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Look into my ocelli! Simple eyes.

Ocelli (singular: ocellus) are simple eyes and are found on many different kinds of insects (such as bees, wasps, flies and dragonflies) and spiders. In adult insects, like the beautiful Jewel wasp shown below, photographed by Johan J.Ingles-Le Nobel, there are usually three dorsal ocelli located on the top of the head: two small lateral ocelli and a larger middle one called the median ocellus.

Jewel wasp (Hedychrum confusum). Photograph by Johan J. Ingles-Le Nobel. Flickr Creative Commons

They are most pronounced on flying insects, but they have an ancient lineage and are also found on more ‘primitive’ insects, like collembolla (springtails) which can have six to eight ocelli (see below).

Cluster of ocelli Sminthuroid springtail Photo by D E Walter

The triplet of photoreceptors on the heads of adult insects each have a lens and are used as wide-field, light detectors; but they are generally thought to be incapable of resolving images. In other words, although they have a lens which collect light, and projects it onto a layer of light-sensitive cells, the image is ‘under-focused’ and lacks detail (Krapp, 2009). Ocelli are more like  a photometer – providing the insect with information on light levels – than a camera. They are very useful for telling the insect which way is up (the bright sky) and which way is down (the darker ground). Detailed resolution of images is left to the compound eyes.

The common darter (Sympetrum striolatum) juv male. Lateral ocellus marked. Photograph by Raymond JC Cannon.

In flying insects like flies, bees and dragonflies, the ocelli help the insect to ‘maintain its orientation in space; to stabilize its flight; and to keep its gaze level’ (Krapp, 2009).

Unidentified dragonfly hovering in flight. Thailand. Photograph by Raymond JC Cannon.

Dragonflies are a bit of a special case, in that the lens of the median ocellus is ‘unusually thick and that its vertical curvature is sufficiently strong to form a focused image on the retinal receptors’ (Van Kleef et al. 2005; Strange et al., 2002).

Dragonfly ocelli by André Karwath. André Karwath aka Aka [CC BY-SA (https://creativecommons.org/licenses/by-sa/2.5)%5D
In contrast to the median ocellus of the dragonfly, relatively little is apparently known about its lateral ocelli, but amazingly, if you stimulate them, the dragonfly rolls its head about (!), i.e. from left to right (Berry et al., 2007). In contrast, a light-stimulus applied to the median ocellus causes the dragonfly to rotate its head forwards, i.e. around the pitch axis (Berry et al., 2007).

The red-veined darter or nomad (Sympetrum fonscolombii) mature male. White arrow shows the Vertex; black arrow points at the lateral ocellus. Photograph by Raymond JC Cannon

All three ocelli are visible in the following photograph (below), with the curved median ocellus marked by the arrow.

Scarlet Grenadier (Lathrecista asiatica) female with median ocellus marked by white arrow. Photograph by Raymond JC Cannon

As always, there is a lot more to this story than I have covered in this blog. Flying insects control their bodies whilst flying by evaluating a wide range of sensory inputs, including: compound eyes, ocelli, antennae, and even wind-sensitive hairs. They have had many millions of years to perfect their control systems. One advantage they get from simple eyes, is to be able to detect very fast movements, and because unlike the compound eyes, the information does not have to pass through, and be processed by the brain, they can respond very quickly: “the neurones in the median ocellus go from photoreceptor to the motor centre and they’re able to do the calculation in one step” according to Josh van Kleef.

References

Berry, R., Van Kleef, J., & Stange, G. (2007). The mapping of visual space by dragonfly lateral ocelli. Journal of Comparative Physiology A193(5), 495-513.

Berry, R., Stange, G., Olberg, R., & Van Kleef, J. (2006). The mapping of visual space by identified large second-order neurons in the dragonfly median ocellus. Journal of Comparative Physiology A192(10), 1105-1123.

Berry, R. P., Stange, G., & Warrant, E. J. (2007). Form vision in the insect dorsal ocelli: an anatomical and optical analysis of the dragonfly median ocellus. Vision research47(10), 1394-1409.

Krapp, H. G. (2009). Ocelli. Current Biology19(11), R435-R437.

Ribi, W., & Zeil, J. (2018). Diversity and common themes in the organization of ocelli in Hymenoptera, Odonata and Diptera. Journal of Comparative Physiology A204(5), 505-517.

Stange, G., S. Stowe, J.S. Chahl, and A. Massro. 2002. Anisotropic imaging in the dragonfly median ocellus: a matched filter for horizon detection. J. Comp. Physiol. 188:455–467.

Van Kleef, J., James, A. C., & Stange, G. (2005). A spatiotemporal white noise analysis of photoreceptor responses to UV and green light in the dragonfly median ocellus. The Journal of general physiology126(5), 481-497.

 

rcannon992 View All

I am an entomologist with a background in quarantine pests and invasive invertebrates. I studied zoology at Imperial College (University of London) and did a PhD on the population dynamics of a cereal aphid (Metopolophium dirhodum) in the UK. I spent 5 years with the British Antarctic Survey studing cold hardiness of Antarctic invertebates and 17 years with the Food and Environment Research Agency. My main interests now are natural history, photography, painting and bird watching.

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