Spider silk colors at 100x

Images taken in a controlled environment or with a posed subject. All subject types.

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Chris S.
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Spider silk colors at 100x

Post by Chris S. »

In a recent thread (Spider web "diffraction"), LordV introduced us to vibrant colors produced by light shining through spider silk, under certain--though fairly common--conditions. In the field, this effect is easy to see or photograph on a clear, sunny day, when there is no haze or cloud to diffuse and decohere the sun's light. Simply aim your eyes or camera at the web, with the sun on the other side of it, just outside your field of view, and you should get web colors.

I wanted to take a closer look, using the Bratcam under studio lighting.

Although I posted the first image below in LordV's thread, I'll repeat it here to facilitate comparison. Pictured is a length of structural silk, one of the "guy-wires" holding a large orb. Structural silk is one of the kinds of silk produced by spiders; other sorts of spider silk can have different chemical, mechanical, and --perhaps--optical properties.

Here is the web I shot, lit from nearly-behind by a small (8mm diameter) light source (fiber optic light guide from a halogen illuminator) placed outside the camera's field of view. (About 11 degrees above and to the right of the optical axis, about 37cm distant.) As such a light source is semi coherent, this may represent a studio approximation of optimal field conditions.

For all images in this post, the lens is a Mitutoyo 100x/0.70 Plan Apo. Unless otherwise noted, no color enhancement has been done.

Image


The above image tells only a partial tale. Fascinatingly, the colors change wildly as the focus point is moved forward and backward. Here is a composite of images where only the point of focus is changed. At the top of the composite, point of focus is on the far side; in the segments below, focus moves to the web itself, then between web and camera. (Outlying color bands have been cropped out.)


Image


Here is a video made from the images used for the above composite, along with many more from the same series. The distance covered is 162 microns--about eight times the diameter of the silk strand. The video starts with focus in the air beyond the far side of the web, and ends with focus in the air on the near side of the web.

https://vimeo.com/107438370

(Some may detect the false appearance of horizontal subject movement. This an effect of utilized aperture, as demonstrated and explained in TheLostVertex's thread, Perspective shift from lighting change. The apparent movement occurred in both the vertical and horizontal dimensions, but I used Zerene Stacker’s alignment capability to correct for most of the vertical movement. I could also have done this for the horizontal, but it would have required cropping, and I didn’t want to leave out information.)

Although color-generation seems most pronounced when the light source is small, partially-coherent, and located behind the web, less-vibrant color is also generated when spider silk is lit in other ways. The image below resembles bright field on a standard microscope. Here I illuminated a white card behind the web, and shaded the web from light sources other than the lit-up card. (Ignore much of the dark purple color, which is probably axial chromatic aberration in the lens. Though the lens is an apochromat, it can produce a little color in a situation like this.)

Image

The following image resembles darkfield on a traditional microscope. Here, I lit the web very diffusely through a ring of white paper wrapped around the lens, pushed forward until it almost touched the web, while keeping the background unilluminated.

Image


Here is a pixel crop of the "quasi-brightfield" image, with no color enhancement:

Image

The same pixel crop, with color vibrance and saturation jacked up to facilitate analysis:

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Here is a pixel crop of the "quasi-darkfield" image, with no color enhancement:

Image

And again, the same pixel crop, with color vibrance and saturation jacked up to facilitate analysis:

Image


I don't think the colors shown here match the simple diffraction produced by a prism or rainbow. Diffraction is clearly involved, but I think that more complicated diffraction models are at work, and are acted upon by interference phenomena. If anyone has further insight, it's most welcome.

Technical details:

This shows the spider silk mounted in front of the camera. I cut a piece of structural silk with scissors, and used adhesive tape to attach it to a gizmo I'd made to hold microscope slides. At one point, I bumped the silk and caused it to detach on one side. I found it easy to reattach, but structural spider silk is stretchy, and apparently, I applied slightly different stretching force in attaching it the second time as I did the first--so the diameter of the silk differs between shots.

Image

I can't name the species of spider that produced this silk. It was a largish, brown-looking nocturnal orb-weaver of a sort quite common here in NE Ohio, often referred to as a "barn spider." Every year, several of them build orbs on the eaves of our house. I thought about sacrificing her and sending her to an arachnologist for ID, but could not bring myself to do it. Having studied her beautiful web--and earlier, brushed two of her orbs off my face, as she kept building them across an outdoor stairway I use at night--she seemed a bit like a collaborator. After grappling with this decision for a couple of days, I went out to get photographs of her, only to find her gone. But I've photographed what I presume are members of her species in the past, and certainly will have the chance to do so again.

