How do bees see?

When a beekeeper looks at the head of his bees and sees the two large, immobile compound eyes positioned on either side of the head, as well as the three ocelli located on the forehead or vertex, he inevitably asks himself the question: with these two large eyes, can my bees see the same things as I do, or do they perceive the world differently? And why are there additional eyes on the head?

1. The visual system of bees is very different from ours

As nature does things well, one may ask for what reasons and in which situations the bee’s way of seeing its environment is advantageous or disadvantageous. Scientists say that no animal sees the same thing; there are as many forms of vision as there are types of eyes and species—more than a million. The diversity of eyes found in nature exists because the vision of each species is adapted to its way of life. Eyes are needed to flee, to hunt, to move, or to attract. The richer an animal’s interactions with its environment, the more complex its sensory organs are, and this is particularly true of vision, especially in bees.

Bees have a visual system that is very different from ours. They possess compound eyes, made up of multiple facets, each acting as an independent light receptor. On average, these eyes contain about 5,000 such facets, called ommatidia, between which fine hairs are found.

When an object is sufficiently close, the light rays that strike an ommatidium directly pass through the lens and stimulate the receptors located at the base of the organ.

The image of the object is then reconstructed in the brain from all these signals, like a mosaic.

left: human view; right: reconstructed bee view

Bees have five eyes, including two large compound eyes made up of several thousand hexagonal facets.
The number of facets (or ommatidia) per eye is:

4,000 to 6,000 for workers
3,000 to 4,000 for the queen
7,000 to 8,600 for drones

Each facet perceives light independently of its neighbours. The brain integrates the information received by each facet to form a mosaic image.

Compound eye, cross-sectional diagram of an ommatidium

Each ommatidium functions as an independent visual receptor that captures the portion of the visual field directly in front of it, but no image is formed there; instead, the image is reconstructed from the information transmitted by all facets. An object within the bee’s visual field emits rays in all directions and stimulates the eye as a whole, but only the ray aligned exactly with the axis of the rhabdomere (retinal rod) is recorded.

2. How does the bee see its environment?

Each ommatidium captures only a single point of light from the overall image. The combination of all these light points creates a grainy, canvas-like image, similar to the halftone patterns used in newspapers or magazines. We do not know exactly how the bee’s brain interprets these images, as it has one hundred times fewer visual neural connections than the human brain; this reduction is nevertheless essential given the insect’s small size.

The resolving power of the bee’s eye is lower than that of most vertebrates. It has been measured that the bee resolves only about one-sixtieth of what the human eye can see. This means that two distinct obstacles that a human can distinguish separately at a distance of 18 m would be distinctly perceived by the bee only from about 30 cm. The farther away an object is, the fewer facets perceive it and the harder it is to decipher. The complexity of the eye prevents high precision. If humans had a compound eye with similar capabilities, it would need to be about 1 m in diameter. Conversely, if the bee had a photographic eye like that of humans to achieve the same result as a compound eye, it would be heavier than the bee itself.

Distance perception is therefore one of the weaknesses of bee vision. The farther an object is, the fewer facets capture its radiation. Another limitation concerns shape perception. Because bee eyes are immobile, their rounded structure distorts the perception of objects around a central point that is more or less accurate.

These weaknesses are compensated by a very high sensitivity to movement. The sensitivity of the facets allows bees to perceive more than two hundred images per second, whereas humans discern only about twenty. This makes it easy to detect other insects, avoid predators, and locate flowers while flying.

In addition, bees have three ocelli located on the top of their head. These are simple eyes that do not form images but are sensitive to changes in light intensity.

This helps the insect when leaving the hive and stabilising its flight relative to the sky and the ground.

3. Does this mean that bees see less well than humans?

The answer is not straightforward, because bees see differently and we still do not know how their brains interpret images. However, the compound eye has a major advantage. When an object moves within the visual field, the ommatidia are activated or deactivated in turn. As a result of this cumulative effect, insects can much better assess whether an object is moving or stationary and react accordingly. For example, foraging bees have been observed to visit wind-swayed flowers more readily than motionless ones. Image decoding is more efficient with a compound eye than with the human eye, as it offers much greater speed of analysis and a higher frequency of sequential vision.

Changes in the visual field occurring at a frequency above 20 times per second are perceived by the human eye as a continuous image. In bees, the rate of sequential vision is well above 100 times per second. Thus, a Hollywood film would appear to a bee as a succession of still images and black frames. Sleight-of-hand tricks are easily deciphered by bees, as hand movements are slower than perception by their compound eyes.

The advantage of a compound eye for insects that forage or hunt is, for example, their ability to fly through dense forests without collision, capture other fast-flying insects, or escape predators. Compound eyes are therefore particularly well suited to detecting very slight changes in an image or movement within a very short time.

