The tree in our front yard had fewer caterpillars this summer, so the yard had fewer thread-waisted wasps. Instead of a daily swarm of wasps under our tree, I saw one or two a week.

The wasps dug burrows as usual, but I didn’t see any of them return to their burrows with prey.

Twice I waited over an hour as wasps searched through leaf litter on the ground and branches overhead. Both times the wasps were still hunting when appointments called me away.

My wasp failures were disappointing, but it’s always worthwhile to spend a few hours sitting quietly in the yard. Last year as I waited on the wasps, I found a wolf spider carrying her army of spiderlings. This year I found flies.

The flies caught my attention because they seemed as interested in the wasps’ activity as I was. They watched as intently as I did.

As the wasps dug, three or four flies positioned themselves within a few inches of the developing burrows. Each time a wasp carried a pinch of excavated dirt away, the flies zoomed in and flew quick figure-eight patterns over the burrow. When the wasps returned and resumed digging, the flies lit nearby and watched until the wasps left again.

The longer I watched, the more convinced I became that the flies were kleptoparasites. They were waiting to deposit their larvae in the wasps’ larder, alongside the wasps’ hungry larvae.

The behavior is well-documented. It’s one of those complicated, clever twists of nature that fills me with questions. How do the flies learn to follow the wasps? Generation after generation, flies see a thread-waisted wasp and something whispers deep within their experience. Follow it. And they obey. Why?