Reading the Akka doc : http://doc.akka.io/docs/akka/2.2.3/AkkaScala.pdf its states in section
2.2.1 Hierarchical Structure
The only prerequisite is to know that each actor has exactly one supervisor,
which is the actor that created it.
But at the top of the hierarchy tree the parent actor has no supervisor ?
It is very well explained in akka doc (see The Top-Level Supervisors section), a little excerpt from it: