It would be helpful for your peace of mind, don't you agree, if we could see a picture of these little insects so that we can identify them?

Please know that caterpillars are not predators, nor are they after you, nor are they in an attack mode. With the exception of a very few species that have stinging hairs, they are harmless to everything but the plants that they are feeding on.

There are very few caterpillars that hang out in 'gigantic swarms' . The pretty tent caterpillars, such as the Forest Tent Caterpillar or Eastern Tent Caterpillar are a very common example.

(Sharing your location with us is important. Perhaps you do live in a part of the world where caterpillars ARE something to be afraid of, lol.)

Please look them up and let us know if one or the other is your beast. If so, they are harmless to you, your home, and even your plants. They may cause temperary defoliation to your tree, but it will grow back.

Pesticides are not recommended for these insects. They have a very long list of natural predators who will reduce their numbers in very short order. You can do a great deal more harm than good by overreacting with pesticides.

Just want to comment that Western tent caterpillars are extremely plentiful and active right now in the PNW. And do tend to "drop out of the sky" from overhanging tree branches when they are of a certain age and activity level. But as rhizo indicates, they are harmless to you, won't be around long (birds think it's a caterpillar smörgåsbord!!) and generally do minimal damage to any trees they are feeding on.