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A wasp armed with a toothed, saw-like drill

  • Wasp oviposition. Image: Laksmikanth Kundanati.

Wasps lay eggs using an egg laying organ called the ovipositor. Some wasps lay eggs inside figs and need to drill through the fig fruit to do so. Researchers from IISc have unearthed the mechanism that these wasps use — they have teeth like projections on the ovipositor, like a saw. And that’s not all, these teeth are coated with zinc. Insights gained from this study may help us to build tools that aid in robot assisted surgery, and novel mechanisms to bore through hard surfaces.

This release was picked up by The New Indian Express (Wasps Give Newer Insights into Robotic Surgery) and Delhi Daily News (Wasps use zinc-tipped ovipositor to drill into fruits to lay eggs).

Boring into various surfaces to lay eggs is an integral part of a female insect’s life. To successfully drill holes, and lay eggs in safe places, the insects have to overcome some key biomechanical challenges. Researchers from the Mechanical Engineering department of the Indian Institute of Science (see below for names and affiliations) have shone light on how the egg laying organ of parasitoid wasps double as a reliable drilling machine.

Figs have a special fruit called a syconium, which has multiple flowers lined along the inside. Each fig has its own wasp species, the pollinator wasp, which enters the syconium through a special opening meant for it and lays its eggs. The flowers get pollinated by the female wasp walking around inside. There are other wasps, called parasitoid wasps, which don’t pollinate or benefit the fig plant in any way. They sit outside the fig, drill a hole through the outer covering and lay eggs inside. The larvae of parasitoid wasps feed on the pollinator wasp larvae, making the inside of a fig a populated warzone.

Since a pollinator wasp has to enter the fig through a hole made for it, it has a smooth ovipositor. However, a parasitoid wasp that has to bore a hole through the fig needs to have a strong ovipositor that can be used to drill multiple times in its life. Through advanced techniques like Scanning Electron Microscopy and Atomic Force Microscopy, researchers have deciphered what materials make up the two ovipositors.

In a paper appearing in the ‘Journal of Experimental Biology’, the researchers describe the structural differences between the ovipositors of the two types of wasps. The parasitoid wasp (Apocrypta sp.) has a body just 2 mm long, with a 5 mm long ovipositor. Along the ovipositor are teeth-like structures, similar to a double-edged saw; the teeth are also reinforced with a considerable amount of zinc at the tips. The pollinator wasp (Ceratosolen fusciceps), with much less demanding requirements, has a relatively smooth, spoon-like structure.

“There is a mutualism that exists between the pollinators fig and the wasps that has evolved over several millions of years. The parasitoid is one that tries to take advantage of this situation and this has evolved with her trying to access the pollinator larvae so she may parasitize them to ensure the evolutionary success of her offspring”, explained senior author Dr Namrata Gundiah.

Using a video recording, the researchers could assess how the parasitoid ovipositor resisted buckling forces while boring. Parasitoid wasps have bigger and stronger ovipositors than pollinator wasps. In both the wasps, the ovipositors are bigger in the tip region (remember in this case ‘big’ is comparable to the width of a strand of spider web silk). Similarly, the tip, one that should actually drill the hole, is much stronger than the inner regions of the ovipositor.

“Earlier studies have shown the presence of zinc and manganese in the ovipositor tips of other wasps. Hence, this [searching for metals] was the first step we embarked upon in our study”, said Dr Gundiah. Though the presence of these metals is suspected to make stronger tips, researchers are yet to find conclusive evidence for that.

Before deciding on the site for laying eggs, the parasitoids do a reconnaissance of fig syconia. They have mechanical and chemical sensing systems that enables them to identify appropriate surfaces just by tapping the possible sites. Once the location is decided, the wasp rises her abdomen, drops her saw-like ovipositor to the surface, and starts drilling through a vibrationary mechanism. Total time for drilling varies from about two to seven minutes, depending on many factors including the stiffness of the surface, the number of eggs laid, and the location of the pollinator larvae.

A closer look at the video recording of the egg laying process revealed that the ovipositor of the insect buckled many times during a single boring event. Through this, the wasp understands that the ovipositor can’t take any more load, and retracts partially. This feedback mechanism enables the insect to avoid fracture during the boring process. The researchers also found out the load at which the ovipositor begins to buckle.

Though the it is a study of an insect, it is completely relevant to engineering. “We are basically trying to understand material characterisation and mechanics of a biological system. The knowledge we gain here can be applied elsewhere like, for developing tools that aid in novel surgical methods”, says Laksmikanth Kundanati, the first author of the paper.