For female fig wasps, the best place to lay eggs is in the developing fruit of the fig plant. And for parasitic fig wasp females, a fig that contains the larvae of other insects is that much better. By parasitizing these wiggly grubs, she gives her own eggs the best start.
In order to bore her way through the tough, woody unripe Ficus racemosa fruit to find larvae already developing within, the female is equipped with a really long, skinny ovipositor -- a needle-like appendage for injecting eggs. In Apocryta westwoodi grandi, it’s 7 to 8 millimeters long, 15 microns in diameter, and comes with a sharp tip.
Now, Lakshminath Kundanati and Namrata Gundiah from the Indian Institute of Science in Bangalore show how these parasitoid wasps have dramatically different ovipositors than pleasant pollinator wasps, like Ceratosolen fusciceps, who lay eggs in the soft flowers of the fig fruit. “Our first question was why don't we look at the different adaptations that these two species undergo?” Gundiah says.
Using scanning electron microscopy, the duo took a high resolution look at the tips of the insects' ovipositors. They discovered that the end of the parasitoid wasp's ovipositor looks like a drill bit -- complete with little teeth to bore through the fruit’s woody exterior. The end of the pollinator wasp's ovipositor, on the other hand, resembles a spoon.
To see what the drill bit was made of, the researchers focused a beam of electrons on the tiny tip and recorded the X-ray spectrum that’s emitted. Turns out, the serrated structures are enriched with zinc for hardness, enhancing the wear resistance of the drill bits. When they used an atomic force microscope probe to see how hard the zinc-enriched teeth were, they got a hardness value of 0.5 GPa. "That is almost as hard as the acrylic cement used for dental implants,” Gundiah explains. Sensory structures at the tip guide the ovipositor to the best spot for egg laying.
Here’s a video of a parasitoid mom at work. Spoiler alert: there’s a surprise attack about 30 seconds in to remind you that her whole arduous task needs to be done fast.
"There are many different challenges for this one tool," Gundiah tells Popular Mechanics. "It must be hard but flexible so that the female wasp can curve and bore it through the fig. And the wasp must be able to use it repeatedly and efficiently without it wearing down or fracturing." Not to mention the sheer size of the tool: How do you keep something that long and thin from buckling?
The wood-borer's ovipositor also has tiny pits along the shaft, right about where it bends as the tip is driven into the fruit; this allows the structure to flex without breaking. The team also filmed the tiny wasps as they impaled unripe figs on the Bangalore campus. They watched the ovipositors bend and flex, and then they calculated the buckling forces the structures can tolerate: up to 7 micronewtons, just so you know. “It can’t fracture when it buckles, so it’s a very cleverly made design,” Gundiah tells National Geographic.
The work was published in the Journal of Experimental Biology this week.
Image: Lakshminath Kundanati via The Company of Biologists