Colorful display of newly described stick insects confounds scientists

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  • Most stick insect species blend into their surroundings to avoid predators.
  • But the males of two newly described species from madagascar, Achrioptera manga and Achrioptera maroloko, are brightly colored.
  • Some scientists believe this allows them to attract females, even at the risk of being spotted by predators.
  • Their distinctive hues make them potential flagship species for the biodiversity-rich regions where they were discovered: the forests of Montagne des Français and Orangea.

If in the midst of a forest you chance upon a crawling twig, you may be in the presence of a stick insect. This group of insects, which evolved specifically to blend into their surroundings, this month yielded two new additions from Madagascar with a key difference: the new species exhibit a striking array of colors that make them stand out.

“Nearly all of the 3000+ known species of stick insects try to be inconspicuous and just look like twigs,” Sven Bradler of the University of Göttingen, Germany, who co-authored a recent paper on the discovery, said in a statement. “There are a very few, very large exceptions — and we have just discovered a couple more of them.”

For Frank Glaw, an authority on Madagascar’s reptilian population at the Bavarian State Collection of Zoology and lead author of the paper, the discovery was a long time in the making. Fifteen years ago, Glaw and a team of surveyors chanced upon two brightly colored giant stick insects during a herpetological survey in the forests of Montagne des Français and Orangea in north Madagascar.

“Beside several new species of amphibians and reptiles and some rediscoveries of reptile species not seen for around 100 years, we also discovered two giant stick insect species, which were extremely impressive and colorful, and this triggered my interest in them,” Glaw told Mongabay.

These specimens differ from known stick insect species in morphology and coloration, but it was dissimilarities in their DNA barcodes — segments of the DNA that are matched with reference databases — that helped clinch their designation as new species in the genus Achrioptera. The team named the newly described species A. manga and A. maroloko. Manga means “blue” in Malagsy while maroloko is the word for “colorful.”.

Achrioptera maroloko, one of two new Madagascan stick insect species discovered by Frank Glaw, Sven Bradler and colleagues. Maroloko means “colorful” in the Malagasy language. Image by Frank Glaw.

Stick insects belong to the Phasmatodea order of insects, and the new species fall in the Achriopterini family, whose members are large, richly colored and endemic to Madagascar and the Comoros archipelago. There is one other genus in the Achriopterini family, Glawiana, and it has only one known species: Glawiana glawi — named after Frank Glaw.

Found mostly in the tropics and subtropics, stick insects go to great lengths to remain unseen by predators such as birds and reptiles. They mimic the swaying motion of leaves and branches in their movements; at other times they lie absolutely still for hours. Their nocturnal habits make them even harder to spot for predators — and for scientists, too. To study them better, Glaw and his colleagues bred the stick insect species in captivity.

Given how heavily stick insects rely on subterfuge to survive, scientists like Glaw are baffled by why the males of some species are so vividly colored. One theory leans toward evolutionary advantage: in the world of drab, dry stick insects, the brightly colored males attract the females. However, this jeopardizes their camouflage, so any mating advantage conferred by the bright hues must outweigh the high risk of becoming food. Another theory posits that the flashy colors may serve as a warning for predators, signaling toxicity.

“Our data point to the still unresolved question why the adult males of many Achrioptera species are colorful, whereas the females are not. I guess and hope that our results will stimulate further research to resolve this question,” Glaw said.

The splendid coloring of the male giant stick insects means they could potentially emerge as one of the flagship species to promote the unique biodiversity of Madagascar, especially Montagne des Français and Orangea, according to Glaw. “Both these areas are unique in terms of their biodiversity and harbor a large number of micro-endemic animals and plants,” he said.

Banner image of Achrioptera manga, one of two new Madagascan species discovered by Frank Glaw, Sven Bradler and colleagues. Image by Frank Glaw.

Malavika Vyawahare is the Madagascar staff writer for Mongabay. Find her on Twitter: @MalavikaVy

Citation

Glaw, F., Hawlitschek, O., Dunz, A., Goldberg, J., & Bradler, S. (2019). When giant stick insects play with colors: Molecular phylogeny of the Achriopterini and description of two new splendid species (Phasmatodea: Achrioptera) from Madagascar. Frontiers in Ecology and Evolution, 7, 105. doi:10.3389/fevo.2019.00105

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This story first appeared on Mongabay

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