With regard to colour, bees have trichromatic vision. Each ommatidium contains nine receptors, four sensitive to green, two to blue and two to ultraviolet. Perception of red is limited.

4. Is the eye with which nature has endowed the bee best adapted to its way of life?

Based on the physiological characteristics of an organism, its way of life and its habitat, certain deductions may be attempted regarding which factors influence its life and survival. However, these do not necessarily correspond to reality. Nature’s successful strategy lies in developing ways of life with a high variability of traits: genetic variations or mutations from which specialisations arise that allow perfect adaptation to the environment.

All insects are equipped with compound eyes, yet they colonise extremely diverse habitats requiring a wide range of visual systems. There are diurnal and nocturnal flying insects, fast flyers and slow walkers. Despite these differences, the basic anatomy of their eyes is identical, although not always ideal. This is often the case for the multitude of insects of all kinds and sizes that populate our environment.

For bees, the most important aspect is to perceive moving objects in time—other bees, predators, and the ground passing beneath them as they fly at about 7 m per second.

Ultraviolet vision gives flowers and landscapes surprising colourations.

It was Karl von Frisch (1886–1982), Nobel Prize laureate in 1973, who demonstrated colour vision in bees through many experiments conducted over several years.

Sensitivity to ultraviolet light was an extraordinary discovery. Flowers that appear uniformly coloured to us look very different to bees. Ultraviolet light reveals lines converging from the petals toward the centre of the flower where nectar is located. Some time ago in Berlin, a new method was developed to measure floral spectra, allowing reconstructions using false colours (blue for ultraviolet, green for blue, red for green). The relative excitations of retinal cells are reproduced while taking into account the resolution capabilities of the bee’s eye at each point of the image. In this way, an adequate description and analysis of the plant’s signal is obtained, as well as a translation of the interaction between the flower as a signal emitter and the bee as a signal receiver. For example, an orchid that appears uniformly reddish to us is in fact coloured with a multitude of shades captured by the retinal cells.

The upper part is perceived by green-sensitive cells, the lower part by blue and ultraviolet cells. This makes it possible to understand how the bee deciphers a flower.

Partial reconstruction of a bee’s vision

5. Conclusion: What to know for beekeeping practice

It cannot really be said that bees see better than humans. They possess the advantages of a compound eye, namely greater speed of analysis and a higher frequency of sequential vision. For example, they can instantly analyse whether an object is moving or not. However, they also have major disadvantages, including shallow depth of field and therefore very low visual acuity.

We now know how bees perceive their environment and what this particular type of vision is used for. However, we are still unable to explain how the bee’s brain translates these images or what consequences result from this, as it has one hundred times fewer neural connections than ours, given the bee’s small size.

Bees distinguish three colours: blue, green and ultraviolet.
Other colours appear to them as black. Gradations of blue and green appear shifted in hue compared with our vision. For example, yellow appears to them as more or less pale green. Ultraviolet cannot be perceived by our retinas (once again highlighting the technical superiority of bees over humans).

Flowers illuminated by ultraviolet light reveal patterns that help guide bees to nectar. These nectar guides are sometimes visible to humans on flowers such as pansies. They also allow red flowers (e.g. poppies), which are less visible to bees, to compensate for this weakness in pollination.

Bees also “see” red.
Contrary to what is often written in beekeeping books or websites, it is incorrect to say that bees do not see red objects and that hives should therefore not be painted red. This also depends on reflectance (or absorbance) in the ultraviolet range, which completely escapes our vision. One cannot determine whether a red paint absorbs or reflects UV simply by relying on our own perception of red.
Bees do not see in the dark.
A small amount of light enters the hive through the entrance and allows some visual orientation, but inside the hive bees rely mainly on their other highly developed senses. Since bees cannot see in the dark, they do not leave the hive at night, except when hives are moved at night. In that case, they do not fly but walk along the outer walls of the hive. If they fall at night during transport, they cannot find their hive again.
The colour black displeases bees.
Black is a colour that bees do not distinguish precisely; many colours, including red, appear black to them. Ignorance may give rise to fear. A human dressed in black or black animals may frighten bees. A very old Slavic oral tradition reports that bee colonies have remembered that black bears are their most dangerous predators. A human dressed in black or black animals may remind them of the bear, triggering an innate defensive or even aggressive reflex.

White is also poorly distinguished by bees, as it appears as a bright glare. Faced with this somewhat blurred halo, the bee loses its bearings. This is probably one reason why beekeepers’ clothing is white—although white is also more comfortable to wear at the apiary in summer.