Hi. My name is Kyle Burks and I’m a new graduate student here at the Frost. I’m working towards my PhD in entomology. Recently I’ve been working closely with István trying to understand how wasps can steer their ovipositor, the stinger-like egg-laying device.
Parasitoid wasps often have incredibly long ovipositors. Instead of using these structures for defense or killing prey, they use them for laying eggs inside of another insect. Their larvae develop in these hosts and eat them alive from the inside before bursting out, just like in the movie Alien. Some of these wasps are capable of bending and steering their ovipositors as they drill through trees or into other insects’ burrows looking for a host to lay their egg inside of. But how do they flex and curl these long needle-like structures since there are no muscles in the shafts of the ovipositor?
The “needle” of the ovipositor is not the syringe-like tube I had always imagined. It’s actually a series of three inter-locking shafts. The dorsal shaft is usually the largest and provides support for the ventral shafts, which slide back and forth. The tip usually has barb-like little teeth, and as one shaft slides back, the teeth bite into whatever the wasp is drilling through as the other shaft presses further, and the wasp “drills” through the substrate with the ventral shafts. This motion allows wasps to drill through things as tough as trees because the backwards motion of one shaft braces and supports the forward motion of the other shaft, and the ovipositor drills forward with little net force. This drilling action is accomplished by muscles inside the wasp’s abdomen (remember: there are no muscles in the ovipositor shaft; all of the muscles associated with the ovipositor are inside the abdomen).
Recently, we made an interesting discovery. I was dissecting two wasps from the family Gasteruptiidae, and noticed that one had died with its ovipositor in a curved position, while the other had a straight ovipositor. When I dissected out the ovipositor and the abdominal muscles that operate it, I noticed a small bulge at the base of the curved ovipositor. It turned out that the bulge was a small hardened plate, called a sclerite, like the ones that make up the exoskeletons of insects.
But what is so interesting about this sclerite at the base of the ovipositor? The underside of all insects’ abdomens are composed of overlapping plates called sternites, and this tiny sclerite is actually a modified sternite! In these particular wasps that I dissected, the seventh sternite (S7) has been shrunk to only a tiny fraction of the size of the other sternites and is located inside the abdomen at the base of the ovipositor. More interesting still, the lower shafts of the ovipositor pass through this tiny sclerite. When the muscles that would normally connect the sternites to each other to allow for movement of the abdomen (see István’s blog recent blog post) pull on the seventh sclerite, the shafts of the ovipositor are also tugged. This causes the ovipositor to bend!
I think it is fascinating the way that evolution can modify common structures in insects. Theses modifications can be to such a degree that the modified structure barely resembles the unmodified structure and can serve a completely new function!