Sparkling camouflage

Jewel beetles are invisible thanks to gem-like wings

jewel beetle is invisible in vegetation

The green jewel beetle Sternocera aequisignata is protected against the gaze of predators not only by its colour, but also by its iridescent shine, Karin Kjernsmo and colleagues demonstrate.

dress embellished with wing cases of jewel beetlesJewel beetles were often to be seen in the ballrooms of Victorian England. That is, their wing cases, the hardened front wings; they were applied on expensive dresses as decoration and glittered like gems. With such dress, a lady could show up.

The beetle wings are so beautiful because they are shiny and iridescent, that is, their colour changes when they are illuminated or seen at different angles. The entire body of the beetle has that iridescent shine. It is an effect of the nanostructure of its exoskeleton, with multiple layers reflecting light. The wing cases of jewel beetles are durable and colourfast, and it is no wonder that they have often been used in jewellery and clothing; jewellery with jewel beetle wings is still being made today.

Surprisingly, jewel beetles don’t have their sparkling appearance to stand out, but to hide in plain sight, Karin Kjernsmo and colleagues prove.

Nail varnish

A well-known jewel beetle is the Asian emerald green Sternocera aequisignata. The beetles are on the menu of birds and should not catch the eye when they dwell in vegetation. Green is a well protective colour. The researchers wondered whether the iridescence makes the beetles still more difficult to detect.

They first wrapped dead mealworms with either a wing case of Sternocera aequisignata or a model. Five different models were used: pieces of resin shaped like a wing and varnished with green, blue, purple or black nail polish, and a high-gloss photo of a wing case that had the different colours, but not the iridescence of real wings. In a forest environment, they pinned the mealworms on plants. It was clear that birds found mealworms wrapped with real beetle wings less often than mealworms wrapped with one of four different wing models. So, wrapped with wing cases, the prey was more safe. Only the black models offered the same safety.

Glossy background

The next question was whether birds had greater difficulty detecting the iridescent wing cases, or whether they refrain from taking them. Human test subjects answered this question. They were asked to walk past plants on which wing cases and the five different models had been placed and to search for the objects. It turned out that the real wing cases were harder to detect than the models, again with the exception of the black ones. On a glossy background, such as a wet leaf, the real beetle wings did not stand out at all.

The conclusion is that the iridescent wing cases of jewel beetles that shine so brightly on ball gowns have a camouflaging effect on plants. Perhaps that explains why iridescent colours are common among insects. Just like the colour black.

Willy van Strien

Photos
Large: Sternocera aequisignata. Ian Jacobs (via Flickr, Creative Commons CC BY-NC 2.0)
Small: 19th century dress embellished with wing cases of jewel beetle. B (via Flickr, Creative Commons, CC BY-NC-SA 2.0)

Sources:
Kjernsmo, K., H.M. Whitney, N.E. Scott-Samuel, J.R. Hall, H. Knowles, L. Talas & I.C. Cuthill, 2020. Iridescence as camouflage. Current Biology, online January 23. Doi: 10.1016/j.cub.2019.12.013
Eluwawalage, D., 2015. Exotic fauna and flora: fashion trends in the nineteenth century. International Journal of Fashion Design, Technology and Education 8: 243-250. Doi: 10.1080/17543266.2015.1078848

Conspicuous, but also undetectable

Brightly coloured frog is invisible against background

Colourful poison dart frog is invisible from a distance

In spite of its bright colouration, predators have difficulty detecting the poison dart frog Dendrobates tinctorius from a distance, according to research by James Barnett and colleagues. The colourful animal turns out to have a cryptic colouration.

The bright colour patterns of poison dart frogs function as a warning signal to predators: don’t eat me, I’m poisonous. Natural enemies learn that they’d better leave these colourful prey alone.

Yet, the striking appearance of these frogs does not offer them complete protection, as James Barnett and colleagues point out. An inexperienced predator that doesn’t yet understand the message may attack and kill such frog. Moreover, some predators are insensitive to the poison, and others are so hungry that they ignore the warning and take the risk. So, a poison dart frog needs additional protection.

It has. Additional protection is provided by the same bright and distinctive colour pattern, which appears to function, surprisingly enough, as a cryptic colour that minimizes detectability. At least, this is the case in the poison dart frog Dendrobates tinctorius.

Camouflage

At close range, the frog clearly stands out against its natural background of leaf litter on the soil of rainforests in French Guiana, the researchers show, thanks to colours that are hardly found in that background: yellow and blue. The salient colouration is a clear signal.

