The Science Behind Photoreception (Part 4 of 5); Superposition Compound Eyes
Now, now, here we are again, trying to understand the uniqueness of the photoreceptive system better. In the last three articles, we have learned a few things about photoreception:
- The mechanism of vision for each animal is different which provide them with a set of unique ability.
- The eyes can be categorised into two types which are single-chambered eyes and compound eyes.
- Pigment cup eyes, pinhole eyes, lens eyes, corneal eyes and concave-mirror eyes are all classified under single-chambered eyes system.
- Compound eyes can be divided into two types which is apposition and superposition eyes.
If you haven't read any of the article related to this series, you can click one of the links below, and you will be brought to the respective post.
Now its all come down to the last type of photoreceptive system which is the superposition compound eyes. Let's rock on!
Superposition Compound Eyes
This type of photoreceptive/optical system has a certain degree of similarity with the apposition eyes in terms of its superficial appearances. Anatomically, it has multiple facets which are arranged outside of a structure which is convex in shape. However, in terms of anything else other than appearance, these two type of eyes are very different. They have a few tweaks in the structure which would allow them to produce an image which has a superposition attribute. This particular tweaks would be explained later in this article. Superposition compound eyes can be found mostly in animals which are active at night or living in a dim environment. Some example would be moth and a few species of crustaceans which inhabits the midwater area in the oceans. As you can see, the similarity between the creatures which exhibit this kind of optical system is they were living in the dark and dim environment. If let say for the sake of an argument, the environmental brightness was to scale up; this optical system would undergo a series of changes which would transform it into apposition eyes. Quite cool isn't it?
The composition of superposition compound eyes is almost similar to the apposition compound optical system, but some mark structural (anatomical) differences would be:
- The existence of a clear, transparent and vast region located just below the optical components
- A retinal layer located in between the surface and the centre of curvature of an eye.
The image which was formed on apposition eyes is inverted relative to the actual object. In superposition eyes, however, a single erect image would be developed by superimposing all of the contributed rays from a specific number of facets available. In some way, we can say superposition compound eyes have a pretty similar function with a single-chambered eye system in that the number of images projected onto the retina is one and it is not inverted.
Optical Mechanism
Superposition eyes can be classified into three types, based on their optical mechanisms which will allow the formation of an image for vision.
- Lens-based (Refracting superposition)
- Mirror-based (Reflecting superposition)
- Combined Lens-Mirror (Parabolic superposition)
The refracting superposition optical mechanism was discovered back in the 1880s, by an Austrian physiologist named Sigmund Exner. He speculated that, for an eye to be able to produce a superposition image, some geometrical requirements need to be satisfied. When a light is shone onto the eyes, it will pass through an element at a specific angle relative to its axis and would emerge on the same side of the axis at a similar degree of angle. However, this mechanism contradicts with the normal behaviour of a lens; an image should be formed on the opposite side of the axis relative to the entering ray of light, not on the same side.
The only structure which is capable of creating the required path is a two-lens inverting telescope. The only catch is, the degree of optical power measured in the outer and inner refracting surfaces of the lens in a superposition eye is too low for them to be operated as a telescope for vision. There must be some sort of cylindrical structure which is equipped with gradients of refractive power to bend any passing light continuously inside the eye. This proposed concept would have the same structure of lens cylinder which is found in the horseshoe crab that aids in the refraction of light for vision. The arrangement of the lens cylinder will produce two equivalent lenses:
- A small image would be developed by the first lens halfway through the structure
- The second lens would convert the small image into a parallel beam
Throughout the process, the direction of the incoming light would reverse making the output parallel beam to emerge on the same side of the axis of the entering ray of light, thus producing an image. This satisfies the condition for creating a superposition image, and the idea was proved to be valid in 1970 with the utilisation of interference microscope which is capable of revealing the distribution of refractive index across a minute objects.
Some creatures such as a shrimp, crayfish, prawn and lobster don't have a lens in their optical system, but they were able to produce an image which complies with the superposition criteria. According to Exner's, without a lens or a lens cylinder, a superposition image should be unattainable. Scientists have also realized that the shape of the optical structures found in the aforementioned creatures is square rather than circular. It was made from jellies which has a low-refractive-index, and the functional mechanism of this eye cannot be determined. At least not until 1975. Klaus Vogt, a German neurobiologist, found that this square jelly optical structure is filled with multiple layers of reflective surface. There are a series of mirrors aligning to the surface of the eye at a right angle that could change the direction of incoming light by reflection instead of refraction which would result in the formation of an erect, superposition image. This mechanism was later known as "Reflecting Superposition" which use mirrors as its principal structure for the bending of light.
Parabolic superposition
In 1988, a Swedish zoologist named Dan-Eric Nilsson discovered a new optical mechanism used by a Macropipus crab. It used both of the lens and mirror as part of its mechanism to achieve visions. Just like the mechanism found in an apposition compound eyes, the lens in this mechanism would display an image at the clear zone while any incoming oblique light would be reflected by a parabolic mirror that is arranged along the crystalline cone just below the lens.
