It was just another day. Way too humid, mosquitos abundant, the forest already feeling like the Ohio river valley swamp jungle it becomes every summer. I was prowling around as usual off the trail, hand pulling invasive Garlic mustard and Stickseed. And then a large, slender insect caught my eye, its shiny wings glittering as it danced around a tall stump. Something long was dangling behind, and when it landed on the stump I realized it was a large ichneumon wasp of a species I’d never seen before.
Though I didn’t know it then, this was a female Black giant ichneumonid wasp, Megaryssa atrata. And she was at this moment preparing to oviposit an egg deep in the stump on the larvae of another wasp, the Pigeon horntail, Tremex columba. In the image above she walks about on the stump, tapping with her antennae in a process called “antennation”, in order to locate a larva. I can’t begin to explain how this works, but in rather dense language of a research study:
“How Parasitoids Use Vibrational Sounding And Vision
In Multisensory Location Of Their Concealed Pupal Hosts” Sabine Fischer 2002
…Members of the hymenopteran families Ichneumonidae and Orussidae have evolved an active host-location strategy called vibrational sounding (antennation). Females “scan” substrates for hidden hosts by transmitting vibrations via their antennae and receiving the reflected signals via the subgenual organs in their tibiae. This echo-location on on solid substrates enables the detection of density differences which may be caused by the concealed body of the host or by its feeding tunnels or pupation chambers.
If you’re beginning to feel awed by the complexity of Magaryssa’s egg laying process – it gets even better. In fact downright strange. M. atrata has the one of longest ovipositors in the genus Megaryssa – 5 to 6 inches! For this slender wand to penetrate even rotting wood, it requires special support.
As I watched, the female’s rear abdominal segments began to unfold, as though an alien being was emerging. She had located a larval host for her egg, and was preparing to drill.
To summon the force required for piercing wood, she possesses a very specialized set of tools. The pale green disc is an intersegmental membrane which enables a 270 degree unfolding of the last abdominal segment. It stretches to form a translucent bag, providing support and stability as it pushes the stylus into the wood.
Her ovipositor has two blades that work together – one drills while the other holds the bit in place. The blades then switch roles with the shaft that drilled taking hold while the other cuts deeper. (Inspired by the ovipositor of this wasp, researchers from Imperial College London are developing a steerable needle technology for difficult medical operations.)
Megaryssa’s ovipositor actually consists of three filaments. The central part is the ovipositor, capable of drilling through wood. Though very thin, it is a tube and the egg moves down the minute channel in its center during egg laying. The other two other thin filaments are protective, bowing out around the wasp during egg laying.
Ovipositing her eggs is the culmination of a female Megaryssa’s short life, and it’s not an easy task. The tip of the ovipositor almost always gets stuck to tiny irregularities on the wood surface. She can lose her balance and then will need to start from the beginning since the exact point of drilling is crucial and must be recalculated for accuracy. Due to the length of her ovipositor it takes over an hour to lay one egg. And she’s at risk especially from avian predators during this time.
Eventually I decided to leave her to her task; pulling myself back from 20 minutes of intense focus felt odd, like returning to the everyday world after an exotic vacation. But now, possessing the memory of one more astounding life form that inhabits this ordinary little fragment of a forest.