In two new species of giant stick insects from the dry forests of Madagascar, males turn blue or multicolored at sexual maturity.
Achrioptera manga, an adult male from Foret d’Orangea, Madagascar. Image credit: Glaw et al, doi: 10.3389/fevo.2019.00105.
The two newly-discovered species belong to the genus Achrioptera, a group of extremely large and exceptionally colorful stick insects endemic to Madagascar and the Comoros Archipelago.
“Most stick insects resemble sticks and, typically, their coloration is highly cryptic,” said Dr. Sven Bradler from the University of Göttingen in Germany and co-authors.
“Only very few species show conspicuous coloration when undisturbed, though some use their conspicuously colored wings to startle potential predators.”
“The genus Achrioptera is an exception, since the males of most large species permanently present their extremely colorful bodies to the environment and potential predators.”
Dr. Bradler and colleagues demonstrated that the well-known species with blue males from Montagne des Français and Foret d’Orangea in the far north of Madagascar, previously attributed to Achrioptera fallax, represents a completely new species, named Achrioptera manga.
They also discovered a second new giant species in these forests, Achrioptera maroloko, which is among the largest insects — females reaching up to 9.5 inches (24 cm) total length — on Earth.
“Genetic tests confirm that the quirky individuals are in fact two new species,” Dr. Bradler said.
“The reclassification places members of this group as close evolutionary relatives to other Malagasy stick insects, rather than cousins from overseas as previously thought,” he added.
“This is a potentially major finding, as it challenges the prevailing view that sticks insects colonized Madagascar multiple times.”
Achrioptera maroloko, an adult female bred in captivity from ancestors from Montagne des Français, Madagascar. Image credit: Moritz Grubenmann / Glaw et al, doi: 10.3389/fevo.2019.00105.
The discovery also prompted the researchers to wonder: what reproductive advantage do these males gain from their bright colors, that is worth exposing themselves to predators?
They bred the new giant stick insect species in captivity to observe their behavior.
“Males of one species started mating attempts only when they achieved their bright blue color,” said Dr. Frank Glaw, from the Bavarian State Collection of Zoology in Germany.
“This might suggest that the males use their bright coloring to attract a mate. However, it is hard to believe the males could find a mate before being eaten — unless their bright coloring acts as a deterrent to predators.”
“Males searching for a mate have to move about more, so pretending to be a stick becomes tricky. Better perhaps to plump for the opposite: a brightly colored warning.”
“Bright colors — suggestive of toxicity — keep safe vivid members of other typically camouflaged species, like lividly colored Malagasy frogs.”
“In support of this, all stick insects have neck glands that repellant substances, and these are typically well-developed in brightly colored species. Alternatively, like the frogs some giant stick insects may have developed the ability to accumulate toxins from their food.”
The discovery is reported in the journal Frontiers in Ecology and Evolution.
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Frank Glaw et al. When Giant Stick Insects Play with Colors: Molecular Phylogeny of the Achriopterini and Description of Two New Splendid Species (Phasmatodea: Achrioptera) from Madagascar. Front. Ecol. Evol, published online April 2, 2019; doi: 10.3389/fevo.2019.00105
