Why Do Most Living Things Only Have Two Eyes

In our impression, it seems that only ghosts and gods have multiple eyes, such as Erlang Shen in traditional Chinese mythology:

Erlang Shen has 3 eyes. Image source: kuaibao

Or Argus Panoptes, a giant with one hundred eyes in Greek mythology:

The hundred-eyed giant Argos Panoptes. Image source: Pinterest

All humans and other animals (whether cute and tame rabbits or wild cheetahs) are two-eyed. Therefore, there seems to be nothing wrong to conclude that “most animals have two eyes”.

However, are there really no animals with multiple eyes?

Is the Prototype of the Hundred-eyed Giant Scallop?

Perhaps you will say: I know this one. Scallops have a pile of eyes …

Scallop can have up to 200 eyes. Image source smithsonianmag

Yes, scallops have a lot of tiny eyes (about 1 mm in diameter), which are lined up along the edge of their shells. Compared with ours, the eyes of scallops are not only large in quantity, but also different in structure. They rely on a concave mirror composed of guanine crystal to focus and reflect light, while human eyes use a convex lens pattern.

Insects with Both Compound Eyes and Ocelli

Apart from scallops, most insects, which are enormous in quantity, also have multiple eyes. You must have thought I was talking about the compound eyes of insects. For instance, the compound eyes of dragonflies are just made up of tens of thousands of individual ommatidia.

The compound eyes of dragonfly. See watermark for image source

Even so, compound eyes are not unique to insects. They are ocular organs of arthropods, which can feel the shape, size and color of objects. Many crustaceans have compound eyes as well, such as shrimps and crabs.

The compound eyes of crustaceans. Dynamic picture from: Awesome Animals

Although compound eyes have poor image resolution, they provide animals with a wide angle of view, and the ability to recognize fast-moving objects. Thus, it is always difficult to photograph flies.

By saying insects have multiple eyes, we don’t simply refer to their compound eyes consisting of multiple hexagonal ommatidia. After all, the ommatidia that make up compound eyes don’t seem to function alone. What we want to introduce here is ocelli of insects.

Er... do insects have ocelli apart from compound eyes?

Yes, the ocelli of insects can be divided into dorsal ocelli and stemmata. The dorsal ocelli are generally located between two compound eyes of insects, and have a simple structure. The main function is to assist the compound eyes to judge visually. As a general rule, insects have 2-3 dorsal ocelli.

Three dorsal ocelli of wasps. Image source: Wikipedia

Three dorsal ocelli on the head of hornet. Image source: Wikipedia

Dorsal ocelli of different insects. Image source: Researchgate

After talking about dorsal ocelli, let’s move on to stemmata. Stemmata generally refer to the ocelli of fully metamorphosed insect larvae. There are typically 1-7 pairs of stemmata. Stemmata have part of the functions of compound eyes. They can recognize the trajectory and shape of objects, but have poor vision.

The stemmata of caterpillar. Image source: Insect Physiology

The stemmata of lepidoptera larvae. Image source: Wikipedia

Spiders that Pretend to be Insects

Spiders, often mistaken for insects (they belong to the class Arachnoidea, phylum Arthropoda, not the class Insecta), also have lots of eyes.

Salticidae with several pairs of eyes. Image source: Wikipedia

Most spiders have eight eyes (all ocelli, no compound eyes). These eyes have a strict division of labor. The largest two eyes in the middle can get a clear color images and judge distance, while the other pairs of eyes can capture information in regards to the movement of objects.

From a certain angle, the spider’s eyes glistens metallically. Image source: ThoughtCo

Different spiders have different pairs of eyes in different sequences.

The unique position of eyes of this kind of spider renders it a good field of vision. Image source: ThoughtCo

There are also spiders with all of their 8 eyes on the obverse side. Image source: ThoughtCo

There are also spiders whose eyes are not so big. Image source: Infinite Spider

8 eyes of tarantula. Image source: Infinite Spider

Leucauge venusta, the arrangement of its eyes is unique. Image source: Infinite Spider

Even the Fish Join in the Fun

All of the animals mentioned above are invertebrates, which are evolutionarily distant from the vertebrates we are familiar with. However, there is a kind of fish that has evolved a structure that resembles multiple eyes.

As you might have guessed, it is the high-profile Anableps anableps. They actually have only two eyes, but possess the magical function of four eyes.

Anableps anableps. Image source: Wikipedia

How does this happen? It turns out that the eyes of Anableps anableps are on the top of their head, and divided into two parts: dorsal and ventral eyes. There is a distinct demarcation line in the middle. Each eye has two pupils and two corneas for the purpose of observing the conditions abovewater and underwater respectively, which highly matches the special habit of living on the water surface of Anableps anableps. 

The special eye structure of Anableps anableps. Image source: Awesome Animals

You May Have a Wrong Idea about “Most”

After reading these examples, you may say yep, there are indeed some animals that don’t have two eyes, but all common animals are two-eyed. It’s okay to say “most animals have two eyes”, isn’t it? 

This is a common myth. When we say “most” animals, we often refer to vertebrates, mainly including fish, amphibians, reptiles, birds and mammals. Almost all of these animals have two eyes (with the exception of Anableps anableps), but they cannot be considered as “most”.

As a matter of fact, the phylum Arthropoda in invertebrates is a phylum that has the largest number of species in the animal kingdom. In the class Insecta alone, more than 1 million species have been discovered. Apart from insects, common shrimps, crabs and spiders are also typical representatives of the phylum Arthropoda, and many of these animals are not two-eyed.

The phylum Chordata, in contrast, has approximately 66,000 species. The difference in quantity is so big that perhaps we should say “most animals are not two-eyed.”