Thanks to everyone who weighed in on the mystery plant! I added a note at the end of that post, but the consensus seems to be that the plant is redberry (Rhamnus crocea, or possibly R. ilicifolia).
Here’s another one, this time from the Sonoran Desert of southern Arizona.
You can see some of the incomplete linear leaf mines in the above photos, and here is a complete one that nearly covers one of the smaller leaves:
Exactly a month after I collected some of these mines, this 2-mm agromyzid fly emerged:
This is the only adult fly I got, and unfortunately it was a female, which means that Owen Lonsdale was only able to identify it to the genus Calycomyza. For most agromyzid flies, species-level identification requires examination of male genitalia.
For reasons I don’t now remember, I had guessed that this plant was something in the aster family (Asteraceae). Mines have been described for a dozen Calycomyza species that feed on members of the aster family, and all of them make blotches rather than linear mines like the one from which this fly emerged. The only Calycomyza species that comes to mind that produces linear mines is C. malvae, which feeds on various members of the mallow family (Malvaceae). It certainly seems plausible to me that this plant is some kind of mallow, having seen desert mallows with leaves ranging from this to this. Any thoughts on what it might be?
I may or may not be on the right track in this case, but noticing what kinds of insects and mites are living on a plant can be a nice quick way to identify the plant. I often recognize black cherry (Prunus serotina) by the Eriophyes cerasicrumena (Eriophyidae) galls on the leaves, or poison ivy (Toxicodendron spp.) by the distinctive leaf mines of Cameraria guttifinitella (Gracillariidae), before I am close enough to see the distinguishing features of the plants themselves. I at first thought this vine was some kind of morning glory (Convolvulaceae) until I saw some milkweed aphids (Aphis nerii) on it and realized it must be something in the milkweed family (Apocynaceae), although I still haven’t figured out exactly what it is.
I should note that even if this plant does prove to be a mallow, that doesn’t necessarily mean the fly is C. malvae. That species is only known from eastern North America, as far as I can tell. But if not that species, it would seem to be something very closely related. I think very few people have ever bothered to look for leaf-mining flies in the Sonoran Desert, so it could easily be an undescribed species. Someone will have to collect some more of these mines and rear some adult males to get to the bottom of this.
Added 11/12/2013: See update at beginning of this post.