Spiderweb colors violate the rainbow rule

Spiderweb (overview). Backlight [1]

According to the laws of physics, rainbows caused by sunlight always appear in the section of sky directly opposite the sun [2]. Here I show that spiderwebs violate this rule by pictures taken of spiderwebs with backlight resulting in rainbow-like colors as well.

Kruisspin / Garden Cross Spider / Araneus diadematus 

Detail. length ~ 1 cm / 0,4 inches

Straight lines demarcate a sector of the spiderweb circle
that lights up selectively. The rest is faintly visible and colorless.
The lines meet at the center of the web,
and seem to point towards the sun.

detail of previous picture. Click to enlarge.

So the part of the spider web circle that lights up seems to be a segment which points to the sun. Furthermore, as shown in the previous blog, the threads of the web consist of many small segments with different colors. Contrary to the previous blog, these pictures are taken towards the sun. The fact that colors are seen at all contradict the physical rules for the formation of a rainbow by the sun. A possible explanation could be that the spider web threads are not made up of water droplets but of proteins...   

Garden Cross Spider
This little spider had nothing to eat for a week.
Finally she caught a fly! Enjoy your meal!

An adult Garden Cross Spider
built its web on the barn door.
Fortunately we could still use the door!

 

Notes

1. The main light source, the sun, is behind the spider, facing the camera and photographer. Note: in Dutch this is called 'een tegenlicht opname'.
2. Thanks Rolie Barth and Matt Young for pointing this out. See for example wikipedia article Rainbow.  

Previous blog

• Accidental discovery of the ultra finestructure of the web of the Garden Cross Spider [+additional pictures].  29 June 2026

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Accidental discovery of the ultra finestructure of the web of the Garden Cross Spider [+additional pictures]

Kruisspin / Garden Cross Spider / Araneus diadematus ©GK
One radial line is visible.

Yesterday my attention was drawn by an orange coloured spider illuminated by the sun. I was forced to underexpose the photo by 2 stops to get the right exposure of the spider. It appeared to be the common Garden Cross Spider (Kruisspin). But what me surprised the most was one thread (fiber) of the web:

Here is a detail of the thread (cut into two parts). A remarkable pattern of very fine stripes with all the colours of the rainbow emerged. I've never seen such a pattern. I expected a continuous thread. Furthermore, lengthwise along the thread, I see 3 dark lines. That would imply that the thread is not one thread, but a combination several (four?) threads.  

detail, 2x zoom, high contrast, b&w.

Why in heaven's name this very detailed structure? What is going on? How does the spider manage to create this pattern? And what is its function (if it has a function at all)?

Afterwards I realized that I have been extremely lucky. First, the sun must illuminate the web at the right angle to make the thread and the pattern visible. Spider webs are supposed to be invisible to be effective! In the shadow, you see absolutely nothing. I was also lucky that a thread was nearly parallel to the camera sensor, which keeps it in focus over a sufficient length of the thread. Secondly, I underexposed the photo to get the spider right. As a side effect the pattern appeared. I almost never underexpose a photo. Normal or overexposure make the fine details disappear. Thirdly, I was lucky there was no annoying wind that could move the web. Because web threads are so thin, the slightest breeze brings them out of focus. Wind is the enemy of macrophotography. Finally, it was a clean and fresh web in the garden, so no house dust attached. 

 

The science behind it

I know that scientists have analysed the chemical composition of the threads. The threads are made of Spidroins proteins that form the majority of spider silk fibers. 

Major Ampullate Spidroin Structure. The repetitive domains of major ampullate (dragline) silk consists of alternating regions of polyalanine and glycine-rich sequences. These repetitive domains are important for the strong and elastic features of the dragline silk fibers ( Xu and Lewis,1990). (Synthetic Spider Silk Production)

It seems very unlikely that what we are seeing here are alternating regions of  Alanine and Glycine (Amino Acids) rich regions in the proteins. More likely we see crystalline and non-crystalline regions in the threads? Maybe different spidroins? I have really no idea. There is a lot to explore...

Surprisingly, I did not find similar pictures of the fine structure of spider web threads in the relevant Wikipedia pages, Spidroins, Spider silk and Spider web.