--Chris

NikonUser
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Post by NikonUser »

Most interesting. I wonder if part of the effect is due to the nature of the strand. Orb Weavers have 6 finger-like projections (spinnerets) through which the liquid silk emerges. Each spinneret has numerous microscopic spigots through which the liquid is 'ejected'. Thus what ends up as looking like a single strand could be constructed from 6 x n individual strands.
NU.
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No man can be truly called an entomologist,
sir; the subject is too vast for any single human intelligence to grasp.”
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lauriek
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Post by lauriek »

I suspect I've missed some previous discussion here, but I just wanted to mention, to my eye I'm seeing what I would definitely assume are interference patterns in your second shot, the backlit set.

And I was going to ask about what NU mentioned, is one strand actually a single coherent homogeneous thing, or is it more like a piece of string, made up of sub-strands woven together?

I'm aware of the multiple spinarettes, having photographed spiders bottoms, but I had it in my head that these worked together to produce a single strand, I can't remember if I read this or just assumed it..

Chris S.
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Post by Chris S. »

NikonUser wrote:Most interesting. I wonder if part of the effect is due to the nature of the strand. Orb Weavers have 6 finger-like projections (spinnerets) through which the liquid silk emerges. Each spinneret has numerous microscopic spigots through which the liquid is 'ejected'. Thus what ends up as looking like a single strand could be constructed from 6 x n individual strands.
NU, I much appreciate your responding. Without doubt, you're far more familiar with spiders than I. While I can reliably differentiate a salticid from a saltine, my knowledge of arachnids goes little deeper.

My tentative sense has been that I’m looking at a single strand of spider silk, rather than an aggregation of thin strands—even though such aggregations appear to be very common. But I’d gladly be corrected, and have certainly pondered the converse. Let me explain my thinking.

The Guardian.com shows six types of silk being produced by an orb weaver: Inside the silk factory: How spiders use their silk—graphic. If this graphic is correct and representative of the spider whose silk I examined, the structural silk I photographed emanated from the orb weaver’s major ampullate gland.

According to the Australian Museum’s Silk: the spider’s success story, ten types of silk glands and their spigots are known in spiders, with individual species having up to eight silk glands, each with different properties and uses. Quoting this source:
  • "Each gland opens on the spinnerets either via one or two spigots (ampullate glands), several spigots (cylindrical glands), or many spigots (pyriform and aciniform glands). In addition, the cylindrical and aciniform glands open onto two spinnerets." (Emphasis mine.)
So if I'm indeed examining silk produced by the major ampullate gland, it should be either a single strand or pair of thick strands, yes?

Looking on the Internet for SEM images of spider silk formation, I saw fascinating images of silk such as you describe, coming from numerous spigots attached to the piriform gland--which, as you say, involve many strands that twist or otherwise agglomerate together: Here is an example. However, this is a quite different sort of silk from the structural silk I examined, isn't it?

For Internet searches specific to structural silk, the search phrases "major ampullate silk" or "major ampullate gland" produce far more returns than I've had a chance to read, though a sad proportion of these are behind paywalls.

However, Spider Silk Composites and Applications, by Craig Vierra et al, is both open access and interesting.

But I'd note that no reference I have so far seen shows SEM images of structural silk sized much within an order of magnitude as thick as what I examined. I'd be delighted if someone points out a suitable reference that I haven't seen. And it appears that even though quite a bit of scholarly attention has been focused on spider silk, the subject is vast, and has barely been examined.
lauriek wrote:. . .I just wanted to mention, to my eye I'm seeing what I would definitely assume are interference patterns in your second shot, the backlit set.
Laurie, thanks also for your thoughts! I strongly agree that interference patterns are evident. If anyone has not yet viewed the video linked to in my post, it shows the interference patterns pretty dramatically.

My local orb weavers, of the type whose structural silk I examined, are gone for the season, so I don't have access to additional web specimens. But in some other year, I think it would be very interesting to compare SEM, X-ray diffraction, and optical imaging on adjacent portions of a single strand of structural silk. I don't have SEM or X-ray diffraction instruments, but may attempt to interest a few friends that do.

Cheers,

--Chris

Craig Gerard
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Post by Craig Gerard »

Chris,

Appreciate the work that went into this project. A most interesting study.


Craig
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abpho
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Post by abpho »

Wow. Thanks for the info. That was very interesting. I never knew that.
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TheLostVertex
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Post by TheLostVertex »

Excellent posts Chris! I love spiders and find all of these images very interesting.

My first thought after viewing the images was that perhaps the silk is acting as a diffraction grating and is producing lots of very strong interference from small details in the silk. The video looked that way to me as well. When watching the video I felt like I could see details in the patterns that otherwise would not be very noticeable. Almost like a DIC image.

There are a lot of brown spiders called barn spiders. Very common ones that should be in your area are Neoscona crucifera and Araneus cavaticus. N. crucifera is very common around here earlier in the year, but I would imagine they come out at different times depending on the climate of the location.

I hope you are able to convince some of your friends to join in this investigation at a later date. :)

flyer2o12
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Post by flyer2o12 »

Once upon a time I accidentally imaged spider silk on a SEM. I noticed that each silk strand of the web was in fact made up several intertwined fibres of silk.

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