As a part of the A-Z Photographic Glossary of Biological Terms challenge, today it is P for Photoreceptor as illustrated by this male Eastern Carpenter Bee (Xylocopa virginica) from Maryland in the US.
“I’ve got my eyes, all five of them, on you!” This bee does indeed have five eyes: three ocelli (simple or single lens) which provide information on light intensity; and two compound eyes. Compound eyes are the principle visual organs of most insects; they are found in nearly all adults and in many immatures. As the name suggests, compound eyes are composed of many similar, closely-packed facets called ommatidia which are the structural and functional units of vision. The number of ommatidia varies considerably from species to species: some worker ants have fewer than six while some dragonflies may have more than 25,000. Each omatidium contains a photoreceptor cell which refracts incoming light down onto visual pigment which in turn absorb certain wavelengths of light and generate nerve impulses through a photochemical process similar to that of vertebrates and within a few milliseconds, sends it’s view of the image to the brain of the bee.
The brain combines all the images of these thousands of photoreceptor cells and forms a detailed picture of it’s subject in a sort of mosaic composition. Since insects cannot form a true (i.e. focused) image of the environment, their visual acuity is relatively poor compared to that of vertebrates but with each of these thousands of photoreceptor cells sending images to the brain, their ability to sense movement, by tracking objects from ommatidium to ommatidium, is superior to most other animals.
For more detailed information of photoreceptors in insects, take a deep breath and then jump into this article from North Carolina State University.