Ocelli Sense Length of Daylight

Biological Strategy

Insects

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Sense Time and Day Length

Being aware of time and day length is important for living systems to appropriately time such activities such as flowering or releasing seeds, migrating, and reproducing. When these activities are done at the wrong time it wastes energy, and resources, and may even jeopardize survival. To determine time, living systems sense light signals. Processing these signals goes beyond simply sensing light; it entails determining whether there is enough light (in quantity or duration) to support a given activity. The rockcress plant, for example, uses light as an environmental cue to time flower development. Delaying blooming protects its reproductive parts from freezing or from blooming before enough pollinators are active.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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The ocelli of insects sense day length via a small lens and pigmented retinal cells.

Image: Roger Smith /

This shows a wasp head with two of the three ocelli showing up between the back parts of the compound eyes. The ocelli are shiny black.

Image: Janice Carr / Centers for Disease Control and Prevention /

Image: Janice Carr (photo credit) and Connie Flowers, Pamela Munn (content provider) / Centers for Disease Control and Prevention /

“Each ocellus usually consists of a small lens backed up by several pigmented retinal cells, which can determine the quality and source of light and usually perceive something moving nearby. Ocelli usually look like small dark dots, and are often grouped in a triangle on the back of an insect’s head. They enable the insect to judge the length of daylight, for example, by which it may regulate its whole life cycle. Spiders’ eyes form extremely good images and have, for their size, excellent resolution.” (Foy and Oxford Scientific Films 1982:122)

Last Updated August 18, 2016

References

Book

The grand design: Form and colour in animals

BLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, London | 26/02/1983 | Sally Foy

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