Have spiders eyes
Sensory performance - seeing or why do spiders have so many eyes?
Last update: 10/27/2019
Most spiders have eight eyes, a few have six, and some cave species have no eyes at all. However, few spiders have good eyesight.
The arrangement of the spider eyes
The first thing that strikes you when you are dealing with spiders and looking them in the eye is that the individual families differ not only in the number but also in the position and size of the eyes.
The eyes are arranged in a characteristic way and can also be used to determine the family membership of a spider species.
As a rule, the spider's eyes are arranged in two rows, at least if the rows can also be thought of as curved. If you now draw a horizontal line between the two rows of eyes, one speaks of the eyes below the line of the front and of the eyes above the line of the rear row of eyes.
If you then differentiate between central and side eyes, you have the designation for the spider eyes together, i.e. front central eyes, front side eyes, etc.
The size of the spider's eyes is very different between the individual species and can range from very small to spotlight-like dimensions.
The spider spiders (Philodromidae) and the crab spiders (Thomisidae), for example, have quite small eyes. Significantly larger dimensions take at least the rear median eyes in the wolf spiders. In the jumping spiders (Salticidae) the anterior median eyes have the dimensions of small headlights. In our domestic latitudes we have already reached the end. However, this is topped by the landing net spider (Deinopis spinosa). This nocturnal spider has developed a special form of hunting. She hangs upside down just above the ground and holds a net between her legs. If a prey crawls under her, she throws out the net at lightning speed and wraps the prey in it. Good night vision is a prerequisite for this type of hunt.
Why do spiders have eight eyes?
An interesting question, but one has to say: it is not that easy to answer. Evolution does not take place purposefully according to a plan, rather it is so that each individual spider is exposed to biotic and abiotic factors. Certain properties under given environmental conditions then represent a fitness advantage. The individual survives, can reproduce and thus brings his genes into the gene pool.
In order to get closer to the answer, where the fitness advantage of many eyes lies, one has to examine their function.
First of all, let's take a look at the functions of the eyes and what visual performance they produce. The world of spiders is rich in shapes. The eyesight of the different spider families is different, so general statements are always a bit problematic. But in order not to get lost in the details, we do it anyway.
Most spider species have poor eyesight. They tend to use their senses to smell, hear or taste in order to find their way around their environment. Of course, this does not mean that the eyes are functionless, so they can distinguish between certain light and dark stimuli. For example, weaving spiders are prompted to rebuild their web towards evening. Fast movements can also be perceived in order to get to safety from predators in good time.
Nonetheless, there are also spiders for which the sense of sight plays a more important role. These are, for example, the wolf spiders or the jumping spiders.
Major and secondary eyes
An important distinction between spider eyes is that of main and secondary eyes. The distinction is made because the structure of the main eyes differs significantly from the structure of the secondary eyes.
The main eyes are always the front median eyes. They are forward facing and posable. More precisely, the retina is mobile because it can be moved laterally by muscles. (1) Due to this ability, the field of vision of the spiders is expanded considerably.
Digression - spinning with six eyesSpiders with only six eyes (Dysderidae, Oonopidae, Sicariidae) lack the anterior median eyes. The Dysderidae, for example, includes the rather impressive Dysdera crocata (2). If you are lucky, you will meet the animal in your own cellar. It is unmistakable: large, bright red front body, strong legs, yellowish gray rear body, powerful jaw claws. The Sicariidae also includes the genus Loxosceles, which includes both Loxosceles rufescens (3), which occurs in Europe, and the brown recluse spider (Loxosceles reclusa), which occurs in the USA and whose bite can have unpleasant consequences (4).
Most spiders' secondary eyes have what is known as a tapetum, while the main eyes do not. Jumping spiders and lynx spiders have no tapetum (5).
"The incident light passes the retina, is reflected on this layer [the tapetum, Marko Leson] and passes the retina a second time". The tapetum is therefore a reflective layer within the eye that increases the light output in the twilight (6).
Spider eyes glow in the dark
Since the main eyes do not have a tapetum, they appear black. The secondary eyes, however, can shine back like headlights if they are illuminated in the dark. This can be seen well with wolf spiders. On the one hand because they have comparatively large eyes and on the other hand because they have a pronounced tapetum in the secondary eyes. So if you go looking for spiders with a flashlight at night, with a bit of luck you may suddenly be surrounded by a multitude of small, luminous dots. All spider eyes that look in our direction ... At (7) and (8) you can find interesting reports about glowing spider eyes.
