At first glance I had taken that (largely pale-colored) protuberance (appearing to be a sliver coming off the dried leaf stem and touching the caterpillar near its "cheek") as simply a piece of plant debris. And, having read your title for the post, I was wondering where the oviposition image was...and thinking maybe you were just inferring oviposition, or had seen but not captured it in a photo. (Yes, I can sometimes be blind as a bat ;-)

But then I followed the protuberance back from the caterpillar's "cheek" rightward below the tachinid's head, between its forelegs, and towards its black-bristly base behind the leaf stem and finally joining the rear of the abdomen behind the head of the second fly!

I had no idea that female tachinids had such a long ovipositor that they could extend in and out of their abdomen (like an old-fashioned hand telescope).

Excellent observation and photo-documentation!

Moved from Tachinidae.

Moved from ID Request.