The ‘Fearsome’ Spiny Leaf Insect
When PhD student Yasaman Alavi first saw a spiny leaf insect, it was not exactly love at first sight. “To be honest, I was a bit sceptical of working with them,” she says. “I had never seen insects that big.”
Spiny leaf insects (Extatosoma tiaratum) are large and a little bit otherworldly. The females grow up to 16 centimetres long, have leaf-like legs and spines along their back. The males are slender and have three ocelli (single-lens ‘simple’ eyes) between their two regular eyes. Combine this with their oddly triangular heads and they resemble fearsome little aliens.
And Hollywood agrees. While they’re actually harmless herbivores, these spiny leaf insects were used to incite terror in the 1984 classic Indiana Jones and the Temple of Doom.
Trickery, Toffee and Antish Antics
Yet Yasaman has grown to love the species. “It makes me so sad that I will have to stop working on them if I move away, that I can’t take them with me,” she says. And I must admit, I find myself equally impressed by the spiny leaf insects. Aside from their extraterrestrial appearance, they have elaborate ways of defending themselves and making young.
These critters are masters of camouflage – their legs perfectly imitate the eucalyptus leaves that they eat. “They’re very cryptic,” Yasaman explains. “I have to be careful [in the lab] not to throw them away with the dry leaves!” When they are threatened, spiny leaf insects mimic scorpions, coiling up their abdomen and making clicking sounds. They then emit a chemical secretion. But luckily for us humans, their defensive odour actually smells like toffee!
To disperse her young, the female spiny leaf insect climbs to the bottom of the tree and flicks her eggs to the ground. Atop the egg is a knob, called a capitulum. Attracted by the capitulum, ants collect the eggs and carry them underground to their nests. Amazingly, when the eggs hatch, the nymphs (young insects) look and behave just like the ants! In time, the nymphs scamper up a tree, begin a series of moulting phases, and, eventually become reproductively mature spiny leaf insects. And this is precisely where Yasaman’s research comes in.
“The cool thing about this species is that females can start laying eggs as soon as they’re mature, irrespective of whether they’re mated or not,” Yasaman says. Spiny leaf insects are able to produce unisexually, through a process called parthenogenesis. This is where a female produces viable offspring from an unfertilised egg. The spiny leaf insect is one of the few species that reproduce via ‘facultative’ parthenogenesis, meaning females can reproduce sexually and via parthenogenesis.
Little is known about the evolution of parthenogenesis. So Yasaman is using a variety of techniques to understand this phenomenon: from molecular biology to behavioural ecology. She is also comparing the fitness of spiny leaf insects born from sexual reproductions with those born from parthenogenesis.
From Tehran to the Tropics
Most of Yasaman’s PhD is lab-based. But in 2014, she visited rainforests around Cairns to collect 15 spiny leaf insects for her research. The species is nocturnal so, armed with a torch, she joined a group of entomologists in the rainforest at night. “It was creepy,” she recalls. “There were many pythons … and we met people who were searching for venomous snakes!”
Tropical Queensland is certainly a long way from Tehran, where Yasaman grew up. But even in Iran’s bustling capital, she had encounters with wildlife. Yasaman’s brother, a vet, would bring home cats, squirrels, rabbits and even an owl once! She says, “I was always interested in animal behaviour – what they do and why they do that”.
While studying her Masters in Sweden, Yasaman took an animal behaviour course. “I remember I went up to the lecturer at the end of class and said, ‘I can actually do this, as a profession?’ Because it looked so fun, I thought nobody can do this as a profession!”
I felt relaxed as soon as I entered the School of Biosciences’ insect lab to check out Yasaman’s research. There are crickets in containers, and their collective trilling sounds like meditation music – all night-forest noises and suburban calm.
Yasaman shows me to the spiny leaf insects – the nymphs that are just two days old do look just like ants! And the adult females, so rotund and spiky, sway in some imaginary breeze.
Yasaman’s affection for spiny leaf insects is clear. Yet she is still iffy about one thing. “The males are still a bit…” she pauses before going on. “Only the males fly, but they don’t fly for so long, so they try to land on the closest object. Sometimes that object is your head”.