The other day I went out to pick some spinach for breakfast, and I noticed this fly resting on one of the leaves:
This is a lauxaniid fly in the Minettia obscura species group, which I recognize mostly by the orange-tinted wings. I often see them resting on leaves, but they usually only let me get one imperfect photo before flying away. This one clung steadfastly to the leaf even after I picked it and continued to fill my hand with other spinach leaves. As I was walking back to the house, it wandered away from its perch and revealed that it had been sitting on this leaf mine, which was small enough to be obscured by its body:
This is a very early mine of Pegomya hyoscyami (Anthomyiidae), another fly. The larvae would devour the entire leaf within a few days if I let them, but as long as we eat spinach regularly we’re able to stay ahead of these flies for the most part (and the occasional leaves that become too infested for us to want to eat go to the chickens instead). Anyway, the lauxaniid moved to the underside of the leaf and proceeded to probe the Pegomya eggs with its mouthparts.
It continued to do this even as I walked inside, set down the rest of the spinach leaves in the kitchen, and placed the fly’s leaf on my desk to take these photos. (By the way, all the tiny spheres on the leaf are
calcium oxalate crystals that originate from the leaf’s stomata salt bladders—see Eric LoPresti’s comment below.)
As you can see, by the time I started taking pictures it had stepped away from the eggs a bit, but still showed no interest in abandoning the leaf. Finally, I took the leaf back outside and had to blow on the fly as hard as I could about five times before it finally decided to take off.
As I’ve mentioned previously, the larvae of some lauxaniids (including some Minettia species) are leafminers in decaying leaves on the ground. Others feed externally on decaying plant matter. I suppose this one was attracted to the compromised tissue where the Pegomya larvae had begun to mine the leaf, but it seemed odd that it was so unwilling to leave this leaf behind.
Yesterday I went for a walk in the woods and spotted one of the mystery leafminers I’ve been trying to rear for the past few years—an unknown scathophagid fly on painted trillium (Melanthiaceae: Trillium undulatum).
I had never encountered this phenomenon before, but just three days later, here was another lauxaniid sitting on a leaf mine! I believe it was another Minettia, though not in the obscura group [Edit: John Carr has identified it as M. lupulina].
I didn’t have my good camera/lens with me, but the fly obligingly sat there while I took several photos until I’d gotten the best shot I could with the camera I had.
It stayed on the leaf after I picked it, departing only after I rolled the leaf up and slipped it into a rearing vial.
So what’s going on here? It’s tempting to think these flies are laying eggs on or in the leaf mines and their larvae will develop as secondary invaders in the mines, maybe after the original miners have left. For that to be the case, of course, both of these flies would have to be females, and I’m not familiar enough with these to be able to tell from the photos. Naturally, I ate the spinach leaf and destroyed whatever evidence might have been there, but I suppose I’ll hang onto the trillium leaf for a while after the scathophagid larvae exit it and see if any lauxaniid larvae appear.