Wasp Bores Into Fruit With Metallic “Drill Bit”

To lay her eggs, the female parasitic fig wasp has to pierce the tough skin of unripe figs. Luckily, she has a built-in power tool: A drill-bit-like appendage that’s thinner than a human hair and tipped with zinc, a new study reveals.

“If you look at this structure, it’s so beautiful in the sense that it’s hard but maneuverable, which is a tough challenge” for a drilling tool, said study leader Namrata Gundiah, a mechanical engineer at the Indian Institute of Science in Bangalore, India. (Watch a video of a parasitic wasp at work.)

A parasitic wasp prepares to drill into a fig. Photograph by Lakshminath Kundanati

“These kinds of structures seem to bore so efficiently—that’s what is really amazing about this system,” she said.

Other wasp species have been found to have similar adaptations, but Gundiah was able to show for the first time that the zinc makes the wasps’ drill bits harder, according to the study, published May 28 in the Journal of Experimental Biology.

Tough Tip

Gundiah, who became intrigued by parasitic fig wasps after seeing them in a David Attenborough documentary, already knew that the insects used long structures known as ovipositors at the rear of their bodies to inject eggs into young figs.

The eggs hatch into wasps that in turn parasitize the larvae of other insects growing inside the figs. (Also see “Pictures: Wasps Turn Ladybugs Into Flailing ‘Zombies.'”)

Gundiah and her graduate student Lakshminath Kundanati wondered what made this wasp’s ovipositor tips hard enough to drill through the fig’s skin.

They also wanted to know if the ovipositor tips of the parasitic wasp (Apocrypta westwoodi grandi) were structurally different from those of a pollinator wasp species (Ceratosolen fusciceps) that lays its eggs in the fig tree’s soft flowers rather than in the hard figs.

The researchers had easy access to both species of wasps from fig trees on their Bangalore campus, and they collected more insects from the wild. They then used electron microscopes to take a high-resolution look at their ovipositor tips. (Also see “Nearly 200 New Species of Parasitic Wasps Discovered in Costa Rica.”)

The parasitic wasp’s ovipositor tip looked like a drill bit, with teethlike indentations that could help bore into the tough fruit. The pollinator wasp’s tip was spoon-shaped.

Using the electron microscope and an x-ray detector, the researchers also identified the x-ray signature of zinc on the ovipositor tips. Gundiah said, “We see it very consistently only at the tip and not anywhere else.”

She used what’s called atomic force microscopy to calculate the hardness of the zinc-enriched tip, and found that it was about as hard as the acrylic cement used in dentists’ offices.

Natural Inspiration

Even with a zinc-hardened tip, the long ovipositor is so slender that the researchers wondered how it could bore into figs several times in the wasp’s life without breaking.

When Gundiah and Kundanati filmed the wasps boring into figs, they observed the ovipositor bend and flex but not break. By calculating the buckling pressure on the structure, Gundiah learned that “it can’t fracture when it buckles, so it’s a very cleverly made design.”

Gavin Broad, senior curator at the Natural History Museum in London in the U.K., said in an email that a several of the study’s findings have been seen before in other wasps, but some of its most original revelations involved the calculation of the buckling pressure on the ovipositor.

“This is an important feature of long ovipositors—that they must be flexible in order to both move in different directions through a [material] and take the pressure when being forced through tough areas,” he said.

Gundiah said that the wasp’s drill bit could serve as an inspiration for better microscopic boring tools, needles, and probes, including ones that could be useful for minimally invasive surgeries. (See: “Cheetah-Cub Robot Created: See Other Nature-Inspired Machines.”)

“I’m trained in studying steel and other kinds of synthetic materials,” she added, “but if you try and apply the same ideas to look at such biological systems, suddenly it opens up so many possibilities of understanding how nature works.”

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Meet the Author
Sandeep Ravindran is a freelance science writer based in Washington, DC. He has written for a variety of publications including NationalGeographic.com, Nature, Popular Science, Wired.com, and the San Jose Mercury News.