by Pavan Kaushik : The few silver linings of the pandemic have been the slight reduction in the destruction of nature. I was walking alone in the woody patches of my campus, whose leaf litter is now left undisturbed due to the pandemic. I saw a mango fruit fallen on the ground. It was half eaten, likely by a squirrel, and was starting to rot. And it was covered with insects. Mostly flies and beetles. There was Drosophila of all shapes and sizes, buzzing around, feeding, boxing, mating, laying eggs on the fruit. It seemed like the happening place for Droso family.
Like any paradise, there’s always the other side. A golden-brown and delicate looking insect landed on the fruit. By the looks of it, it was clear it was an insect parasitoid – likely intending to lay eggs into fly larvae where the larval body becomes food for the baby wasps. With a long black ovipositor, nearly as long the rest of body. The wasp sat there on the fruit, doing nothing. Walked a bit and sat again. No movement. Not even the antenna. Then flew away.
This made no sense. I expected them to lay eggs and move on. I was like, be a good naturalist. Sit and stare. I waited. Within a few minutes, the wasp flew back onto the fruit. Again, doing nothing but sitting, walking a bit and sitting again. Finally, it touched the fruit with the antennae, moved a bit forward, and raised its ovipositor. Then the glorious scene of the outer sheath bending and the fine needle-like ovipositor penetrated the fruit. Like a surgical procedure, it laid eggs, where hundreds of maggots lie within. Another wasp joined her in the oviposition spree.
I was expecting the wasp to use its antennae more, which are a known to be used as a probe. What happened here was more subtle. It seemed very likely that they were using their six legs as a vibration sensor, like a seismograph array, to localise larvae. What I find surprising is their phenomenal ability to track down soft-bodied insects eating soft food. These objects barely give any vibration signals, unlike larvae that eat wood.
One of the reasons insect antennae are so sensitive is due to the Johnston’s Organ. A sensor adept at picking even the slightest mechanical movements, over a large range of frequencies. They are used measure wind speed, sound, feel substrates, etc. Coincidentally, insect antennae have evolved from legs. A similar sensory structure exists in the insect leg, known as femoral chordotonal organ. Could this be what they might be using to probe the soft world to pick the subtle differences between the wind, nearby Drosophila groups buzzing around, and the larvae munching inside the fruit?