Honeybees Have Five Eyes
We are making this bee biology week, where we look at some of the fascinating and unique features of a bee’s body. Their biology is clearly quite different to that of the human body. After all, a honeybee is an insect!
As human beings with two eyes, we might be in awe of the fact that busy bees have 5 eyes.
Honeybees have two 'normal' eyes on the sides of their heads and 3 eyes on the top of their heads, capable of seeing ultraviolet (UV) light.
Their 2 normal eyes are large compound eyes, and their 3 small simple UV eyes are called ocelli.
Similar to other insects, a bee's compound eyes are composed of many individual units. Each unit creates a dot of color and brightness that contributes to the bee’s vision. The combined eyes produce a mosaic of dots similar to digital pixels.
A worker bee eye has about 5,000 of these units. A drone’s eye has about 10,000 units.
Each unit consists of a hexagonal lens. Some hexagonal lenses have a hair between it and the next lens over. There is an opaque area between lenses and each lens within the hexagon is transparent. The sheet of lenses is called the cornea. Each of these thousands of lenses focuses light from a small arc down to the cells beneath.
In this 6:46-minute video by Ian Stell, he gives a brilliant and detailed description of the eye of the bee:
The arrangement of the light filtering system beneath the lens is as follows:
LENS (external layer)
Each group of cells is identified as an OMMATIDIUM and is responsible for one specific colored spot of light.
Let’s follow a beam of light as it passes through the lens:
The crystalline cone is a transparent structure made of 4 cells that directs the light down to a long thin structure where the actual light-sensitive cells are found. The light passes down the narrow center known as the Rhabdom, between long thin cells called retinula cells. 8 retinula cells surround the rhabdom. There is a 9th cell at the bottom of the ommatidium.
The rhabdom has a complex structure. Each of the 8 retinula cells has microscopic projections that pass into the rhabdom, so the rhabdom consists of a mesh of these projections overlapping each other. These projections contain a light-sensitive chemical. When light hits this chemical, it stimulates an electrical pulse from the affected retinula cell.
This impulse is then transmitted by that particular retinula cell by a fine nerve that passes through the base membrane of the eye, into the optic lobe of the brain, first into the medulla and then by connections beyond that to lobula and beyond.
It is important that the retinula cells in each ommatidium are only activated by light coming in through the lens for that specific ommatidium and are not affected by light entering into neighboring ommatidium. To ensure that light does not spread from one unit to another, each ommatidium is surrounded by fine pigment granules in pigment cells. These can find the light to that specific ommatidium. Each individual ommatidium is sensitive to the full range of colors within the bee’s spectrum, which is focused on shorter wavelengths than the human eye. This is so that the bee can see ultraviolet light, which humans cannot see, but bees cannot see red, which humans do see.
These pigment cells give the bee's eyes such a distinctive color. The color vision of bees is produced by a combination of 3 specific color signals, the way an image shows in a digital photo. It is made by a blend of those 3 colors.
Of the 9 cells in each ommatidium, 3 are sensitive to light in the UV range, 2 to light in the blue range, and 4 to light in the green range. The combination of different signals from the retinula cells allows the bee to sense the color of light entering that ommatidium.
In total, the worker bee has about 10,000 ommatidium with both eyes. See their vision quality like a digital camera that produces coarser vision than the human eye. A human can see one hundred times the detail that a bee can see. The bee’s eye is superior to the human eye in that it can see movement that humans can’t see. Bees can see new images faster than 30 times per second. The bee’s eye sees 150 images or more per second in each ommatidium, so the bee is very sensitive to movement even with a lesser quality of clarity. Despite poor resolution, the bee’s eyes are adequate for navigating over several miles. She can avoid obstacles, find flowers, and then find her way home to the hive.
It is remarkable and must be noted that the bee's eye structure is hexagonal, just like the honeycomb cells that bees make are hexagonal, and serve as the underlying structure of their beehive. This is hardly a coincidence.
Enormous thanks to Ian Stell for his brilliant research and incredibly detailed study of the secrets of honeybee eyes and vision, and for sharing this information on his YouTube video, shown above.
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