I didn't even think that it could be an egg; so I don't know. My camera doesn't have enough resolution to get more detail and, unfortunately, I didn't observe it again for a few days, so I missed that part of the development.
It looks very egg-like, don't you think? I replaced the image with a slightly better close up, so, take another look.

I suspect that the pointy upper left quandrant of the greenish thing is where the egg stalk is anchored into the skin of the caterpillar. There is sort of a dimple there in the skin of the caterpillar. I expect that the egg may have swollen through the absorption of fluid from the host, which would make the egg stalk difficult to discern. I suppose, too, that the first larval instar might not eclose from the egg until the host larva has spun its cocoon or at least started doing so. All Tryphoninae have a stalked egg that is anchored externally in the skin of the host larva. The Eucerotinae also have stalked eggs, but in that case the stalks are glued to surfaces of foliage, and the larvae hatch and sit atop the stalk waiting for a primary host caterpillar to latch onto, so I suppose they are analogous to chiggers. Because of the stalked egg, Townes placed the Eucerotinae as a tribe within Tryphoninae, but they have since been given subfamily status.

This picture was taken on 9/22/10 and just seven days later, on 9/29/10 the wasp larva was building a cocoon inside the caterpillar's cocoon and it had gone from less than 1 mm to about 10 mm in length. The caterpillar was reduced to a dried up husk. Too bad I missed taking pictures of the intervening seven days, pretty amazing!

How did these compare with the rate of development in the Hercus fontinalis you reared?

Just as fast or faster. I picked up the caterpillar on July 3 and the larvae were less than 1 mm long, by the fifth they had doubled in size in the morning and doubled again in the evening. The change that day was very dramatic. They were even a little larger, about 5 mm when they abandoned the shriveled dried up caterpillar. They began to build cocoons on the sixth and emerged as adults around July 15.
Well, D. melanogaster does it even faster at 10 days per generation. But I am still impressed.

I presume this rapid development to be necessary for survival--i.e. that they have to consume the host before it putrefies.