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Use of flash with Leucauge venusta

I recently took a series of photos of what I assume is Leucauge venusta. I noticed that the amount of green and the amount of yellow varied slightly on the same spider depending on whether they were in sunlight or shade. Moreover, when you use the flash, large areas of the abdomen turn silver. It would seem that there is some kind of a reflective material that changes the apparent color pattern based on the K value of the light.

Guanine and Guanocytes
Kevin,

Thanks for that response. Perhaps I should have said "as yet, there is no definitive answer from the folks at Bug Guide as to whether this is a matter of structural refraction, pigmentation, or both." I was trying to reflect your comment "are the silvery reflections (found in most/all of the tetragnathids as well as a few other species, too) a result of a pigmentation (as white, yellow, etc. are) or refraction? Perhaps it's a combination of both."

Apparently, I was unclear.

I guess my frame of reference is what I learned about Blue Jay feathers about a thousand years ago in elementary school. The blue is a result of structure, not pigment (or so I was told).

Awash in arachnological ignorance, I would, if pushed, posit that your comment about the hairs is the right direction to look. I suspect that the guanine serves in the same role as the silver on the back of an old mirror; the hairs, or some other aspect of the cuticle(?) then serving as the actual "glass."

Or I could be all wet.

Flash with Leucauge venusta
The photos are contained in a web posting at http://northoftheridge.com/2012/06/orchard-orbweavers/

Editors--if you can think of any other way that I can make the photos available as a set to your site, feel free. I don't mean to use this site to push my material.

 
Guanine / guanocytes
Thanks John. Nice photos.

Just a couple quick notes -- all colors (as we and as the camera see them) change in appearance relative to their illumination. Some surfaces (e.g. reflective, glass surfaces) show more drastic changes. This is due to the colors of the various sources (reflected light from the sky, reflected light from plant vegetation, etc.), the nature of the light sources (specular vs. diffuse), incidence angle, etc., and also, with respect to camera chips and film, their specific response curves.

Also, your comment that "there is , as yet, no answer as to whether this is a matter of structural refraction, pigmentation, or both" is not supported by our comments. We are (I am) unaware of the specific nature of this in spiders, but if you gathered together all of the knowledge about spiders that I am unaware of, it would fill a canyon. :-)

Doing a quick Google search on spider pigmentation, I see that even Wikipedia is "smarter than the average bear": "Only three classes of pigment (ommochromes, bilins and guanine) have been identified in spiders, although other pigments have been detected but not yet characterized. ... Bilins are found, for example, in Micrommata virescens, resulting in its green color. [In many species guanine is] ...accumulated in specialized cells called guanocytes. In genera such as Tetragnatha, Leucauge, Argyrodes or Theridiosoma, guanine creates their silvery appearance."

So there you have it: an accumulation of guanine (a common spider pigment and also a basic building block in DNA/RNA) in these specialized guanocyte cells. I'm sure that with a bit deeper searching (or try Foelix's great book Biology of Spiders), that you could find specific papers dealing with this in more detail.

-Kevin

Source of this silvery color
Brings to mind an interesting question -- are the silvery reflections (found in most/all of the tetragnathids as well as a few other species, too) a result of a pigmentation (as white, yellow, etc. are) or refraction? Perhaps it's a combination of both (white pigment overlayed with refractive layers or hairs)?

 
Silvery color
I had a couple of immature spiders (I'm pretty they were Leucauge venusta) from my collecting the other day, and in alcohol the abdomens were very silvery.

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