A cousin to the eastern tent caterpillar, the forest tent caterpillar (Malacosoma disstria) doesn’t actually make tents; instead, it constructs silken mats along tree trunks and branches. Like its cousin, however, it also congregates in large numbers and the mats can be just as unsightly and large as the tents. The caterpillars are harmless to people, and usually only visually annoying or slightly damaging in trees, although in large groups they can defoliate or even kill trees. They grow to become mid-size (about 1.5cm long) brown moths which do not damage plants.
They live all over the US, but are more common east of the Mississippi River. There is one generation per year, with populations being cyclical, and varying widely from year to year.
This photo shows the dark color phase of the larva; they also come in a lighter blue flavor, but both have the distinctive white spots along the midline. Here’s the lighter blue version:
Below is the adult eastern tent caterpillar moth, a lovely reddish-brown thing with two distinct dark stripes on the wings, a fuzzy red dorsum and a nice tent to the wings when at rest. I believe this one is female based on the antennae. Both sexes have feathery antennae in this species, but the males’ are much larger and more robust.
Guess what showed up the day after the above female? Here’s a male of the same species:
I looked at this pair of images and suddenly doubted my identification, because the female seems to have very angled rear edges to her forewings and the male has straight edges. This may just be due to how these particular individuals are holding their wings, but, looking on bugguide, I do see some differences in wing shape, possibly related to gender. Interesting!