Often overlooked on walks, acorns from previous years are a home to a variety of life...new acorns are used by a moth which lays its larvae inside...a larva will eat the inside of the acorn and pupate inside the hollowed out acorn...and later eclose from the original hole made by the female adult moth to insert the egg inside...so what's left is a perfect little home for ants and spiders. An old acorn can house a colony of approximately 100 ants and their brood.
There are several genera of ants that use old acorns as home. In particular, there are what are known as slavemaking ants. The surest sign that you've found a slavemaking nest is a mix of very small black and very small red ants. There are two varieties of target hosts, one which is black (Leptothorax longispinosis) and one which is red (Leptothorax ambiguus).
If the host or slaves are entirely made up of L. longispinosis, then the slavemaker Protomognathus americanus can be distinguished from its hosts by looking at the head. The slavemaker has a considerably larger head.
The hosts and slavemaker are slow moving ants so no worries about ants running up your arms etc. but there are other varieties that do move quickly.
(There are no common names for these ants other than, I guess, little black ants and little red ants...lol...so I was stuck with the sci names...)
Supercolonies may be made up of several acorns lying close together.
Next time you're in the woods, have a look...you might be surprised......and if you do find mixed colonies...please let me know the location of the woods.
