Meet Cephalotes varians, the Turtle Ant. I only just found out about these wonderful critters from the wonderful blog Myrmecos. These little fellows have a worker variant with this impressive flat-top. The hive inhabits already existing cavities and holes in a tree trunk. When it comes time to defend their nest, the larger, armored workers plug the entries with their heads. They’re a living portcullis, protecting the entry from intruders.
Damn, that is badass!