structures, so that you can't recognize the single ommatidia beneath anymore. This can affect the whole or only parts of the eyes.
In my experience, sometimes when later rehydrating the specimen for pinning/preparation, the delamination can (partly) be revised, but when drying out again, it delaminates again.

You can buy one of these?

In tropical ecology there is this rule of thumb that often holds true: That in many places rare species are common and common species are rare. This implies high biodiversity, but at the same time vulnerability: commerical collecting at a site may consequently wipe out a whole local population or subspieces when trying to satisfy customer's high demand for one particular, spectacular looking, must-have insect.
Most likely such commerical collecting is not controlled at all, let alone managed for sustainability.
For this reason, and some others, I regard this kind of trade with animals taken from the wild, may it be a tropical region, or non-tropical as with this cicindelid/carabid beetle here, with a great feeling of unease, and would not support it with becoming a customer.

--Betty

Tragedy of the commons

[rjlittlefield]

In some specimens, when dehydrating, the outer layer(s) of tissue that cover the omatidia oftentimes delaminate from the underlying structures, so that you can't recognize the single ommatidia beneath anymore. This can affect the whole or only parts of the eyes.
In my experience, sometimes when later rehydrating the specimen for pinning/preparation, the delamination can (partly) be revised, but when drying out again, it delaminates again.

Betty, thank you for prompting me to investigate this further. I had thought that I had not seen this effect before, but in fact I have -- and in one of my own frequently referenced threads!

The beetle I'm talking about is the Ten-Lined June Beetle HERE. In the full-resolution image (HERE), you can see that the eye is interestingly patterned with bands of golden-colored ommatidia.

In that image, you can see from the specular highlight above center that the surface of the eye is smooth and continuous both over the featureless black sections and over the golden ommatidia.

I have just now put that beetle under a couple of scopes at higher resolution, and what I see is very interesting. Under a dissecting scope at 45X, my impression is that the eye appears to have three layers: 1) a smooth transparent outer layer, 2) a cracked and shrunken middle layer that is featureless and black, and 3) a lower layer of golden ommatidia that is visible only through cracks in the featureless black middle layer. However, under a compound scope at 200X (using a Nikon CF M 20X ELWD objective), that impression changes, and what I see looks like only two layers: an outer smooth transparent layer, and an inner layer containing well defined ommatidia in which most of the ommatidia are black with barely visible outlines, and the others, arranged as if in "cracks", contain highly reflective voids or crystals. I'm still not sure what the real structure is -- nondestructive examination only goes so far, and in this case I don't think it's far enough.

I had never examined the eye of my beetle under high enough magnification to appreciate the layered structure, and in the image of Gérard's in this thread, the reflections don't happen to make it apparent that the outer layer is intact. But I suspect that closer inspection of his beetle would show it to have the same structure as mine: multiple layers with the outer one intact, and something resembling cracks on the inner one.

This is an excellent day -- I have learned several new things.

--Rik

[Gérard-64]

Here is a closer view:



PS:According to the seller,this insect has not been varnished

Moreover I have a dilemna concerning the I.D:

1-It was sold to me as a Megacephala Sp. from PERU
2-A friend of mine,Mr Alain Ramel: http://aramel.free.fr/ has identified it as a Megacephala euphratis.
3- I was told on another forum that M.euphratis does not live in PERU...
So I am not sure anymore about the I.D..

If somebody can help...

[rjlittlefield]

Thanks for the closer view. It takes some study, but I believe I can see in the reflections evidence that the top layer is smooth and intact. This looks a lot like my beetle does through the 45X dissecting scope.

--Rik

[Mitch640]

Whatever the black is, there seems to be more of it, behind the eye, and to the right, where the whole structure becomes the head.

[rjlittlefield]

Whatever the black is, there seems to be more of it, behind the eye, and to the right, where the whole structure becomes the head.

In the first image, there's a black ring around the eye, extending down around the base of the antennae. The ring looks a bit "painted on" and it's also apparently continuous with what appears to be the "cracked coating" over the eyes. All this fit very nicely with Gérard's "varnish" description, but I'm pretty sure it's an illusion. Certainly on my beetle, and I think Gérard's also based on his second picture, the islands of black apparently on the eye are actually inside the eye, behind a transparent outer layer. The appearance is much the same as the stuff on the outside, but I'll bet the origin is much different.

--Rik

[morfa]

Very nice images!

I've seen similar effects and I suspect it happens when the eyes dry out.

Here is the eye of another Tiger beetle showing similar (but less pronounced) effects: http://farm3.static.flickr.com/2546/419 ... 04e3_b.jpg

In fresh/living specimens the lighter streaks are not visible.

[rjlittlefield]

similar (but less pronounced) effects

The "cracking" is similar, but the surface of the eye is very much different.

In morfa's example, the ommatidia are still clearly visible as an embossed pattern on the outermost layer.

In Gérard's example (and in my June Beetle), the outermost layer shows no sign of the ommatidia. It is just a smoothly curved surface with small random irregularities.

What took me by surprise was the smooth curve of the outer layer. I had never noticed that in some dried beetles the ommatidia are not represented in the outer layer of the compound eye.

Now I am curious to know whether the outer layer of those beetles' eyes is smooth when the beetle is still alive. It seems unlikely to me that drying could so completely erase the pattern of the ommatidia, if that pattern was present when the beetle was alive. But really that's only based on extrapolation from other quite different systems, so I'm not feeling very confident about the guess.

--Rik