Uff...June is a busy month for me so I've only had a chance to post once and it was only an image. Thanks for holding in there, readers!
Witch Hazel is a flowering plant native to North America (there are two other species found in other parts of the world, however). It is so called because of the structure in the photograph which supposedly looks like a witch's conical hat. Except it's green. And tiny. And it's filled with aphids (seriously, can you get enough of true bugs?! I thought not.)
This structure is what is known as a gall. If you've ever inspected the leaves of trees with any thoroughness you've probably come across galls. They form on the leaves of many local species, especially oaks. These oak galls are generally formed by wasps. For a long time I knew of their existence but had no idea what they were.
These structures protect the developing larvae of insects and a few mites and roundworms. After the eggs of these animals hatch the larvae begin to feed on the plant inducing tumorous cell grown that forms the structure we know as a gall. The gall then serves to protect the larvae as it grows. Most of the gall causing insects are in one family of flies.
But as I alluded to above the witch hazel galls are not caused by flies but by aphids. These aphids have a very complicated life cycle. After emerging from witch hazel galls they move to feed on birch. They remain on birch for six generations but each generation produces a different form of the aphid with progressively shorter legs and antennae. After the sixth generation females are born ready to return to and lay their eggs on witch hazel, once again producing the signature witch hat gall. There is also a second aphid species that form galls on witch hazel on the stems instead of the leaves.
Kricher, John C. and Gordon Morrison (1988). Ecology of Eastern Forests. Houghton Mifflin: NY, NY.