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Spiderwebs reflect UV to attract prey

Jerome Marot

Well-known member
So spiderwebs reflect UV. That is shown in your picture. What the picture does not show is whether a web which would not reflect UV would attract less preys.
 

Asher Kelman

OPF Owner/Editor-in-Chief
So spiderwebs reflect UV. That is shown in your picture. What the picture does not show is whether a web which would not reflect UV would attract less preys.
Jerome,

When UV reflecting spiders are small it’s likely that UV reflectivity of the web could very well protect it by camouflage.

But “attracting insects” seems worthy of bringing in other supportive evidence no so far mentioned.

Asher
 

Jerome Marot

Well-known member
I don't have to, it has been published here: https://www.researchgate.net/publication/232692128_Ultraviolet_reflectance_of_spiders_and_their_webs and my interest was to see if I could prove that....
Thank you for the citation. The following passage may be of particular interest as to the role of UV reflectance of spider webs:

Bird vision is more similar to that of humans, but it often—like that of insects—extends into the UV (Finger & Burkhardt 1994).Consequently, stabilimenta are probably quiteconspicuous to birds. Since birds are only rarely the prey of spiders, it may be concluded that the main function of stabilimentum is probably deterrence against birds, rather than attraction of prey; thus confirming the studies of e.g., Lubin (1975), Horton (1980), Eisner & Nowicki (1983), Schoener & Spiller (1992) and Blackledge & Wenzel (1999). However, before any final conclusions can be drawn, much more must be learned about the way different potential prey and predators perceive spiders and their webs.
 
As a chemist I can't help feeling the UV reflectance could be to prevent the silk strands decomposing...
UV is detrimental to many polymers, & a web that's weakened won't feed the spider.
;)
 

Asher Kelman

OPF Owner/Editor-in-Chief
As a chemist I can't help feeling the UV reflectance could be to prevent the silk strands decomposing...
UV is detrimental to many polymers, & a web that's weakened won't feed the spider.
;)
In terms of existential benefit, being a warning to birds would be immediately life-saving to the spider.

UV resistance might be necessary, only after weeks or months, during which time there will be many opportunities for repair.

I am more interested in what you might know about UV resistance of polymers! That in itself is fascinating for its theoretical mechanisms and for its usefulness.

Plastics that breakdown might be more biodegradable and less hatmful, environmentally.

Polymers that simply resistant to UV damage might make better paints and structural parts!

Asher
 
In terms of existential benefit, being a warning to birds would be immediately life-saving to the spider.

UV resistance might be necessary, only after weeks or months, during which time there will be many opportunities for repair.

I am more interested in what you might know about UV resistance of polymers! That in itself is fascinating for its theoretical mechanisms and for its usefulness.

Plastics that breakdown might be more biodegradable and less hatmful, environmentally.

Polymers that simply resistant to UV damage might make better paints and structural parts!

Asher
Valid points but the protection from birds seemed to be somewhat speculative. UV certainly affects many chemicals, and it could well be that after initiation a free radical type breakdown the materials of the web are less suitable for the spider to reclaim. (I believe they frequently rebuild their webs)

I don't have a great deal of dealings with polymers, but my interest in UV photography has led me to investigate a bit.
Pure Acrylic is moderately transparent to UV, but is usually doped with an additive to protect it (making it useless for my purposes).

UV light is more energetic than visible or IR, so initiates many more reactions

The old Cheap supermarket plastic bags did breakdown fairly quickly (several weeks/months) on exposure to sunlight, but this effectively only has an affect if you use them for storing things in the shed. In the environment they are often buried or under water :(
 
What makes you say that, Mike?

Think of gamma rays.

100kvp will go through bone!

10 MV will go though entire body, killing cancer cells in its path!

UV doesn’t penetrate more than a few mm!

Asher
I say it from years of studying chemistry. Most organic compounds are not affected by visible light, yet UV of the right energy will ionize them leaving a reactive intermediate that can undergo fresh reactions.

The degree to which UV penetrates depends on the substance, it's concentration, and the specific wavelength.
UV at 350nm will get through many of my lenses, even if it is reduced in intensity.
The 80mm El Nikkor recommended for UV on Dr Scmitt's blog transmits over 50% at this wavelength, considerably more than a much smaller 25mm c-mount lens.
Once we get below ~190nm air becomes quite effective at blocking UV - our high grade UV-visible spectrometer at work can't measure such wavelengths because of the air in the way. It's not an issue for my work as any of the samples I might be analyzing will block the same wavelengths to such an extent that impractically small amounts would be needed.

With lenses there's frequently an extra factor thrown into the transmission physics. Lenses coatings are designed to use interference effects to reduce surface reflections, & these often block UV too, but I'm reasonably sure the two lenses I used for examples do not have sophisticated multi later coatings, possibly singly coated but probably not even that.

When you go to extremely high energies the energy transfer is much more difficult, atoms can't just gain a little temperature to use up any excess energy over the precise activation energy a reaction needs.
 

Asher Kelman

OPF Owner/Editor-in-Chief
Forget about chemistry for a moment. When we use the term, “energetic”, we’re considering the energy stored in the wave. Yes, you are correct in that UV might like to interact with certain organic bonds, but the UV part of the electromagnetic spectrum actually carries far less energy per incident photon, (considering light as particles), than orthovoltage or mega voltage gamma rays!

With higher energy radiations, interactions are mostly with water, knocking out electrons that then cause whatever damage or reactions we observe. UV, visible and other low energy radiations, each match to different classes of organic compounds capable of absorbing the energy, resulting in excitement and emission of light at a different wavelength as the excited electrons crash out of orbit, or else chemical bonds are broken and new highly unstable reactive species are transiently formed.

But in terms of radiations we commonly use in medicine and industry, honestly, UV is real low in its stored energy!

Asher
 
It is indeed, but I'm not able to take photographs with higher energy sources.
I now have a flash modified for UV - though I've only used it for UV induced fluorescence to date.

Regulations on the sale of ionizing sources & specialist detector needed make more energetic forms of imaging out side my hobbyist range.

I don't think spiders need to be overly concerned with gamma rays either.😁
 
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