Each is approx 1 inch long with a black head and a maroon & white lobed body. A sticky web-like material was underneath this mass of writhing beauties hanging on the side of a tree. (Pleasant Hill, Missouri)
Your caterpillars are Walnut caterpillars.
These larvae do feed on hickories, pecan, and walnut. Trees can withstand 2 or 3 consecutive years of heavy defoliation before they die. Severe infestations do not occur very often. There are over 13 different parasites that keep this insect under control. So, unless the tree has had infestations for several years it is best to let Nature keep the population in check and not to treat.
This insect is found in eastern North America west to MN south into OK, TX and then Mexico.
There is one generation in the north and up to 3 in the south. During molting, larvae in the group usually congregate on the trunk of the tree. When they are ready to pupate they drop to the ground and burrow into the soil. So, they are not congregating to pupate but to molt. The walnut caterpillar overwinters as a pupa.