I think I have identified the dew drenched mystery caterpillar from August 18th. After going through about fifty pages of photos on the Bug Guide site and other sources I have decided this fuzzy caterpillar is the fall webworm (Hyphantria cunea). Fall webworms are in the in the lepidopteran family Arctiidae (now placed in the Subfamily Arctiinae of the Family Erebidae) which includes the woolly bear.
Fall webworms feed on many species of deciduous hardwood trees and shrubs. I usually find them or rather evidence of them in the form of leafless branches on apple trees shrouded in silk full of frass. Seeing these full grown solitary larva feeding on milkweed, chicory, knotweed, and raspberry is a bit perplexing. Do the mature larva need to feed on these plants for some reason? Other than the personal observations of a few people there does not seem to be any published information on fall webworms eating leaves of herbaceous plants. All technical references say it eats leaves of deciduous trees but they are not the final word on the subject.
Other Arctiinae are out and about now, too, such as the familiar woolly bear which is very common this year. I recently found another caterpillar, shown in the three photographs below, that is probably the larva of Arctia caja, the great tiger moth. It is feeding on the yellowing leaves of Apocynum sibericum (dogbane). In three days it ate two entire leaves and started on a third. Dogbane is a toxic plant with chemicals similar to those found in milkweed so for this caterpillar to eat it is quite a feat. The caterpillar may be sequestering dogbane toxins which will make the adult moth inedible. Or perhaps the leaves are less toxic as they begin to die.
A third Arctiinae larva I found and is shown in the final photograph. This one has tufts of yellow and black hairs and was feeding on apple leaves. A check of the photos at the Bug Guide web site and the book Tiger Moths and Woolly Bears show it to be the caterpillar of the spotted tiger moth (Lophocampa maculata). It feeds on a wide variety of deciduous trees and shrubs.
My knowledge of the moth and other insect diversity here is slowly increasing each summer. Identifying and cataloging the species has become a project connected with my gardening activities. I set up many patches of plants, some annuals, some perennials, to attract bees, moths, and butterflies. Other insects come in as well to feed on the plants and flowers and with them come their various predators.