 

June 30 / July 1: some edits in the text, +1 picture.

 

Additional pictures

8 July 2026

[4684-detail] original positions. GK.

[4669-detail] main thread. GK.

[4658-detail] original positions. GK

[4657-detail] original positions. GK

 

[4654-detail] original positions. GK.

All pictures with a factor -2 to -3 stops under-exposure. Kruisspin / Garden Cross Spider. They are not enlarged. Macrolens Sony 90mm. Threads have direct sunlight from behind the photographer. No artificial light source. All threads except one [4669] are connecting threads. See here for the anatomy of a spider web.

What stands out is that no threads are the same.  Can all this be explained by pure optics??? That is: no internal structure of the threads???

 

9 July: Meanwhile, I have made pictures with the sun in front of the camera ('tegenlicht'). They also show the usual patterns depending on the angle of the sun's rays. Direct sunlight is necessary. The sun must not be wholly or partially covered by clouds. Also, the cross bands are never visible across the entire web at the same time. This can be interpreted either as differences in the structure of the threads themselves or as a difference in the angle at which the sunbeams strike the web. The fact that the visibility of structures highly depend on sunlight is in no way an argument against the existence of structures. In biology structures are always made visible by chemical or optical techniques. For example, chromosome bands are only visible when stained, but are absolutely real.

 

Further Reading

• Optical effects on spider webs by Dietrich Zawischa (beautiful pictures with explanations)
• Accidental Observations by Dietrich Zawischa is a collection of remarkable optical effects in everyday life. (With thanks to Matt Young) (22 July 2026)

Are there biological facts that the theory of evolution does not explain and does not need to explain?

The theory of evolution is the overarching unifying theory in biology. Are there biological facts that the theory of evolution does not need to explain? Or does the theory of evolution have to explain every fact about life? According to the theory of evolution every species and every individual of a species is the result of evolution. That would imply that every feature of an organism must somehow be explained by the theory of evolution. 

Aerodynamic forces on a bird (Videler, 2012)

Even at the time when I was a student and attended evolutionary biology courses and did lab experiments on evolution, I had the vague idea that the theory of evolution was incomplete, although at the time I couldn't say why, or what exactly was missing (here). There is so much to explain, and "A change in gene frequencies over time" and "Differential reproduction of heritable variation" seem far too simple for the task. Yes, there was a Second and Third Evolutionary Synthesis, but still a lot was missing. Obviously, the planetary, geological and cosmological context of evolution is often missing in the textbooks. Last year I blogged about the fact that evolution textbooks are suffocating: very low levels of Oxygen!  My list of top 20 evolution books (blog) reflects my preoccupation with the question of what is missing. Furthermore, physicist Rolie Barth pointed out physical factors and constraints in evolution (blog) [1]. At the other end of the spectrum: is evolution supposed to explain human behaviour, intelligence, consciousness, psychology, language, culture, civilization, war? What is the scope of the theory? Where does it end?

A few days ago I had an interesting discussion with google-AI about the question: Are there biological facts that the theory of evolution does not need to explain?  The initial answers seemed incorrect and incomplete. So, I demanded better answers and google-AI came with surprisingly insightful answers:

1. What evolution does NOT need to explain: The physics of how cells physically move, stick together, or split. For example, when a sheet of cells bends into a tube, it follows basic laws of fluid mechanics and surface tension.
2. What evolution DOES need to explain: Why the cells bend into a tube at that exact moment, in that specific location, to form a spinal cord. The timing, the instructions, and the genetic switches that trigger those physical movements are entirely shaped by natural selection.
3. What evolution does NOT explain: The electromagnetic forces that allow a hydrogen bond to form between Adenine and Thymine. That is pure physics and chemistry.
4. What evolution DOES explain: Why a specific three-letter sequence (like UUU) maps to a specific amino acid (like Phenylalanine), and why this map is nearly universal across all life.
5. Evolution does NOT explain: Why a hydrogen bond forms at all, why the universe is made of matter instead of antimatter, or the laws of thermodynamics that govern energy conversion.
6. Evolution DOES explain: How life hijacked those inescapable physical realities and organized them into the intricate, Mendelian, metabolic, developmental masterpiece we call biology.