The function of the spider eyes
At the beginning we already looked at the arrangement of the spider eyes. The field of vision of the individual eyes overlaps. Since a spider does not have a movable head, but only a so-called head-chest piece (cephalotorax) on which the eyes sit, it can get an almost all-round view through the top and side arrangement of the 3 pairs of secondary eyes.
Especially with jumping spiders you can see how secondary eyes and main eyes work together. With a little experiment like you can see in the video you can simulate that.
If you find a jumping spider, you can attract its attention by moving a little distance above its head. It won't take long before she aligns her entire body with the movement and looks with her large eyes exactly in the direction in which the movement was noticed.
The function of the eyes in hunting the jumping spider
When hunting the jumping spiders, the functions of the individual eyes mesh neatly like clockwork. First, the posterior lateral eyes perceive a movement. The spider orients itself in the direction of the prey and fixes the object with its front median eyes. These act like a telephoto lens and deliver a more detailed, possibly even colored image and enable the jumping spiders to recognize what it is about from a distance that is not too great. As soon as the jumping spider knows that it is a prey, it sneaks up and the front side eyes come into play. They are used to estimate the distance to the prey. As soon as the prey is within jumping distance, the jumping spider repels.
Polarized light and the Apulian tarantula (Lycosa tarantula)
The secondary eyes - and also the main eyes - of many spiders can detect polarized light. Wolf spiders and flat-bellied spiders, for example, are able to do this and use it for orientation (9).
Many wolf spiders are different than the name suggests ambulance hunters. The Apulian tarantula builds an earth tube lined with spider silk with an above-ground part. Here she is waiting for prey. As soon as a suitable insect comes near the hiding place, the spider storms out, chases the insect if necessary and seizes it. Then she withdraws directly to her hiding place. What seems so banal is not so easy at all.
If we as humans walk all over the place, arrive at our destination, turn around and see the starting point again, then returning there is no problem. But what if we no longer see the starting point? How do we find our way back then? The tarantula also faces this problem if it has pursued its prey over a few centimeters.
How does she do it? The key word is path integration. In simplified terms, this means that it is able to find its way back to A directly from E despite several deviations from a straight line.
Excursus - path integration
The tarantula's compass is polarized light. Your front median eyes take over the perception of this light. But that alone is not enough. Only in combination with a "distance measurement" does the spider calculate the direct route to the hiding place. In order to find out something about the relevance of the other pairs of eyes, scientists darkened the spider's front side eyes in an experiment. They found that it was no longer possible for the animal to find its way back to the hiding place. They concluded from this that the darkened eyes apparently perceived the things that are necessary so that they can orient themselves in space (11) (12) or carry out the distance measurement.
Conclusion: So why do spiders have so many eyes?
But back to the original question why spiders have so many eyes.
Ecological fitness once again plays the main role: What promotes survival and what promotes the number of one's own offspring?
We can now figure it out a little bit by looking at the various descriptions. Since spiders do not have a movable head but have a large number of predators, the individuals have an evolutionary advantage who have a wide field of vision. So if the arrangement of the eyes allows a panoramic view. For this it can be enough to perceive shadowy movements in order to be able to escape in time if necessary.
The more detailed vision of the main eyes, including the secondary eyes, would not be necessary for this and would not bring any fitness advantage. In fact, the opposite might actually be the case: it would be a waste of energy to have a trait that does not add value to survival or reproduction. For example, it may be less energy-intensive to flee as a precaution than to develop eyes that can see sharply in all directions at 20-30 cm or even further away.
If the spider aligns itself with the moving object, for example to judge whether it is a possible prey, then a pure schema would only be no disadvantage if it were compensated for by other skills. This may be sufficient for ambulance hunters who wait until the prey runs into their arms, but it is hardly an option for jumping spiders. In order to jump on the prey, the distance must be estimated accurately and the prey must be recognized as such from a distance.
Seeing in more detail helps here.
Another important principle in evolution is that of energetic limitation. Energy is not unlimitedly available to the organism. The development of two large super eyes that can measure distance, see in detail and receive polarized light was perhaps associated with excessive energetic costs and several pairs of eyes that perform different tasks rather brought the corresponding fitness advantage.
So evolution is not purposeful. Why exactly 8 eyes have developed with exactly the skills they have cannot ultimately be answered. Other "solutions" could certainly have evolved, examples can be found in the realm of insects. The function, however, can be explained with a lot of research and experiments and from this in turn one can try to derive the fitness advantage.
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