As It Happens·Q&A

Beleaf it or not, these 2 different-looking insects are actually the same species

At the Montreal Insectarium, entomologists have cracked a century-old mystery around an elusive leafy insect. 

Scientists thought Phyllium asekiense were all male — until a Montrealer solved the mystery

A male Phyllium asekiense is pictured on the left, and a female on the right. For years, scientists thought they were 2 different species. (Montreal Insectarium)


At the Montreal Insectarium, entomologists have cracked a century-old mystery around an elusive leafy insect. 

Scientists have long wondered how a species of stick insects appeared in nature with seemingly no traces of females — not even mothers. The seemingly single-sex Phyllium asekiense puzzled entomologists for years, until Montreal's Stéphane Le Tirant solved the mystery. 

Le Tirant is an expert in beetles, but he is fascinated by the leafy insects. He was sent a batch of eggs from Papua New Guinea in April 2018, which he and his team carefully looked after in a breeding program.

When the eggs hatched and the insects grew, he saw what looked like a different species of leaf insects, and put two and two together — what were previously classified as two different species are, in fact, males and females of the same species. 

The insectarium published their findings in the journal ZooKeys in September. Some of the insects are now on display as part of their scientific collection.

Le Tirant, whose first language is French, declined an interview request from As It Happens due to language barriers. His Montreal Insectarium colleague Julia Mlynarek spoke to guest host Helen Mann about the findings. Here is part of their conversation. 

Julia, can you start by describing these two groups of insects? How different do they appear?

They appear very differently. They're actually described as different genera, so different groups, altogether. 

The males tend to look more like sticks. They have ... rolled leaves, so they're very elongated, and they have wings. 

The females look a lot more like leaves that are a bit eaten on the sides, and come in different colours from green to orange and yellows. They are a lot larger and they're a lot flatter looking, so they actually mimic leaves a lot more.

The female Phyllium asekiense comes in different colours, mimicking leaves. (Montreal Insectarium)

Why haven't scientists been as puzzled by the female-only leaf insect as they were by the male-only stick insect?

In the female insects, and especially in stick insects, some species are parthenogenetic, so they are able to lay eggs, viable eggs, even if they haven't mated with the male. In this species that was described as females-only, it was considered that maybe they were parthenogenetic.

The males, they can't lay eggs, so that was a really big mystery.

How did the team at the Insectarium determine that these two apparently different insects were actually the same species?

It's a wonderful story of luck, experience and really good knowledge of insects. 

Stéphane Le Tirant, the entomologist at the Insectarium, received a batch of 13 eggs that the technicians then took, led by Mario Bonneau, to rear them out. They took really great care to hatch these eggs as well as possible. It took several months … and once they hatched, they started rearing them to adulthood, giving them different leaves to eat. 

Once [the insects] started growing up, [the team] noticed that some of them looked a lot more like sticks and some of them started looking more like the thicker leaves that were expected, because they were expecting these thick-leaved females. 

Through Stéphane's knowledge of stick insects, right away he noticed that these really resemble a completely different genus. And so they knew that these species were actually the same thing, not a different species.

And as you said, it came after several months of nurturing these little tiny nymphs. What's involved in raising these things?

First of all, you have to figure out the right humidity levels and the right temperature to raise them, so that these nymphs don't die. You have to make sure you don't manipulate them too much because it can add stress, and so with stress, they can die as well. 

For feedings, what do they eat? What would they like to eat? Because these species are not from North America, they're from Papua New Guinea. Can we give some of the plants ... that we have here? Will they survive and be happy?

There was a lot of trial and error. Luckily, our technicians are absolutely wonderful. They were able to figure it out and grow these species.

[They ate] mostly [guava] leaves and I think they tried some oak, as well. 

Even with all that care, though, only five of these 13 eggs ultimately hatched.

Yes, yeah. In nature, insects tend to lay a lot of eggs and not all of them hatch, so actually hatching five out of 13 is not a bad ratio.

A clutch of 13 eggs laid by a female Phyllium asekiense, a leaf insect from Papua New Guinea. The Montreal Insectarium received these eggs in the spring of 2018, when they were smaller than the size of chia seeds. (Montreal Insectarium)

Why do you think it's taken more than a century to solve this mystery? Why hasn't this observation been made somewhere before?

Leaf insects [and] stick insects, are so cryptic in nature that ... it's very difficult to find them in nature. And so, you have to either really know what you're looking for, or just a lot of luck. I think that [people] were looking for the answers, but maybe they were just looking in the wrong places at that time.

Are the males and females normally found together, if they are found? 

When they do mate, they are definitely found together. But because the females don't fly, they tend to sort of stick around the leaves and move about a bit. The males fly ... so they may not be found in the same areas.

Ecologically, this group is not very well known because, as I said, they're very difficult to observe in nature. They're high up in the trees, in a remote place of the world.

Julia Mlynarek is an entomologist at the Montreal Insectarium, where her colleague Stéphane Le Tirant made the leaf insect discovery. (Montreal Insectarium)

What's the significance of this discovery in the entomology world?

It's just a very cool story to be able to link the males and the females together, which is ideally what we want. It also helps out with species identification. 

Now that we know that the males and the females look very different ... we can go back through all the natural history collections that we have and verify them.

Then, you know, for insect conservation, we can actually see what these species will look like, how common they are, how many males there are, how many females there are.

There's a whole bunch of natural history questions and ecological questions that can be answered now that we know what these species actually look like.

Written by Mehek Mazhar. Interview produced by Chloe Shantz-Hilkes. Q&A edited for length and clarity.

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