After reading through all of the 4 articles which featured various type of eyes of possibly every creature on the planet, we know that the design or mechanism of an eye of a specific creature will relate to their roles in the ecosystem. Prey would have an optic mechanism which would allow them to stay alert to their surroundings to avoid being predated. Predators would have some sort of vision partitioning, which would allow them to see clearly at a specific point on the field of vision in order to be attentive and not distracted by the surrounding. On the next article, a.k.a the last one for this series, we would be having a discussion regarding photoreceptors (structure and function) and the general adaptive mechanism of vision. Thank you for your attention.
Disclaimer: Some of the picture used in this article are taken from a blog which can be accessed through the source's link just beneath the picture. If you're the rightful owner of the picture and has some problems which its usage, do contact me and I will address the issue.
References and further reading materials
- Michael Land (1999, July 26). Encyclopaedia Britannica. Photoreception. Retrieved March 11, 2018, from https://www.britannica.com/science/photoreception
- What-when-how. Eyes and Vision (Insects). Retrieved March 11, 2018, from http://what-when-how.com/insects/eyes-and-vision-insects/
- Wikipedia. Compound eye. Retrieved March 11, 2018, from https://en.wikipedia.org/wiki/Compound_eye
- Lumen. Image Formation by Lenses. Retrieved March 11, 2018, from https://courses.lumenlearning.com/physics/chapter/25-6-image-formation-by-lenses/
- EverGreen. Flying insects. Retrieved March 11, 2018, from http://archives.evergreen.edu/webpages/curricular/2011-2012/m2o1112/web/flying_insects.html
You write
only you left out an important and vast type of creature on the planet: plants! While they don't exactly have eyes the way animals do, they do "see". Have you read:
Baluška F and Mancuso S, Trends Plant Sci. 2016 Sep, Vision in Plants via Plant-Specific Ocelli, https://www.ncbi.nlm.nih.gov/pubmed/27491517
I think that's a hypothetical opinion. The hypothesis was suggested by Francis Darwin in 1907. Even though some studies suggested that the plant might "see", but the mechanism which involved with this hypothetical sight is not well understood. Correct me if I'm wrong, there is not even a mechanism which explain how it works.
This is one of the definition of creature I found on the web. Is plant a creature? It's a living thing of course, but is it a creature? If it is then, it is my mistake
The next definition down from "creature is an animal" that I found states:
In more archaic definition, I found,
In both those cases, a plant is a being, and would therefore fit into the definition of a creature.
As for plants "seeing", it is no longer hypothetical. The definition of vision is,
And while only now is science discovering plant anatomy similar to animal eyes, the ability to use visual cues to understand the environment has been documented in studies by Mancuso and others such as Daniel Chamovitz, Ph.D., Dean, George S. Wise Faculty of Life Sciences and Director, Manna Center for Plant Biosciences. Both of them have written extensively about this in their books.
In fact, plants have all the same five senses we have, and then some. It is estimated that they have at least another 15 to experience the environment. Pretty neat, right?
Indeed they are but the notion of them having an eye is still hypothetical, therefore it is not included. Maybe one day, someone would expand on that theories and eventually, it will be recognised. This article explores photoreception, a mechanism of light detection that leads to vision and depends on specialized light-sensitive cells called photoreceptors, which are located in the eye. If you're saying the act of seeing in a plant is no longer hypothetical, it's fair enough. If you point to me to the direction which would explain the structure /the organ which made vision possible, I'll be happy to include it on my next article; provided the theories have been proved and accepted by the majority of the scientific community. Otherwise, it would be weird to include a half-baked theory in this series. Cheers.
You are correct in saying that plants don't exactly have eyes that are the same as most animals, though there are studies that suggest they have analogous structures:
Baluška F and Mancuso S, Trends Plant Sci. 2016 Sep, Vision in Plants via Plant-Specific Ocelli, https://www.ncbi.nlm.nih.gov/pubmed/27491517.
My first comment was in reference to your statement,
I was just pointing out that you have not included the eye-like mechanism for 99.7% of the creatures on the planet. Plants have photoreceptors.
Brillian Green, p. 42, Stefano Mancuso and Alessandra Viola, Giunti Press 2013
Brilliant Green, Stefano Mancuso and Alessandra Viola, Giunti Press 2013
If you don't know who Stefano Mancuso is, I suggest you look him up. He is considered an expert on Plant Neurobiology and Plant Intelligence. He runs the Laboratorio internazionale di neurobiologia vegetale (International Laboratory of Plant Neurobiology) out of the University of Florence in Italy.
In addition to Mancuso's work, there are a number of studies that talk about plant photoreceptors. For example, there is the phytochrome:
Phytochrome Signaling Mechanisms, Jigang Li,a,b Gang Li,b Haiyang Wang,b and Xing Wang Denga,b,1, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3268501/
For what it's worth, I am not negating any of your work, which I think is really fantastic. You have done an amazing job laying out animal vision.
I study under Mancuso, and it is normal that people leave out the plant world when making statements about the living beings of our planet. Funny how we focus so much energy on the less than 0.03%.
Hahaha. I love your spirit. I've never thought your comment as negating my works. It's fascinating how interested you are in plants. I mean, you can't be that resourceful unless you are passionate about something.
Glad to know the passion shines. You are right, I absolutely adore plants. Not in the traditional sense--I actually have very little house plants--I love them for everything they give us and for how different from animals they are. We talk about diversity, but most of the time we subconsciously look for the similarities. With plants, you have to look beyond.
Well, I do agree with you and thank you for your input. I guess will study your sources and eventually came up with an article about that.