Is there any way to disable the mirror modifier temporarily? I'm currently modelling a creature with a different spikes locations on his back. I use mirror modifier to model the rest of his body because the only different part that is not symmetric are the spikes on the back.

Is it possible to disable the mirror modifier temporarily, so that I can model my creature with different spikes locations without being mirrored, and then enable it again? It would save me lot of time modelling the rest of his body that will be mirrored.
Modifiers have a few switches to control them:



The Camera icon enables or disables rendering using the modifier.

The Eye icon toggles visibility on the 3D viewport.

The Dotted Square (or Box) toggles visibility in Edit mode.

The Triangle, when enabled,  will show the final modified geometry in Edit Mode and can be edited directly. (This last option can only be used when visibility in Edit mode has been enabled)

To learn about all of the other options for modifiers please read:

https://www.blender.org/manual/modeling/modifiers/introduction.html#interface

Note that the mirror modifier will affect all of the mesh when enabled. The elements that you don't need to mirror will need to be a separate mesh. There is no way to mark elements of the same mesh not to be affected by the modifier.

You might want to separate the asymmetrical features from the original mesh, then 
Apply the modifier to make the symmetric mesh a permanent one, and re-join the two meshes back together.