Many adult insects jump to get around, but for most insect larvae, their hardened exteriors make this virtually impossible. There are some notable exceptions though, such as the larvae of the Mexican jumping bean moth (Cydia deshaisiana) that jump inside the seeds of the Sebastiania plant, and the larvae of the Emporia melanobasis moth from South Africa that jump inside the seeds of their host tree, commonly known as the Mexican jumping tree. And now we have one more example to add to the list, as researchers describe a new jumping behaviour in the Calindoea trifascialis caterpillar from Vietnam.
"One day while at the National Park in Vietnam, we collected a lot of caterpillars on leaves and put them in tupperware containers under my bed for the night, to study in the morning," says Kim Humphreys from the Department of Natural History at the Royal Ontario Museum, who discovered the behaviour with Christopher Darling from the University of Toronto. "I was sick all night with fever and fainting spells. Large rats besieged us all night with a lot of noise. In the morning I said to Dr Darling that I was too exhausted to eat breakfast, and that I wanted to stay in bed a couple of hours to get my strength back up. He walked the 3 km to a nearby town to eat breakfast."
Humphreys was still in bed when he heard a noise under his bed again. "I thought, 'Oh no - even in the daytime the rats are still at it.' So I looked under the bed, and ... no rats. The noise was coming from the tupperware containers! So I opened the lid, and could not believe my eyes. I took a few jumping caterpillars, forgot I was sick, and ran the fastest 3 km I had ever run to show Dr Darling."
In the early stages of its life, the C. trifascialis caterpillar will construct a tent from the leaves of the Southeast Asian Dipterocarpus tree and anchor it to the plant. Using the tent as a shelter, the caterpillar spends three weeks fattening up, preparing for its transition into adulthood. When the caterpillar is ready to transform, it rolls itself up in a new leaf, detaches from the host plant, and tumbles to the ground in search of a safe place to build a cocoon and develop inside.
On the ground, the caterpillar is at serious risk of being eaten by ants and parasitoid wasps, so it needs to find a safe place as soon as possible. Sort of like how you'd move inside a sleeping bag or a mermaid costume, it repeatedly thrusts the front part of its body up and back, striking the top of the leaf-roll with enough force that it lifts it up and backwards at the same time. If the leaf-roll gets airborne, the caterpillar will progress about 0.75 cm backwards along the ground with each hop. Sometimes the lift is not enough to achieve a proper jump, but it will always move the leaf-roll backwards.
"No other insect jumps using this method," says Humphreys, who describes the behaviour in today's issue of Biology Letters. "No other insect larva jumps in a shelter it makes, and as far as I know, no other insect larva jumps using an oriented method like this one does. So in my mind, there were lots of interesting questions to be asked and investigated."
Here's one inside a clear roll, so you can see its backwards movements:
The first thing Humphreys investigated was why the caterpillar would risk moving from the safety of the Dipterocarpus tree to the ground, where instances of predation are much higher. He found that the caterpillar actually had a better chance of staying alive on the ground than if it was to stay attached to the plant for an extended period of time in its cocoon, where it would likely dry up and die from sun exposure.
Next he wanted to figure out how the caterpillars knew where to go once they hit the ground - because they can't see a thing outside their leaf-rolls. "But they have tiny eyes and can certainly detect the direction to the brightest light - the Sun," he says. "They face this, and jump abdomen, or posterior, first." This doesn't mean they're necessarily headed towards the safety of the shade, however. They could easily be headed towards a huge patch of sunlight that will fry them to a crisp. So why risk it?
Humphreys suggests that the C. trifascialis caterpillar uses the sunlight like a compass to orient itself, so it can move in a mostly straight line. And this could be the most efficient strategy to find a cool, shady place for the pupa to develop into an adult.
"Imagine you are in a forest, lost, with your water running out. You know somewhere there are roads leading to safety. You can’t see where though, because there are so many trees. You have no map, but you have a compass," he says. "To get to safety, should you just wander around, hoping to chance upon a road? Should you use the compass and just go straight, in any direction? Or should you do something in between: a little random, a little straight? This is something like the dilemma the insect has to face. But is a compass without a map any good? The answer seems to be yes."
Earlier this year, a team led by Marie Dacke from Lund University in Sweden reported a similar strategy in another insect that needed to orient its movements in darkness. On moonless nights, the African ball-rolling dung beetle (Scarabaeus satyrus) orients itself by the light of the Milky Way. The starlight didn't turn out to be as effective a compass as the moonlight, but it still got the beetles and their dung to where they needed to be more than three times faster than if they had no light at all.
First video by Chris Darling, second by Kim Humphreys.
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