Time
We see time as a major theme in this film from the get go. The alchemist's story begins in the 16th century and ends in the 20th, this reveals how the desire for time plays a major role in the story. del Toro also uses the man dressed as a clock at the New Year's Eve party to represent time running out and/or getting longer as Jesús has begun to live eternally. De la Guardia also represents the idea of never wanting to do, and having a hunger to live forever so great that he is willing to commit murder in order to prolong his venture into the afterlife. Finally, Angel is biding his time as he waits for his uncle to die so he can inherit his fortune, this is why he cares very little as to whether the cronos is recovered or not.
Monsters
As a director, del Toro uses this theme in nearly all of his films. He questions the authenticity of someone being a monster based on the outward appearance of them. Thus, when Jesús is transformed into a walking monster by all accounts, the idea of being a monster is flipped on its head. This is something we've seen with Frankenstein, and del Toro shows how men are the true monsters represented in Angel and De la Guardia.
Mythology
The mythology elements in the film relate to the insect and the cronos in which it lives. del Toro creates a mythology that allows for the audience to believe that this could be quite possible as he ties the fact that mosquitoes and bugs are able to be exhumed from thousands of years of being fossilized. And, they are connected to water which is then connected to Jesus Christ. And, from here we are given the equation which produces eternity. The mythology also creates the need for De la Guardia as he believes in it more than salvation and living eternally once he has died.
