The net represents entanglement, confusion, treachery, and deception. It seems to be the most widely used of the Oresteia's symbols and appears in multiple situations such as Zeus' metaphorical casting of it over Troy (355-60, Agamemnon), Cassandra's hysterical claims of the net like death trap awaiting Agamemnon (1125-29, Agamemnon), and Orestes' angry account of the entrapment and murder of his father (1114-16, The Libation Bearers). Along with these specific references to the net, there is also an ongoing theme in the Oresteia of characters weaving plots and lies in order to ensnare their intended victims.
The Serpent / Snake
The snake is very similar to the net in that its main characteristics are entanglement, and deception. Both symbols usually remain out of sight until it is too late to avoid them. The snake though incorporates a human element. It is the combination of intellect and cunning as a means of entrapment rather than just an object of entrapment itself. The snake implies thought along with deception. An example of this would be Clytamestra (the snake) entangling Agamemnon in the robes (the net) in order to exact her revenge.
Light vs. Dark / Old vs. New / Juridical Justice vs. Retributive Justice
All three of these are major motifs in the Oresteia and, in a way, are all interconnected. Throughout the plays there is a sense of change. Each play brings its characters closer to a new way of being. In the beginning the house of Atreus is covered in the darkness of bloodshed, vengeance, and war. The furies have free reign and torment all those they encounter. Then when Orestes returns, at the will of Apollo (who represents the Sun and illumination), we begin to see a change. It seems to be a transition period between dark and light, old and new. As time passes the need for vengeance fades, even though it is still clearly present, and thought and reason become more prevalent. Finally, the full transition is made to the new, illuminated way of life when Orestes is found innocent, the cycle of retributive justice is replaced by juridical justice, and the Furies are appeased.
These are just a few of the major themes and symbols in the Oresteia and are far from complete in their analysis. If you have anything to add, or just completely disagree with what is being said please comment.