Bees have two distinct sets of eyes, each with their own function. They have two compound eyes on the sides of their heads, and three simple eyes called ocelli on the tops of their heads. The ocelli are simple photo-receptors (light detecting organs) consisting of a single lens and several sensory cells. Unlike compound eyes,ocellido not form a complex image of the environment but are used to judge light-intensity, detect movement, maintain stability and to navigate. As will be discussed in this post, bees can see two types of light that humans cannot, polarised light and Ultra-Violet (UV) light. Polarised lightwaves arelightwaves in which the vibrations occur in a single plane and being able to see this mean that bees can always detect where the sun is, even when it is cloudy. They are able to use this polarised light as a navigating system and can then communicate these directions to the colony through the ‘waggle dance’.
Like all bees, Honey Bees are thought to have extremely sharp vision due to their compound eyes, with queen bees having around 4,000ommatidia (or facets) in each of their compound eyes with eachommatidium containing a cluster of photoreceptor cells; a worker bee having about 7,000 ommatidia and a drone having about 8,500. This higher number is because the drone needs to be able to find the queen be during her mating flight - although his antennae also have about 30,000 receptors that help him to detect the queen’s pheromones (compared to the 1,600 that the queen herself has, and the 3,000 of the worker bee).
A bee’s-eye view of the world is quite different to our, not only because of their size and because they can fly, but because they can also see a different part of the light spectrum than we can. They quite literally see things in a different light!
Human eyes are ‘trichromatic’ meaning that we have three photoreceptors in our eyes, receptive to red, green and blue and can see colour combinations of these three. this means that our visible range is red, orange, yellow, green, blue, indigo and violet. We cannot see either the infrared end beyond visible red, or the ultraviolet end beyond violet.
Bees are also thought to be trichromatic, but that bees cannot see red because they don’t have a photoreceptor for it. However, they can see the ultraviolet (UV) end of the light spectrum and so so they make colour combinations from blue, green and ultraviolet.
Flowers can therefore look very different to bees, and many have indications on their petals, known as nectar guides, that direct the bees towards the centre of the flower where their reward, either nectar or pollen, is concentrated . These nectar ‘bulls-eyes’ are often only visible to creatures such as bees, as many flowers such as sunflowers, primroses and pansies have nectar guides that can only be seen in ultra-violet light.
For example, the picture above shows what an Evening Primrose would look like to us, and how it looks usingUV-Induced Visible Fluorescence Photography (UVIvFP) indicating how a bee might see it - although bees cannot see red, so what they may see is black indicators instead or like this UVIVFP photograph of an Evening Primrose from Craig Burrows:
Whilst bees can sense polarised light in a way that humans cannot, it is also thought that they can sense the electrostatic field in flowers in a way that we cannot. As bees fly they develop a positive electrostatic charge, whilst flowers are slightly negatively charged, allowing pollen to stick to them better, and a lack of this charge can indicate to them that another bee has already consumed the nectar and pollen.
In this way these UV nectar guides are essential in ensuring that bees are able to find the pollen and nectar that they require for food, and in turn continue to spread the pollen that ensures the continued survival of the plants in a mutually beneficial cycle.
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