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He was a stick, she was a leaf; Together they made history

In the spring of 2018 at the Montreal Insectarium, Stéphane Le Tirant received a clutch of 13 eggs that he hoped to hatch into leaves. The eggs were not ovals but prisms, brown paper lanterns barely larger than chia seeds.

They were laid by a wild female Phyllium asekiense, a leaf insect from Papua New Guinea belonging to a group called frondosum, which was only known to female specimens. Phyllium asekiense is an amazing foliar insect, present in both summer greens and autumn browns. As Royce Cumming, a graduate student at City University of New York, puts it, “Dead leaf, living leaf, semi-dried leaf.”

Mr. Le Tirant, responsible for the collections of the insectarium since 1989, specializes in beetles; he estimates he has 25,000 beetles in his private collection at home. But he had always nurtured a passion for leaf insects and had managed to breed two species, a small one from the Philippines and a larger one from Malaysia. A Phyllium asekiense – rare, beautiful and above all alive – would be a treasure in any insectarium.

In the insect breeding laboratory, Mario Bonneau and other technicians nested the 13 eggs on a mesh sieve on a bed of coconut fibers and sprayed them often with water. In the fall, and over the course of several months, five eggs hatch into slender, black nymphs. Technicians treated the baby nymphs with the utmost care, moving them from tree to tree without touching the insects, only whatever leaf they were clinging to.

“Other insects, we catch them,” said Le Tirant. “But these little leaf insects were so precious, like the jewels in our lab.

The technicians offered the nymphs a buffet of fragrant guava leaves, bramble and salal. Two nymphs refused to eat and soon died. The other three munched on bramble, moulted, chewed, moulted and moulted again. A nymph turned green and large, just like her mother.

But to Mr. Le Tirant’s amazement, the other two grew thin and stick-shaped and even sprouted a pair of wings. They oddly resembled the leaf insects of Nanophyllium, an entirely different genus whose six species had only been described from male specimens.

Mr. Le Tirant emailed a photo to Mr. Cumming, who confirmed what had now become clear: the two species were in fact one and the same. The newborns had solved a century-old mystery of the missing Nanophyllium female.

“Since 1906 we have never found anything but men,” Cumming said. “And now we have our last solid proof.”

Mr. Cumming and Mr. Le Tirant recently reunited the long-lost companions – broad-leaved females and slender males – into one species, Nanophyllium asekiense, in the journal ZooKeys.

It is actually quite common for leaf insects – which are a family in the broader order of stick insects – are only known to one sex. Many stick insects exhibit extreme sexual dimorphism, with females being unrecognizable from their male companions.

In 2018, Paul Brock, a scientific associate at the Natural History Museum in London who edited a draft of the new document, solved a similar mystery among stick insects. He and his colleagues described the first male Acanthoxyla, a genus of New Zealand stick insect thought to be exclusively female, from a specimen found on a car in Cornwall, England.

“Leaf insects are a particular challenge because they are so rarely found in nature,” said Dr Brock.

Leaf insects are almost impossible to see in nature, and scientists can’t study what they can’t see. One of the few foliar insect experts in the world, Mr. Cumming has never seen a leaf insect in the wild, only specimens in captivity or in museums. Dr Brock has seen wild stick insects, but never a wild leaf insect.

Mr. Le Tirant, who has made numerous insect collecting trips, saw only one leaf insect in the wild. While researching with a local collector in Malaysia, Mr. Le Tirant discovered it after hitting a tree with his large collecting net, which shook many leaves and a leaf insect. “If I was alone I would never have seen a single leaf bug,” he said, shaking his head at his fortune. Mr. Le Tirant brought the insect back to Montreal, where he lived, died and still resides, in a drawer of the insectarium.

Even if someone could distinguish a leaf insect from its arboreal brethren, there is almost no chance that the insect will be in the company of its mate, let alone in the act. While the winged males fly from tree to tree, the flightless females spend their entire lives at the top of the canopy, out of reach and sight, swaying in the breeze like the leaves do. “By chance, someone could be pulled from a tree,” Cumming said.

How, then, to match leaf insects with their companions? With field observation a non-starter, entomologists have resorted to hypotheses. Two decades ago, Dr. Brock was the first to suggest that the female Nanophyllium could be found in the frondosum group. He was examining a pair of male and female Papua New Guinea leaf insects whose uneven legs looked eerily similar.

“It would be a simple task nowadays, undertaking DNA bar coding,” said Dr Brock. But enough evidence was lacking: the female did not have her front legs, and only one species of Nanophyllium had been formally described.

In 2017, Mr Cumming decided to see if he could prove Dr Brock’s hypothesis. He and Mr. Le Tirant spent several years exploring museum specimens, resulting in 21 newly described leaf insect species. M. Cumming, M. Le Tirant and their colleagues spent two years writing an article identifying the common morphology of female frondosum and male Nanophyllium. The similarities were small but certain – two knots on the back of the head and lobed leaflet-shaped legs.

Their article had already passed a peer review when Mr. Le Tirant’s nymphs grew and unexpectedly provided unwavering proof. “We had to rewrite everything,” Cumming said. Mr. Brock is delighted that the puzzle has finally been solved.

At the Montreal Insectarium, the two male nanophylliums flew day and night for four months and died before their sister matured. She lived for nine months, laying 245 eggs in pastel Easter Eggs: blues, yellows and beige. “To have eggs from a female in so many colors?” Said Mr. Le Tirant. “This is something very special, something that I have never seen in the past for a leaf insect.”

Very few of her eggs hatched and no nymphs survived. But Mr. Le Tirant kept all of his eggs, hatched and unhatched, on pins and in jars.

Although the pandemic prevented Mr. Cumming and Mr. Le Tirant from meeting in person, they quickly became friends and will soon complete a larger project to review the evolutionary history of leaf insects.

Mr. Le Tirant still marvels at his luck – at the hatching of the eggs and knowing Mr. Cumming a few years before Mr. Le Tirant was able to retire, giving Mr. Le Tirant the chance to study seductive insects towards the end of a long career devoted to beetles. “You can study rocks all your life, or you can study diamonds,” he says. “What a fabulous insect.”

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