But from a distance, things are different. Predators no longer are able to discern the pattern and the colours blend together to form an average hue that matches the background colour. So, the colouration turns out to function as distance-dependent camouflage; it makes the frog invisible to birds, snakes and mammals that are not at very close range.

Model frogs

That may be hard to believe, but experiments show that it really is like that. The researchers made frogs of plasticine (modelling clay) and gave their models either a natural colour pattern or painted it plain yellow or brown-and-black. They assembled different backgrounds: leaf litter, paper with a leaf litter print, bare soil and paper in a homogeneous colour. In the field, they put model frogs on different backgrounds and assessed how often wild avian predators attacked these models.

As expected, background had no effect on the number of attacks on yellow models, while brown-and-black animals were safer on leaf litter or paper with a leaf litter print than on other backgrounds.

And what about the frog models with a natural colouration?

Just like the brown-and-black animals, they had the best chance to remain undetected on a leaf litter background.

The bright colour pattern of the poison dart frog Dendrobates tinctorius thus has a dual function. At close range, it is a warning signal, while the colours blend to form a cryptic colour when viewed from a distance.

Willy van Strien

Photo: Dendrobates tinctorius. ©James B. Barnett

Source:
Barnett, J.B., C. Michalis, N.E. Scott-Samuel & I.C. Cuthill, 2018. Distance-dependent defensive coloration in the poison frog Dendrobates tinctorius, Dendrobatidae. PNAS, online June 4. Doi: 10.1073/pnas.1800826115

An uninvited guest

Frog breeds safely and undisturbed among leafcutter ants

Lithodytes lineatus breeds among leafcutter ants

Leafcutter ants ignore the frog Lithodytes lineatus when it breeds in their nests. They simply do not notice him, André Lima Barros and colleagues show, because the frog is chemically camouflaged.

Ants behave aggressively against intruders in their nests, but the South American Leptodactylid frog Lithodytes lineatus isn’t molested. In fact, he is at home in the huge colonies of leafcutter ants. Years ago, Andreas Schlüter reported that he had heard frog males calling from the interior of a leafcutter ant nest to attract females. Upon inspection of a nest, he found an adult frog within and numerous tadpoles swimming in a little pool. Obviously, the frogs breed in leafcutter nests.

Odours

It is clear why they willingly live there. Adult frogs, eggs and larvae are safe from predators, for the ants prevent these from entering the nest. Moreover, the nest has an agreeable humid microclimate.

But the question is why the ants, eager to evict all intruders from their nests, do tolerate these animals.

Now, André de Lima Barros and colleagues show that the frogs are chemically camouflaged. In their skin, they synthesize compounds which apparently imitate the odours with which the ants communicate. Since the ants rely on odour perception, the frogs go unnoticed: a good example of mimicry.

Not a burden

The researchers placed frogs of different species close to a nest entrance. When the experimental frog was a Lithodytes lineatus, the ants never attacked, but when it belonged to another species – either a species that is closely related to Lithodytes lineatus or a species that looks exactly the same as this frog – the ants became aggressive and bit the unwanted guest, that tried to escape quickly.

Next, the biologists prepared an extract from the skin of Lithodytes lineatus and coated a frog with it that normally would be chased away by the ants. Impregnated with skin extract of Lithodytes lineatus, the frog elicited no response.

So, Lithodytes lineatus can enter a leafcutter nest unharmed thanks to chemical camouflage. The uninvited guest is not a burden to the ants whatsoever, as he doesn’t touch the ants nor their brood. As he eats all sorts of other critters, such as assassin bugs and crickets, he may help the ants to keep the nest free of such enemies, in return for a safe place to breed.

Willy van Strien

Photo: Lithodytes lineatus, outside ant nest. Andrew Kay (via Flickr, Creative Commons CC BY-NC-SA 2.0)

Sources:
De Lima Barros, A., J. L. López-Lozano & A. P. Lima, 2016. The frog Lithodytes lineatus (Anura: Leptodactylidae) uses chemical recognition to live in colonies of leaf-cutting ants of the genus Atta (Hymenoptera: Formicidae). Behavioral Ecology and Sociobiolology, October 20 online. Doi: 10.1007 / s00265-016-2223-y
Schlüter, A., P. & K. Löttker Mebert, 2009. Use of an active nest of the leaf cutter ant Atta cephalotes (Hymenoptera: Formicidae) as a breeding site of Lithodytes lineatus (Anura: Leptodactylidae). Herpetology Notes 2: 101-105.