Insect eyes are amazing structures, and they are built in different ways that allow different insects to see particular things or carry out certain actions.
Bees and hoverflies have two sets of eyes – compound eyes and simple eyes. The compound eyes are the big, noticeable eyes at the side of the insect’s head, and they’re made up of lots of tiny photoreceptor cells, which are in groups called ommatidia1. They are used by bees and hoverflies to see the environment they’re in, like colours and UV nectar guides on flowers, which guide insect to the nectar and are invisible to us. You may have noticed that honey bees, unlike other bees, have hairy compound eyes. These hairs are present to catch pollen when the bee is foraging so that they can collect as much pollen as possible2. The simple eyes, which are three little bumps in a triangle on top of a bee or hoverfly’s head and are also called ocelli, are made up of one lens and they detect light so that the insect can find the direction they want to go in, relative to the sun.
Butterflies, like bees and hoverflies, also have compound eyes, which are again made up of lots of tiny lenses and allow them to perceive different colours, including UV. This also allows them to see nectar guides on flowers. They have such good vision that they can see both polarised light and UV, and one butterfly, called Graphium sarpedon, has eyes that have a whopping fifteen forms of photoreceptor, whereas we only have two (called rods and cones)3,4. Butterflies also have two ocelli, unlike bees and hoverflies which have three, but they have the same role.
Dragonflies have huge compound eyes that meet at the top of their head, and these contain ommatidia5. Damselflies, which are closely related to dragonflies and belong to the order Odonata, also have ommatidia, but unlike dragonflies, their eyes are located at the sides of their head and do not meet. The difference in the positioning of their eyes is due to the way they hunt. Dragonflies approach their prey from underneath, so they need to have good vision near the top of their head, whereas damselflies approach prey directly in front of them, so they need to have good vision at the front of their eyes6. If you’ve ever watched dragonflies and damselflies hunting, you might have noticed the difference in their hunting techniques. Both damselflies and dragonflies also have three ocelli, like bees and hoverflies, and again, these have the same function, which is to detect light to orient themselves.
You might have seen some of these insects ‘looking’ at you before, as they can have a dark spot on their eye that follows your movement. This isn’t actually them looking at you (sadly), but it’s something called a pseudopupil. This is present because the ommatidia don’t reflect any light when something is in front of the insect and therefore they appear black, but the other regions of the eye do reflect light, so they appear lighter.
Next time you get up close to an insect, have a good look at their eyes, and see if you can spot any of these features. They’re great to photograph too if you’re into macro photography, and this can help you see them in more detail.
References
- The Amateur Entomologists’ Society. Ommatidia [Internet]; n.d. [cited 2020 Nov 17]. Available from: https://www.amentsoc.org/insects/glossary/terms/ommatidia
- Yi H. Honeybees are really hairy, so they can carry as much pollen as possible [Internet]; 2017 Apr 10 [cited 2020 Nov 17]. Available from: https://qz.com/954240/honeybees-are-really-hairy-so-they-can-carry-as-much-pollen-as-possible/
- Morrel V. This butterfly has extreme colour vision [Internet]; 2016 Mar 08 [cited 2020 Nov 16]. Available from: https://www.sciencemag.org/news/2016/03/butterfly-has-extreme-color-vision
- Kolb H. Facts and Figures Concerning the Human Retina [Internet]; 2005 May 01 [cited 2020 Nov 17]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK11556/
- Jones R. Why do dragon and damselflies have such big eyes? [Internet]; 2014 July 31 [cited 2020 Nov 17]. Available from: https://www.discoverwildlife.com/animal-facts/insects-invertebrates/why-do-dragon-and-damselflies-have-such-big-eyes/#:~:text=Their%20eyes%20are%20made%20up,in%20other%20smaller%2Deyed%20insects.
- University of Minnesota. Glimpse into ancient hunting strategies of dragonflies and damselflies [Internet]; 2016 Jan 16 [cited 2020 Nov 18]. Available from: https://www.sciencedaily.com/releases/2020/01/200116121845.htm#:~:text=in%20dragonflies%20with%20eyes%20that,at%20the%20same%20time)%3B