Additionally:

• Lipid Bilayers: Amphiphilic lipid molecules automatically assemble into complex, spherical, semi-permeable cell membranes when exposed to water, driven entirely by hydrophobic physical forces.
• Membraneless Organelles: Inside the cell, proteins and RNA's undergo liquid-liquid phase separation—similar to droplets of oil separating in water. This thermodynamic behaviour spontaneously forms highly organized, functional compartments (such as the nucleolus) without needing an explicit genetic blueprint directing every coordinate.

These examples can be expanded with:

Microtubules: the self-assembly of microtubules. The proteins themselves are inherited, but they self-assemble into microtubules. 

Biochemistry: See also the primacy of biochemistry as opposed to genetic determinism [2].

Bio-electricity: "Without bioelectricity, your cells couldn't communicate, your muscles couldn't move, and your heart wouldn't beat." [3]:

• The Cellular Battery: Membrane Potential
• Nervous System Communication
• Muscle Contractions (Powering Movement and the Heart)
• Wound Healing and Tissue Regeneration
• Specialized Biological Superpowers (Electroreception, Electrogenesis)

Quantum effects: photosynthesis. 

Mechanical forces: Venus flytrap [4].

Van der Waals forces that hold molecules together (Gecko).

Hydrogen bonds (DNA bases).

Aerodynamics (birds, bats, insects), hydrodynamics (fishes, dolphins, sharks, blood flow).

Gravity: Effect on body size (scaling laws).

Geomagnetic field: is used by pigeons for navigation

Ultrasound navigation (bats)

etc. 

 

It seems very reasonable that evolutionary theory doesn't need to explain physical and chemical phenomena, because it is a biological theory. Physics and chemistry do explain physical and chemical phenomena. A biological theory can take them for granted, isn't it?

However, there is a problem. For a full scientific explanation of the evolution of life on earth, one cannot do without additional non-biological explanations. But organisms are undivided wholes. They can't be subdivided into biological, chemical and physical parts. So, must we conclude that the theory of evolution, as a biological theory, by definition is incomplete? [5]. Could it be that the theory only has to explain how biological things have evolved, and not all the details of how they work precisely? Would it be enough to demonstrate which genes and which mutations are involved in creating a specific trait and how it contributes to fitness? Does the theory need to include aerodynamics to explain the origin and evolution of flight? Finally, if new scientific discoveries are made on a daily basis, how can the theory of evolution ever be complete? I am afraid that the question of the scope and completeness of the theory of evolution is a rather complex and elusive question. Maybe, several quite different questions. Anyway, the question is too wide-ranging and too tricky to settle in one blog post. 

Notes

1. See also this blog where Rolie Barth explains that "Protein evolution can draw from a reservoir of a limited number of physically possible structures".
2. Nick Lane say about the primacy of biochemistry contra genetic determinism: Genes evolved simply to stabilize and optimize these already functioning biochemical pathways, rather than starting from scratch to build the cell. Also: The electrical origins of life!  
3. According to https://gemini.google.com 
4. Fast cell wall softening causes Venus flytrap closure, Science, 11 Jun 2026
5. Note: Could that be the cause of my gut feelings about the incompleteness of the theory of evolution half a century ago? 

Preliminary Conclusions

17-19 June 2026

• The Theory Of Evolution is a high level abstract paradigm that generates research strategies to investigate the details of structure and function of species.
• The Theory Of Evolution can be so compact because all the details are supplied by the biological subdisciplines, chemistry, physics and so on.
• Another reason why the Theory of Evolution is so compact is that it does not predict 'the brute facts of life': the existence of Kangaroos, Penguins, Anteaters, Hummingbirds, Bats, Octopuse, Sundew, Ocean sunfish, Platypus, Eukaryotes, Sex, Genetic Code, Split genes, the structure of DNA, etc. (historical contingency).
• The two hardest problems of The Theory Of Evolution are the origin of life and the origin of consciousness.
• Common descent predicts deep similarities of all life, and indeed this has been found on the genetic, biochemical, developmental and anatomical level, although the extent and specifics of similarities cannot be predicted in advance and often have surprised scientists.

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