The coloured wings of wood tiger moths, seen with moth and bird eyes

Miriam J. Henze, Olle Lind, Johanna Mappes, Bibiana Rojas, Almut Kelber

Wood tiger moths* (Arctia plantaginis) may confuse you: while all have black-and-white forewings, males have yellow or white hindwings, while female hindwings are red or orange. This colouration makes the moths stand out on natural backgrounds – not only for our eyes. Birds also easily detect them, but as the moths taste bad (don’t try), birds learn their warning colouration, and soon learn not to attack them. However, this would be even easier if all moths had the same colour.

The different colours of the wood tiger moth. Upper row: a yellow and a white male. Lower row: an orange and a red female. Photo: A. Kelber
The different colours of the wood tiger moth. Upper row: a yellow and a white male. Lower row: an orange and a red female. Photo: A. Kelber

So here is the problem: Why does the species have so many colours?

To answer this question, we looked at the moths with their own eyes, and with the eyes of a bird that preys on them. We find that the moths have three types of light detector cells (photoreceptors), most sensitive to the ultraviolet, blue and green part of the light spectrum, similar to the well-known honeybee. We combined this knowledge with measurements of their wing colours in established colour vision models.

A wood tiger moth female resting on green vegetation. We combined measurements of their wing colours (graphs with grey shading show fore- and hindwing colours of a red female) and of the sensitivity of light-sensitive cells in their eyes (UV, Blue and Green) in models of visual detection and discrimination. Photo: B. Rojas
A wood tiger moth female resting on green vegetation. We combined measurements of their wing colours (graphs with grey shading show fore- and hindwing colours of a red female) and of the sensitivity of light-sensitive cells in their eyes (UV, Blue and Green) in models of visual detection and discrimination. Photo: B. Rojas

This tells us that the moths themselves see the yellow and white males as having clearly different colours, mostly because the white males also reflect a lot of ultraviolet light. But red and orange females do not appear to them as distinctly coloured. All colours can be easily discriminated from green and brown backgrounds.  Birds, as expected, can detect all these colours without difficulty.

We conclude that females may use the white and yellow colour of males for mate choice, while males cannot reliably discriminate females by colour. More work is needed to better understand the complex relationship between the moths’ colouration and the moths’ and their predators’ vision and learning.

Generally, a better understanding of colour vision helps us to understand the colouration of animals.

Read the paper here.

* The wood tiger moth, Arctia plantaginis, like many moths, has its name from its larval stage: the caterpillar is orange and black like a tiger. Arctia means bear, because the caterpillar is also hairy, and plantaginis indicates that the caterpillars eat leaves of plantain (in Latin, Plantago). The adult moth does not feed. Males spend their short life finding a female, and females, after mating, lay eggs and die.

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